Decreased free fraction of thyroid hormones after prolonged Antarctic residence H. LESTER KENNETH
REED, DAVID BRICE, D. BURMAN, MICHELE
K. M. MOHAMED M. D’ALESANDRO,
SHAKIR, AND JOHN
T. O’BRIAN
Department of Environmental Medicine, Naval Medical Research Institute, Bethesda 20814-5055; Endocrine Division, Department of Medicine, National Naval Medical Center, Bethesda, Maryland 20814-5010; Department of Family Practice, Naval Hospital, Jacksonville, Florida 32214; and Endocrine-Metabolic Service, Department of Medicine, Walter Reed Army Medical Center, Washington, DC 20307-0001
REED, H. LESTER, DAVID BRICE, K. M. MOHAMED SHAKIR, KENNETH D. BURM,AN, MICHELE M. D’ALESANDRO, AND JOHN T. O’BRIAN. Decreased free fraction of thyroid hormones after prolonged Antarctic residence. J. Appl. Physiol. 69(4): 1467-
1472,1990.-We investigated the effects of Antarctic residence (AR) on serum thyroid hormone and cardiovascular responses to a 60-min standard cold air (OOC)test (SCAT). Serum total thyroxine (TT,) and serum total triiodothyronine (TTa), free Tq (FT*) and T3 (FTS), thyrotropin (TSH), and percent free fraction of Tq (%FT)) and T3 (%FT3) were measuredin normal men (n = 15) before and after each of three SCATS. The SCAT was first carried out in California and then repeated after 24 and 44 wk AR. Mean arterial pressure(MAP) and sublingual oral temperature (T,,) were measuredbefore and during each SCAT. The SCAT did not alter thyroid hormones before or after AR. The %FT4 decreasedfrom 0.0334 t 0.0017to 0.0295 t 0.0007%(P < 0.002) with 44 wk AR but without a significant change in TT, or FT, for the same period. The %FT3 also decreasedfrom 0.2812 & 0.0128 to 0.2458 t 0.0067% (P < 0.005) after 44 wk AR. FT3 decreased(P < 0.003) but TT3 and TSH were unchanged with 44 wk AR. The decreasein %FT* and %FT3may be theoretically accountedfor by a 10%increase in either the capacity or the affinity of the serum binding proteins. The SCAT in California increasedMAP and did not change T,,. After 44 wk AR, the SCAT no longer increased MAP but did lower T,,. The shift in the T,, and MAP response to the SCAT is consistent with the associatedoccurrence of cold adaptation during AR. We describe for the first time a decreasein the free fraction of both serum TS and T4 present with extended polar residence and independent of a SCAT, further characterizing the recently reported “polar T3 syndrome.” cold adaptation; blood pressure; temperature; thyroxine; triiodothyronine HUMANS WHO LIVE in the polar regions of the globe are exposed to extremes of low temperature, low relative humidity, increased electromagnetic radiation, changing photoperiods, and social isolation. Changes in physiology associated with prolonged circumpolar residence may increase our understanding of the reported human responses to midlatitude winters. Studies conducted in midlatitude regions suggest circannual or seasonal elevations in winter measurements of blood pressure (33) and incidence of stroke (12) and heart attack (36) and increased frequency of mood disorder (29). Iodothyro-
nines may alter cardiovascular function (30), lipid metabolism (16), and mood (1) and adapt to conditions such as overfeeding (6), fasting (20), critical illness (37), and cold exposure (8, 10). Euthyroid men living in Antarctica for greater than five continuous months demonstrate alterations in the hypothalamic-pituitary-thyroid-peripheral tissue axis (24, 25, 27). These changes are characterized by the following: 1) increased pituitary release of thyrotropin (TSH) in response to intravenous TSH releasing hormone (TRH); 2) intact pituitary sensitivity to triiodothyronine (T3); 3) increased production, clearance, and distribution volume of [ 1251]T3; and 4) variable decrease in serum total T3 (TT3) and free T3 (FT3) (24, 25, 27). No significant change in serum total or free thyroxine (T4), unstimulated TSH, and thyroid-binding globulin (TBG) is observed. The contributing mechanisms- and physiological significance of these thyroid hormone changes are presently unknown. Our recent reports and those of others suggest that both acute (31) and extended cold exposure may alter the serum binding of thyroid hormones (24, 25, 27, 31). These conclusions are derived indirectly, primarily from the nonspecific T3 resin uptake test (24,25,27,31) and analysis of free thyroid hormones by radioimmunoassay (RIA) (31). The determination of the dialyzable fraction of thyroid hormones provides a measurement of free hormone concentrations more accurate than these other methods (7). The direct analysis of the percent free fraction should help characterize the mechanism of any changes in free hormone concentrations. The purpose of this study was to measure the role of Antarctic residence (AR) in modifying hormonal and physiological responses to cold air. To carry out our objective, we measured changes in the dialyzable fraction of thyroid hormones and the thermal and cardiovascular characteristics associated with I) an experimental 60min standard cold (0°C) air test (SCAT) (2, 11, 21) and 2) the interaction of the SCAT with 24 and 44 wk AR. MATERIALS
AND
METHODS
Fifteen healthy euthyroid men, all members of the US Navy 1983-84 annual winter-over party at McMurdo Sound, Antarctica, were studied from August 1983 to 1467
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September 1984, with each subject serving as his own control. The protocol was approved by our Institutional Review Board, and all participants provided signed informed consent. The men were similar with respect to age [24.5 t 0.9 (SE) yr] and body weight (81.3 t 2.7 kg).
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for an average of 4.3 t 0.7 h/day. Indoor fluorescent lighting was used during the austral winter months. Indoor temperatures ranged from 19 to 27OC.All subjects maintained routine sleep cycles of 8 h/day. Blood samples and physiological measurements were taken just before each SCAT as described.
Acute Cold Air Exposure Blood Analysis
Three standardized 60-min SCATS (0 t 2°C) were performed with each subject seated in an environmental chamber (wind speed ~4.5 m/s) (2, 11, 21). One SCAT was carried out in California (control) initially and then repeated after 24 and 44 wk AR. SCATS were performed between 0800 and 1100 h local time in an identical fashion for each testing period. Subjects, wearing a cotton T-shirt, shorts, socks, and shoes, had a Teflon catheter inserted into the brachial vein for free-flowing blood sampling. Pre-SCAT blood samples were obtained in the postabsorptive state 20 min after catheter placement while the subject was seated in a warmed laboratory (22°C) before entering the climatic chamber. Post-SCAT blood samples were obtained from the same catheter after 60 min of cold air exposure. Subjects were quiet during the cold air exposure and were not allowed to smoke, eat, or move excessively. The sublingual oral temperature (T,,) was recorded with a digital display analog input circuit thermocouple (IVAC, San Diego, CA) after local oral thermal equilibrium. All subjects were asked to breathe through the nares with mouth closed while the T,, was recorded. Changes in respiratory rate were not clearly evident. The subjects were not allowed to eat or drink for 20 min before temperature measurements, thus minimizing variables that might affect T,,. Auscultatory blood pressure and pulse from the brachial artery were measured by the same investigator pre-SCAT and after 15, 30,45, and 60 min.of each cold air exposure. Mean arterial pressure (MAP) was calculated as [diastolic pressure + (pulse pressure/3)]. Serum from blood coagulated at room temperature was frozen at -3O”C, and all samples were transported to Bethesda, MD, for processing and then analyzed together at the completion of the 1-yr study.
Serum TT4 and TT3 were measured by RIA (Nichols Institute, San Juan Capistrano, CA). The percent free fraction of T4 and T3 (%FT4 and %FT3) were measured by equilibrium dialysis (Nichols Institute) by use of a modified method of Sterling and Brenner (32). After dialysis, contaminating free iodine was removed by antibody precipitation. The percent dialyzable fraction of total hormone is referred to as the percent free fraction (normal %FT, -0.03% and %FT3 -0.3%) (19). The free hormone concentrations were calculated as the product of the total hormone concentration and the percent free fraction of the hormone. We tested the effect of freezer storage on this assay of thyroid hormone binding. The sera of 10 subjects different from those in the present study were analyzed for %FT, before and after 21 mo of storage at -30°C. We did not find an increase in the %FT3 with this duration of storage. Serum TSH was measured by a sensitive and specific enzyme-linked immunosorbent assay technique (34). Serum cortisol and prolactin responses in the SCAT were measured as standard markers of cold exposure (11, 13). Serum cortisol levels were measured using a commercially available RIA kit (Clinical Assays, Cambridge, MA). SI unit conversion for cortisol is nmol = 2.759 X pg. Serum prolactin was measured by a commercially available RIA kit (Organon Teknika, Irving, TX) with an assay detection limit of 4 pg/l. We analyzed TT4, TT3, %FT*, and %FTs with a well-known model of thyroid hormone binding to predict whether the observed changes were primarily due to increases in receptor number or affinity (28). The serum total protein of six subjects was measured pre-SCAT by spectrophotometry to test for hemoconcentration (bicinchoninic acid protein assay, Pierce, Rockford, IL).
Extended
Data Analysis
Antarctic
Residence
The men were studied in August 1983 while in Port Hueneme, CA (latitude 34” 07’ N, longitude 119” 07’ W), before departing for Antarctica, in April 1984 after 24 wk, and in September after 44 wk in McMurdo Sound, Antarctica (77” 51’ S, 166” 37’ E). The 24- and 44-wk time points are similar to previous reports (24, 25). The mean monthly ambient temperatures for California in August (22.7”C) and McMurdo Sound, Antarctica, in April (-23.6”C) and September (-29.2”C) were similar to those of past reports (24, 25, 27). An internationally utilized US Navy diet as previously described (24, 25) was consumed, with calories distributed as -40% carbohydrate, 35% protein, and 25% fat. Energy intake increases with AR at this base from -2,OOO-2,500 to 3,0003,500 kcal/day (24, 25, 27). Each subject wore standard military polar clothing while in McMurdo Sound; the face and hands are commonly exposed during outdoor activity. Subjects were exposed to outdoor temperatures
Statistical analysis was performed by using a randomized block design (n = 15) with one- and two-way (date and time = treatments) analysis of variance (ANOVA), general linear model (GLM), and Dunnet’s test for differences between the one-way ANOVA means (SAS, SAS Institute, Cary, NC, and STAT PAK version 4.1, Northwest Analytical, Portland, OR). The data are expressed as means -I- SE. Significance was determined as P < 0.05. RESULTS
Acute Cold Air Exposure
(SCAT)
Thyroid hormones. The SCAT did not induce any differences in serum TT4, FT*, TT3, FT3, and TSH (Table 1) or in serum %FT4 or %FT3 (Tables 2 and 3). Serum TT3 increased, although not significantly, during the control SCAT and did not change with the SCATS in Antarctica (Table 1).
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THYROID
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IN
1. Thyroid hormone values during a SCAT administered before and 24 and 44 wk after Antarctic residence
wk, and 44-wk SCAT to 357 t 43, 332 t 20, and 399 t 57 nmol/l, respectively, from baseline pre-SCAT measurements of 547 t 80, 467 t 52, and 488 t 48 nmol/l, respectively.
TABLE
Hormone
TT+
nmol/l
SCAT
Control
Pre Post Pre Post Pre Post Pre Post Pre Post
76.4t6.5 86.5t3.6 25.4t2.2 27.8kl.l 1.90t0.08 2.07t0.05 5.24k0.18 5.38t0.25 2.7920.38 2.21t0.27
24 wk
44 wk
84.2t3.0 82.1zk4.5 24.6tl.O 24.721.4 1.95t0.06 1.95t0.06 4.64t0.14 4.78t0.15 2.81t0.35 2.79t0.36
84.1k3.4 83.4t5.8 24.7tl.O 25.5t1.2 1.90t0.05 1.95t0.05 4.67t0.17 4.80~0.23 2.83t0.42 2.88kO.43
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Extended Antarctic
Residence
Thyroid hormones (Tables l-3, Fig. 1). In the absence of hormonal variations during each SCAT, an ANOVA was performed combining pre- and post-SCAT data. TT3, nmol/l %FT, and %FT3 varied with AR [F(2, 53) = 7.39, P < 0.002; Table 2; F (2, 54) = 6.12, P < 0.005; Table 3, FTs,* pmol/l respectively] as did FT3 [F(2, 51) = 6.56, P < 0.003; TSH, MIU/ml Table 11. Serum TT3 values tended to be lower after 24and 44-wk AR, although the difference was not statistiValues are means t SE of 15 men before (Pre) and after (Post) a cally significant. By use of the fitted predictions of the 60-min standard cold air exposure (SCAT) carried out in California model previously described (28), the change in the per(control) and after 24 (24 wk) and 44 wk (44 wk) of Antarctic residence. cent free fraction of the hormones with AR would be Total triiodothyronine (TT3), thyroxine (TT,), and thyrotropin (TSH) were unchanged throughout the study. * Free TB (FT,) decreased similar to a 10% increase in receptor concentration. [F(2,51) = 6.56, P < 0.0031 over the study. T, nmol = 1.287 X pug; TS Alternatively, this same decrease in the percent free nmol = 1.536 x pg. fraction may also be explained by a 5-10% increase in the affinity of receptors for thyroid hormones. Physiological measurements. MAP increased and pulse Prolactin and cortisol measurements. Pre-SCAT serum rate decreased in the control SCAT (Table 4) but were prolactin values were similar for the control, 24-, and 44unchanged during the 24- and 44-wk SCAT [F(4, 56) = wk periods (7.82 t 1.26,8.51 t 0.99, and 9.86 t 1.27 pg/l, 0.977, P > 0.421. There was no change in T,, during the respectively). Similarly, serum cortisol values obtained control SCAT, but it decreased significantly during the before the SCAT were not different in the control and 24- and 44-wk SCAT [F (4, 56) = 10.93, P < O.OOOOl]. 24 and 44 wk pre-SCAT measurement. The serum total Neither hyperventilation nor increased anxiety was protein concentrations measured before the SCAT were noted during the 24- and 44wk SCAT, minimizing their not different before or after the 24- and 44-wk periods potential effects on T,, and MAP. The subjects appeared (85.6 t 5.2, 92.7 t 7.7, 77.0 t 4.5 g/l, respectively). more relaxed during the 24- and 44-wk SCAT and shivPhysiological measurements (Table 4). Initial body ered less noticeably than during the control SCAT. No weight of 81.3 t 2.7 kg was unchanged after 24 and 44 quantitative measurements of shivering or respiration wk AR (84.0 t 2.7 and 83.2 t 2.8 kg, respectively). MAP rate are available. was increased over control with 24 and 44 wk AR [ F(2, Prolactin and cortisol measurements. The serum pro- 28) = 13.58, P < 0.0002]. After 24 and 44 wk T,, was lactin value decreased equally to near the lower limit of significantly lower than control [F( 2, 28) = 8.17, P < assay detection after 60 min of the control (4.12 t 0.92 0.002], analyzed by two-way ANOVA with date as a pg/l), 24-wk (4.45 t 1.01 pg/l), and 44-wk (4.41 t 0.80 treatment. T,, was weakly correlated with FT3 over the ,ug/l) SCAT. Th e pre-SCAT prolactin values were not course of the study (r = 0.193, P = 0.048), and no different in the control, 24-wk, or 44-wk periods. Serum association was found between T,, and FT4 for the same cortisol also decreased after 60 min of the control, 24- period. FT4, pmol/l
TABLE
2. %FT4 during a SCAT administered before and after 24 and 44 wk Subj No.
1 2 3 4 5 6 7 6 9 10 11 12 13 14 15 Mean
& SE
Values are means Percent free fraction
Control
of Antarctic
residence
24 wk
44 wk
Pre
Post
Pre
Post
Pre
Post
0.049
0.030 0.032 0.031 0.033 0.034 0.032 0.031 0.031 0.027 0.031 0.034 NA 0.029 0.040 0.031
0.028 0.028 0.024 0.030 0.031 0.035
0.029
0.035 0.031 0.027 0.031 0.034 0.028 0.027 0.028 0.026 0.028
0.033 0.030
0.035 0.033 0.031 0.032 0.028 0.031 0.032 0.028 0.037 0.030
0.029 0.031 0.048 0.028 0.0334zk0.0017 t SE of 15 men before of Tq (%FT,) decreased
0.029 NA 0.030 0.031 0.030 0.026 0.027 0.031 0.032
0.030 0.030 0.031 0.031 0.026 0.030 0.030 0.028 0.030 0.028
0.029 0.032 0.037 0.030
0.0318t0.0007 and after a 60-min SCAT carried out in California [F(2, 53) = 7.39, P < 0.002]. Value did not change
0.029
0.029
0.028 0.032 0.028 NA 0.032 0.028 0.031 0.032
0.028 0.028 0.030 0.033
0.030 0.035 0.032
0.029
0.0306t0.0005 and after significantly
24 and 44 wk of Antarctic residence. with SCAT. NA, not available.
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1470 TABLE
THYROID
FUNCTION
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3. Percent free fraction of T3 during a SCAT before and after 24 and 44 wk of Antarctic Subj No.
Control
24 wk
residence 44 wk
Pre
Post
Pre
Post
Pre
Post
1
0.379
2 3 4 5 6 7 8 9
0.272 0.246 0.307 0.280 0.280 0.251 0.268 0.243 0.310 0.248 0.238 0.263 0.401 0.233
0.266 0.252 0.248 0.268 0.274 0.239 0.276 0.275 0.215 0.276 0.247 NA 0.236 0.304 0.209
0.292 0.222 0.188 0.232 0.256 0.273 0.208 NA 0.216 0.264 0.283 0.213 0.239 0.253 0.259
0.209 0.233 0.223 0.316 0.258 0.228 0.234 0.210 0.250 0.280 0.250 0.224 0.268 0.293 0.220
0.237 0.229 0.225 0.312 0.261 0.214 0.240 0.252 0.227 0.262 0.216 0.231 0.255 0.282 0.245
0.301 0.252 0.218 0.269 0.286 0.237 0.230 0.248 0.262 0.212 0.185 0.188 0.242 0.282 0.254
10 11 12 13 14 15
0.2444kO.0087 Mean rf~SE 0.2812t,O.O128 0.2560t0.0069 0.2427t0.0082 0.2464t0.0081 0.2458t0.0067 Values are means t SE of 15 men before and after a 60-min SCAT carried out in California and after 24 and 44 wk of Antarctic residence. Percent free fraction of T, (%FT,) decreased [F(2, 54) = 6.12, P < 0.0051. Value did not change significantly with SCAT. TABLE
4. MAP, heart rate, and TOl.before and during a SCAT before and after 24 and 44 wk of Antarctic 0°C
22°C 0 min
residence
15 min
30 min
45 min
60 min
MAP, mmHg 94.5zk3.4 Control 85.4t2.7 89.622.4 89.5zk2.9 94.0rt3.5 87.0t2.7 88.0t2.6 24 wk 90.8t2.6 91.2t2.2 90.8t2.2 102.0t3.7 42 wk 100.6t2.6 103.4t3.3 101.0t3.5 102.lt3.5 Heart rate, beats/min 66.4A2.7 Control 74.8t3.3 71.4t2.6 68.5t3.0 66.8A2.4 24 wk 76.0t1.3 72.1t1.4 70.421.9 68.121.6 66.5tl.9 74.4k2.3 44 wk 75.7t2.8 74.5k2.3 72.7t2.8 73.0t2.1 To,, “C Control 36.80t0.07 36.76t0.18 36.94&O. 13 36.92t0.14 36.7020.16 24 wk 36.66t0.09 36.5OkO.16 36.21t0.17 36.00&O. 12 35.9920.17 44 wk 36.50t0.13 36.31t0.20 36.2720.21 36.15t0.21 36.03kO.21 Values are means t SE of 15 men in California before and after 24 and 44 wk of Antarctic residence (AR). These measurements were before and during exposure to 0°C at each testing period. Mean arterial pressure (MAP) increased after 24 and 44 wk AR [F(2,28) = 13.58, P < 0.0002] compared with control and failed to increase further with SCAT [F(4,56) = 0.977, P > 0.4271. Sublingual oral temperature (To,) fell after 24 and 44 wk AR [F(2, 28) = 8.17, P < 0.0021 compared with control. T,, also decreased during SCAT after 24 and 44 wk AR [F(4,56) = 10.93, P < 0.00001].
DISCUSSION
Changes in human thyroid hormone binding to serum proteins in response to an acute cold air challenge are not clearly understood (31). We now report that a 60min exposure to cold air before or after extended AR fails to change significantly thyroid hormone values or the percent free fraction of the hormones. However, AR is associated with a fall in the free fraction of both T4 and T3, adding further characterization to the recently described “polar T3 syndrome” (27). Many reports confirm that thyroid hormone levels are altered by cold exposure, although the response depends on the duration, type, and severity of exposure. Recently, this topic has been reviewed (8). Serum T3 levels are increased during the winter months in Japanese natives living with poor housing conditions and thus exposed to cold climates (18). Ingbar et al. (10) have reported a near doubling in the thyroid clearance of 13’1 and a similar increased production of organic iodide in euthyroid men living in an environmental chamber at 13°C for 12 days. Recently, Solter et al. (31) showed that serum total T3
0.400 g ; ii
0.400
T
0.360 1
0.320 0.280 0.240
%FT4
%FT3
LT iz *Pi 3 $ aI E t+
0.200
FIG. 1. Dialyzable free fractions of T4 (%FTd) and T3 (%FT& in California before Antarctic residence (m) and after 24 (a) and 44 wk (I@ of Antarctic residence (means * SE). %FT, declined (P c 0.002), as did %FT3 (P c 0.005) with AR. %FT4 values have been multiplied by 10 to make the graph more legible.
and T4 are decreased in a large group of male and female factory workers dressed warmly and exposed chronically to air temperatures of -10 to 8OC. This exposure for 8 h produces a decrease in total T3 and increases in both FT* and FT3. However, shorter exposures of adults to cold
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THYROID
FUNCTION
air (*OOC) failed to elicit major changes in thyroid hormone status (13, 21, 35). The serum values of FT3 from paired subjects vary only slightly with polar residence (24, 25). When a small number of nonpaired subjects are grouped together and compared, the differences may not be distinguishable (27). Thus the mechanisms for these changes are likely multifactorial and dependent on subject variability. Possible contributions to a decreasing FT3 value during extended AR include a new steady-state condition with 1) increased hormone clearance (25), 2) increased tissue distribution (27), and 3) altered binding to serum proteins (22). The values of %FT, and %FTs we present are comparable to normal values in previous reports (7, 19, 38). This fall in the free fraction of both T3 and T4 in this study suggests the involvement of new or altered serum proteins that results in a net higher binding capacity or an affinity for thyroid hormones. Because of the difference in T4 and T3 binding constants and serum pools, small fluctuations in serum values of T4 may obscure differences in the %FT4 compared with similar changes in the %FTs. Seventy percent of the receptor sites on TBG, a major T3 serum carrier protein, are not occupied. Thus, measurement of the serum concentration of this carrier protein yields minimal information unless its concentration is dramatically changed. However, measurement of the maximum binding of [1251]T3 would help determine the total number of receptors. We have reported that the electrophoretic pattern of maximum [1251]T3 binding to serum proteins, including TBG, in a similar group of Antarctic residents studied in 1986-87 was decreased by -50% after 42 wk (22). The serum value of TBG in that study tended to decrease after 42 wk compared with basal studies in California but was not statistically lower. Albumin remained constant over the same period of Antarctic residence. This observation, combined with unchanged serum TT4 .and TT3 values in the present study, favors an increase in receptor affinity rather than a 10% increase in receptor number. Our observations do not allow us to identify the cause or to select a mechanism to explain these observations. However, overfeeding (6, 38), fasting (20, 37), dietary iodine deficiency or excess (27), social isolation with possible depression (15), freezer storage at -30°C (7), and changes in photoperiodicity do not alter thyroid hormone concentrations and binding in the manner we describe. The mechanisms responsible for altered thyroid hormone binding associated with cold adaptation must explain decreased FT3 values and decreased free fractions of both T4 and T3. An enriched serum pool of thyroidbinding proteins with slightly increased binding affinities is a reasonable explanation. The hormonal response to acute cold air exposure has been shown to include decreased values of serum prolactin (21) and cortisol (11) and variable changes in TSH (13, 21, 35). In agreement with these reports, we find that serum prolactin and cortisol decline in response to an acute 60-min cold air exposure and that this fall does not change with AR. Exposure of seminude men to cold for 30-60 min elevates MAP by -16% and does not markedly change rectal or oral temperature (17, 23, 26). Repeated cold air
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1471
or water exposures will attenuate this pressor response to cold (17). The blood pressure response to cold in our subjects after 24 and 44 wk AR was decreased to a degree similar to that described for men exposed to a 5-wk cold water adaptation program (17). The attenuation of this cold pressor response may involve adaptation of the cutaneous circulation (2). Repeated cold air or water exposures over several weeks decrease body temperature and the temperature that initiates shivering when an individual is exposed to a cold air challenge (2, 23). Subjects clothed in traditional polar garments experience significant cooling of the face and torso when exposed to -32°C (14). Our findings of a lower oral temperature after 24 and 44 wk AR are in agreement with Guenter et al. (9), Bittel et al. (3), and Budd and Warhaft (5). The fall in oral temperature with cold exposure that we describe is in agreement with Bittel et al. (2, 3) and Briick et al. (4) and is in contrast to Budd and Warhaft (5), who did not perform their studies in a climatic chamber in Antarctica. Our findings, supported by these earlier reports, are consistent with a hypothermic type of cold adaptation characterized by a “greater cooling of body temperature with less metabolic compensation” (3) or “low metabolic response and a more pronounced cooling of the body” (23). The mechanisms relating the changes in human thyroid hormones and hypothermic adaptation to cold after AR are unknown. Existing evidence suggests that much of the human T3 pool is located in muscle (19). Muscle cells with an increased content of T3 have an increased number of mitochondria, increased oxygen utilization, and decreased contractile strength and are thought to have an uncoupling of oxidative phosphorylation (1). The increased [ 1251]T3 pool found with AR is likely distributed in part in skeletal muscles that are involved in shivering. If the set point of thermal equilibrium was shifted to favor heat generated from meaningful muscle contraction, as opposed to shivering, then a subject sitting quietly and exposed to cold may demonstrate a fall in core temperature and thus an apparent hypothermic response to cold. This same subject would require increased energy intake to carry out normal muscle function and would show a fall in the shivering threshold. The role of T4 as a prohormone and T3 as the active agent in bringing about this shift in thermal equilibrium is speculative and needs further investigation. A fall in the %FT3 and %FT, after AR suggests a significant modification of human thyroid hormone transport proteins after extended polar residence. These changes in protein binding may facilitate hormone delivery to specific tissues. The simultaneous development of hypothermic cold adaptation in these individuals provides an area of research that links changing thyroid hormone economy and cold adaptation. We thank the 1983-84 winter-over crew for participation in the study, the National Science Foundation, and Naval Support Force Antarctica for their support. We also personally thank Dr. Louis Homer for model fitting to derive estimates of changes in binding affinity and capacity of serum proteins, Dr. Stephen Lewis and Kathleen Kowalski for review of the manuscript, and Patricia Mu&nix for manuscript preparation. This work was supported in part by Naval Medical Research and Development Command work unit 62233N.MM33C30.004.
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The opinions and assertions expressed herein are those of the authors and are not to be construed as official or reflecting the views the Department of the Navy, Department of the Army, Department of Defense, or the National Science Foundation. This work was presented at the 1987 meeting of the Federation of American Societies for Experimental Biology, Washington, DC. Address for reprint requests: H. L. Reed, Mail Stop 11, Dept. of Environmental Medicine, Naval Medical Research Institute, Bethesda, MD ‘20814-5055. Received 14 February 1990; accepted in final form 14 June 1990. REFERENCES R. D., AND G. R. DELONG. The neuromuscular system and brain. In: Werner’s The Thyroid: A Fundamental and Clinical Text (5t,h ed.). Philadelphia, PA: Lippincott, 1986, p. 885-894. 2. BITTEL, J. H. M. Heat debt as an index for cold adaptation in men. 1. ADAMS,
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A. M. HANNIQUET, changes observed before and after J.-L. Etienne’s journey to the North Pole: is central nervous system temperature preserved in hypothermia? Eur. J.
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N. WARHAFT. Body temperature, shivering, blood pressure and heart rate during a standard cold stress in Australia and Antarctica. J. Physiol. Land. 186: 216-232, 1966. K. SCHEIDEGGER, R. Woo, AND 6. BURGER, A. G., M. O’CONNELL, E. DANFORTH, JR. Monodeiodination of triidothyronine and reverse triidothyronine during low and high calorie diets. J. CZin. Endocrinol. 7. CHOPRA, NGUYEN.
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I. J., A. J. VAN HERLE, G. N. CHUA Serum free thyroxine in thyroidal illnesses: a comparison of measurements by equilibrium dialysis, and free thyroxine index.
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J. CZin. Endocrinol. Metab. 51: 135-143, 1980. 8. FREGLY, M. J. Activity of the hypothalamic-pituitary-thyroid axis during exposure to cold. Pharmacol. Ther. 41: 85-142,1989. C. A., A. T. JOERN, J. T. SHURLEY, AND C. M. PIERCE. 9. GUENTER,
Cardiorespiratory and metabolic effects in men on the South Polar Plateau. Arch. Intern. Med. 125: 630-637, 1970. 10. INGBAR, S. H., C. R. KLEEMAN, M. QUINN, AND D. E. BASS. The effect of prolonged exposure to cold on thyroidal function in man (Abstract). Clin. Res. Proc. 2: 86, 1954. 11. JESSEN, I. The cortisol fluctuations in plasma in relation to human regulatory nonshivering thermogenesis. Acta Anaesthesiol. Stand. 24: 151-154,198O. 12. KEATINGE, W.
R., S. R. K. COLESHAW, AND J. HOLMES. Changes in seasonal mortalities with improvement in home heating in England and Wales from 1964 to 1984. Int. J. Biometeorol. 33: 71-
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Physiol. Stand. 132: 543-548, 1988. 14. LIVINGSTONE, S. D., J. GRAYSON, J. FRIM, C. L. ALLEN, AND R. E. LIMMER. Effect of cold exposure on various sites of core temperature measurements. J. AppZ. Physiol. 54: 1025-1031,1983. 15. MAEDA, K., Y. KATO, S. OHGO, K. CHIHARA, Y. YASHIMOTO, N. YAMAGUCHI, S. KUROMARU, AND H. IMURA. Growth hormone and
prolactin release after injection of thyrotropin-releasing in patients with depression. J. CZin. Endocrinol. Metab.
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505,1975. 16. MULS, E., M. ROSSENEU, J. BURY, P. DE MOOR. Hyperthyroidism
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apolipoprotein A composition man. J. Clin. Endocrinol. Metab. 61: 882-889, 1985. 17. MUZA, S. R., A. J. YOUNG, M. N. SAWKA, J. E. BOGART, AND K. B. PANDOLF. Respiratory and cardiovascular responses to cold
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T. Physiologic and pathophysiologic implications of hormone binding. In: Werner’s The Thyroid: A Fundamental and CLinical Text (5th ed.). Philadelphia, PA: Lippincott, 1986, p. 128135. 20. O’BRIAN, J. T., D. E. BYBEE, K. D. BURMAN, R. C. OSBURNE, M. R. KSIAZEK, L. WARTOFSKY, AND L. P. GEORGES. Thyroid hormone homeostasis in states of relative caloric deprivation. Metab19.
olism 29: 721-727, 1980. 21. O’MALLEY, B. P., N. COOK, A. RICHARDSON, D. B. BARNETT, F. D. ROSENTHAL. Circulating catecholamine, thyrotrophin,
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thyroid hormone and prolactin responses of normal subjects to acute cold exposure. Clin. Endocrinol. Oxj. 21: 285-291,1984. 22. QUESADA, M., H. L. REED, AND D. SMITH. Selective alterations in triidothyronine (Ts) binding to serum proteins during prolonged Antarctic residence (Abstract). FASEB J. 3: A395, 1989. 23. RADOMSKI, M. W., AND C. BOUTELIER. Hormone response of normal and intermittent cold-preadapted humans to continuous cold. J. Appl. Physiol. 53: 610-616, 1982. 24. REED, H. L., K. D. BURMAN, K. M. M. SHAKIR, AND J, T. O’BRIAN. Alterations in the hypothalamic-pituitary-thyroid axis after prolonged residence in Antarctica. Clin. Endocrinol. Oxf. 25: 55-65, 1986. 25. REED, H. L., J. A. FERREIRO, AND J. T. O’BRIAN. Pituitary
K. M. M. SHAKIR, K. D. BURMAN, and peripheral hormone responses to TS administration during Antarctic residence. Am. J. Physiol. 254 (Endocrinol. Metab. 17): E733-E739, 1988. 26. REED, H. L., S. PEPPER, D. ARMSTRONG, F. J. VON TERSCH, AND S. B. LEWIS. Oxygen saturation of brachial venous blood correlates with fingertip temperatures between 11 and 39OC. Auiat. Space Enuiron. Med. 60: 1068-1071, 1989. 27. REED, H. L., E. D. SILVERMAN, K. M. M. SHAKIR, R. DONS, K. D. BURMAN, AND J. T. O’BRIAN. Changes in serum triidothyronine
kinetics after prolonged Antarctic residence: the polar TS syndrome. J. Clin. EndocrinoZ. Metab. 70: 965-974, 1990. 28. ROBBINS, J., AND M. L. JOHNSON. Theoretical considerations in the transport of the thyroid hormones in blood. In: Free Thyroid Hormones. Princeton, NJ: Excerpta Med., 1979, p. 1-13. 29. SACK, D. A., J. NURNBERGER, N. E. ROSENTHAL, E. ASHBURN, AND T. A. WEHR. Potentiation of antidepressant medications by phase advance of the sleep-wake cycle. Am. J. Psychiat. 142: 606608,1985. 30. SKELTON,
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C. L., AND E. H. SONNENBLICK. The cardiovascular system. In: Werner’s The Thyroid: A Fundamental and Clinical Text (5th ed.). Philadelphia, PA: Lippincott, 1986, p. 853-864. SOLTER, M., K. BRKIC, M. PETEK, L. POSAVEC, AND M. SEKSO. Thyroid hormone economy in response to extreme cold exposure in healthy factory workers. J. Clin. Endocrinol. Metab. 68: 168172,1989. STERLING, K., AND M. A. BRENNER. Free thyroxine in human serum: simplified measurement with the aid of magnesium precipitation. J. Clin. Inuest. 45: 153-163, 1966. TANAKA, S., A. KONNO, A. HASHIMOTO, A. HAYASE, Y. TAKAGI, S. KONDO, Y. NAKAMURA, AND 0. IIMURA. The influence of cold temperatures on the progression of hypertension: an epidemiological study. J. Hypertens. 7, Suppl. 1: S49-S51, 1989. TSENG, Y., K. D. BURMAN, J. R. BAKER, JR., AND L. WARTOFSKY. A rapid, sensitive enzyme-linked immunoassay for human thyrotropin. Clin. Chem. 31: 1131-1134, 1985. TUOMISTO, J., P. M;~NNIST~, B.-A. LAMBERG, AND M. LINNOILA. Effect of cold-exposure on serum thyrotrophin levels in man. Acta
Endocrinol. 83: 522-527, 1976. 36. VUORI, I. The heart and the cold. Ann. 37. WARTOFSKY, L., AND K. D. BURMAN.
Clin. Res. 19: 156-162,1987.
Alterations in thyroid function in patients with systemic illness: the “euthyroid sick syndrome.” Endocr. Reu. 3: 164-217, 1982. 38. WELLE, S., M. O’CONNELL, E. DANFORTH, JR., AND R. CAMPBELL. Decreased free fraction of serum thyroid hormones during carbo33: 837-839, 1984. hvdrate overfeeding. Metabolism
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REED, DAVID BRICE, D. BURMAN, MICHELE
K. M. MOHAMED M. D’ALESANDRO,
SHAKIR, AND JOHN
T. O’BRIAN
Department of Environmental Medicine, Naval Medical Research Institute, Bethesda 20814-5055; Endocrine Division, Department of Medicine, National Naval Medical Center, Bethesda, Maryland 20814-5010; Department of Family Practice, Naval Hospital, Jacksonville, Florida 32214; and Endocrine-Metabolic Service, Department of Medicine, Walter Reed Army Medical Center, Washington, DC 20307-0001
REED, H. LESTER, DAVID BRICE, K. M. MOHAMED SHAKIR, KENNETH D. BURM,AN, MICHELE M. D’ALESANDRO, AND JOHN T. O’BRIAN. Decreased free fraction of thyroid hormones after prolonged Antarctic residence. J. Appl. Physiol. 69(4): 1467-
1472,1990.-We investigated the effects of Antarctic residence (AR) on serum thyroid hormone and cardiovascular responses to a 60-min standard cold air (OOC)test (SCAT). Serum total thyroxine (TT,) and serum total triiodothyronine (TTa), free Tq (FT*) and T3 (FTS), thyrotropin (TSH), and percent free fraction of Tq (%FT)) and T3 (%FT3) were measuredin normal men (n = 15) before and after each of three SCATS. The SCAT was first carried out in California and then repeated after 24 and 44 wk AR. Mean arterial pressure(MAP) and sublingual oral temperature (T,,) were measuredbefore and during each SCAT. The SCAT did not alter thyroid hormones before or after AR. The %FT4 decreasedfrom 0.0334 t 0.0017to 0.0295 t 0.0007%(P < 0.002) with 44 wk AR but without a significant change in TT, or FT, for the same period. The %FT3 also decreasedfrom 0.2812 & 0.0128 to 0.2458 t 0.0067% (P < 0.005) after 44 wk AR. FT3 decreased(P < 0.003) but TT3 and TSH were unchanged with 44 wk AR. The decreasein %FT* and %FT3may be theoretically accountedfor by a 10%increase in either the capacity or the affinity of the serum binding proteins. The SCAT in California increasedMAP and did not change T,,. After 44 wk AR, the SCAT no longer increased MAP but did lower T,,. The shift in the T,, and MAP response to the SCAT is consistent with the associatedoccurrence of cold adaptation during AR. We describe for the first time a decreasein the free fraction of both serum TS and T4 present with extended polar residence and independent of a SCAT, further characterizing the recently reported “polar T3 syndrome.” cold adaptation; blood pressure; temperature; thyroxine; triiodothyronine HUMANS WHO LIVE in the polar regions of the globe are exposed to extremes of low temperature, low relative humidity, increased electromagnetic radiation, changing photoperiods, and social isolation. Changes in physiology associated with prolonged circumpolar residence may increase our understanding of the reported human responses to midlatitude winters. Studies conducted in midlatitude regions suggest circannual or seasonal elevations in winter measurements of blood pressure (33) and incidence of stroke (12) and heart attack (36) and increased frequency of mood disorder (29). Iodothyro-
nines may alter cardiovascular function (30), lipid metabolism (16), and mood (1) and adapt to conditions such as overfeeding (6), fasting (20), critical illness (37), and cold exposure (8, 10). Euthyroid men living in Antarctica for greater than five continuous months demonstrate alterations in the hypothalamic-pituitary-thyroid-peripheral tissue axis (24, 25, 27). These changes are characterized by the following: 1) increased pituitary release of thyrotropin (TSH) in response to intravenous TSH releasing hormone (TRH); 2) intact pituitary sensitivity to triiodothyronine (T3); 3) increased production, clearance, and distribution volume of [ 1251]T3; and 4) variable decrease in serum total T3 (TT3) and free T3 (FT3) (24, 25, 27). No significant change in serum total or free thyroxine (T4), unstimulated TSH, and thyroid-binding globulin (TBG) is observed. The contributing mechanisms- and physiological significance of these thyroid hormone changes are presently unknown. Our recent reports and those of others suggest that both acute (31) and extended cold exposure may alter the serum binding of thyroid hormones (24, 25, 27, 31). These conclusions are derived indirectly, primarily from the nonspecific T3 resin uptake test (24,25,27,31) and analysis of free thyroid hormones by radioimmunoassay (RIA) (31). The determination of the dialyzable fraction of thyroid hormones provides a measurement of free hormone concentrations more accurate than these other methods (7). The direct analysis of the percent free fraction should help characterize the mechanism of any changes in free hormone concentrations. The purpose of this study was to measure the role of Antarctic residence (AR) in modifying hormonal and physiological responses to cold air. To carry out our objective, we measured changes in the dialyzable fraction of thyroid hormones and the thermal and cardiovascular characteristics associated with I) an experimental 60min standard cold (0°C) air test (SCAT) (2, 11, 21) and 2) the interaction of the SCAT with 24 and 44 wk AR. MATERIALS
AND
METHODS
Fifteen healthy euthyroid men, all members of the US Navy 1983-84 annual winter-over party at McMurdo Sound, Antarctica, were studied from August 1983 to 1467
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1468
THYROID
FUNCTION
September 1984, with each subject serving as his own control. The protocol was approved by our Institutional Review Board, and all participants provided signed informed consent. The men were similar with respect to age [24.5 t 0.9 (SE) yr] and body weight (81.3 t 2.7 kg).
IN
ANTARCTICA
for an average of 4.3 t 0.7 h/day. Indoor fluorescent lighting was used during the austral winter months. Indoor temperatures ranged from 19 to 27OC.All subjects maintained routine sleep cycles of 8 h/day. Blood samples and physiological measurements were taken just before each SCAT as described.
Acute Cold Air Exposure Blood Analysis
Three standardized 60-min SCATS (0 t 2°C) were performed with each subject seated in an environmental chamber (wind speed ~4.5 m/s) (2, 11, 21). One SCAT was carried out in California (control) initially and then repeated after 24 and 44 wk AR. SCATS were performed between 0800 and 1100 h local time in an identical fashion for each testing period. Subjects, wearing a cotton T-shirt, shorts, socks, and shoes, had a Teflon catheter inserted into the brachial vein for free-flowing blood sampling. Pre-SCAT blood samples were obtained in the postabsorptive state 20 min after catheter placement while the subject was seated in a warmed laboratory (22°C) before entering the climatic chamber. Post-SCAT blood samples were obtained from the same catheter after 60 min of cold air exposure. Subjects were quiet during the cold air exposure and were not allowed to smoke, eat, or move excessively. The sublingual oral temperature (T,,) was recorded with a digital display analog input circuit thermocouple (IVAC, San Diego, CA) after local oral thermal equilibrium. All subjects were asked to breathe through the nares with mouth closed while the T,, was recorded. Changes in respiratory rate were not clearly evident. The subjects were not allowed to eat or drink for 20 min before temperature measurements, thus minimizing variables that might affect T,,. Auscultatory blood pressure and pulse from the brachial artery were measured by the same investigator pre-SCAT and after 15, 30,45, and 60 min.of each cold air exposure. Mean arterial pressure (MAP) was calculated as [diastolic pressure + (pulse pressure/3)]. Serum from blood coagulated at room temperature was frozen at -3O”C, and all samples were transported to Bethesda, MD, for processing and then analyzed together at the completion of the 1-yr study.
Serum TT4 and TT3 were measured by RIA (Nichols Institute, San Juan Capistrano, CA). The percent free fraction of T4 and T3 (%FT4 and %FT3) were measured by equilibrium dialysis (Nichols Institute) by use of a modified method of Sterling and Brenner (32). After dialysis, contaminating free iodine was removed by antibody precipitation. The percent dialyzable fraction of total hormone is referred to as the percent free fraction (normal %FT, -0.03% and %FT3 -0.3%) (19). The free hormone concentrations were calculated as the product of the total hormone concentration and the percent free fraction of the hormone. We tested the effect of freezer storage on this assay of thyroid hormone binding. The sera of 10 subjects different from those in the present study were analyzed for %FT, before and after 21 mo of storage at -30°C. We did not find an increase in the %FT3 with this duration of storage. Serum TSH was measured by a sensitive and specific enzyme-linked immunosorbent assay technique (34). Serum cortisol and prolactin responses in the SCAT were measured as standard markers of cold exposure (11, 13). Serum cortisol levels were measured using a commercially available RIA kit (Clinical Assays, Cambridge, MA). SI unit conversion for cortisol is nmol = 2.759 X pg. Serum prolactin was measured by a commercially available RIA kit (Organon Teknika, Irving, TX) with an assay detection limit of 4 pg/l. We analyzed TT4, TT3, %FT*, and %FTs with a well-known model of thyroid hormone binding to predict whether the observed changes were primarily due to increases in receptor number or affinity (28). The serum total protein of six subjects was measured pre-SCAT by spectrophotometry to test for hemoconcentration (bicinchoninic acid protein assay, Pierce, Rockford, IL).
Extended
Data Analysis
Antarctic
Residence
The men were studied in August 1983 while in Port Hueneme, CA (latitude 34” 07’ N, longitude 119” 07’ W), before departing for Antarctica, in April 1984 after 24 wk, and in September after 44 wk in McMurdo Sound, Antarctica (77” 51’ S, 166” 37’ E). The 24- and 44-wk time points are similar to previous reports (24, 25). The mean monthly ambient temperatures for California in August (22.7”C) and McMurdo Sound, Antarctica, in April (-23.6”C) and September (-29.2”C) were similar to those of past reports (24, 25, 27). An internationally utilized US Navy diet as previously described (24, 25) was consumed, with calories distributed as -40% carbohydrate, 35% protein, and 25% fat. Energy intake increases with AR at this base from -2,OOO-2,500 to 3,0003,500 kcal/day (24, 25, 27). Each subject wore standard military polar clothing while in McMurdo Sound; the face and hands are commonly exposed during outdoor activity. Subjects were exposed to outdoor temperatures
Statistical analysis was performed by using a randomized block design (n = 15) with one- and two-way (date and time = treatments) analysis of variance (ANOVA), general linear model (GLM), and Dunnet’s test for differences between the one-way ANOVA means (SAS, SAS Institute, Cary, NC, and STAT PAK version 4.1, Northwest Analytical, Portland, OR). The data are expressed as means -I- SE. Significance was determined as P < 0.05. RESULTS
Acute Cold Air Exposure
(SCAT)
Thyroid hormones. The SCAT did not induce any differences in serum TT4, FT*, TT3, FT3, and TSH (Table 1) or in serum %FT4 or %FT3 (Tables 2 and 3). Serum TT3 increased, although not significantly, during the control SCAT and did not change with the SCATS in Antarctica (Table 1).
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THYROID
FUNCTION
IN
1. Thyroid hormone values during a SCAT administered before and 24 and 44 wk after Antarctic residence
wk, and 44-wk SCAT to 357 t 43, 332 t 20, and 399 t 57 nmol/l, respectively, from baseline pre-SCAT measurements of 547 t 80, 467 t 52, and 488 t 48 nmol/l, respectively.
TABLE
Hormone
TT+
nmol/l
SCAT
Control
Pre Post Pre Post Pre Post Pre Post Pre Post
76.4t6.5 86.5t3.6 25.4t2.2 27.8kl.l 1.90t0.08 2.07t0.05 5.24k0.18 5.38t0.25 2.7920.38 2.21t0.27
24 wk
44 wk
84.2t3.0 82.1zk4.5 24.6tl.O 24.721.4 1.95t0.06 1.95t0.06 4.64t0.14 4.78t0.15 2.81t0.35 2.79t0.36
84.1k3.4 83.4t5.8 24.7tl.O 25.5t1.2 1.90t0.05 1.95t0.05 4.67t0.17 4.80~0.23 2.83t0.42 2.88kO.43
1469
ANTARCTICA
Extended Antarctic
Residence
Thyroid hormones (Tables l-3, Fig. 1). In the absence of hormonal variations during each SCAT, an ANOVA was performed combining pre- and post-SCAT data. TT3, nmol/l %FT, and %FT3 varied with AR [F(2, 53) = 7.39, P < 0.002; Table 2; F (2, 54) = 6.12, P < 0.005; Table 3, FTs,* pmol/l respectively] as did FT3 [F(2, 51) = 6.56, P < 0.003; TSH, MIU/ml Table 11. Serum TT3 values tended to be lower after 24and 44-wk AR, although the difference was not statistiValues are means t SE of 15 men before (Pre) and after (Post) a cally significant. By use of the fitted predictions of the 60-min standard cold air exposure (SCAT) carried out in California model previously described (28), the change in the per(control) and after 24 (24 wk) and 44 wk (44 wk) of Antarctic residence. cent free fraction of the hormones with AR would be Total triiodothyronine (TT3), thyroxine (TT,), and thyrotropin (TSH) were unchanged throughout the study. * Free TB (FT,) decreased similar to a 10% increase in receptor concentration. [F(2,51) = 6.56, P < 0.0031 over the study. T, nmol = 1.287 X pug; TS Alternatively, this same decrease in the percent free nmol = 1.536 x pg. fraction may also be explained by a 5-10% increase in the affinity of receptors for thyroid hormones. Physiological measurements. MAP increased and pulse Prolactin and cortisol measurements. Pre-SCAT serum rate decreased in the control SCAT (Table 4) but were prolactin values were similar for the control, 24-, and 44unchanged during the 24- and 44-wk SCAT [F(4, 56) = wk periods (7.82 t 1.26,8.51 t 0.99, and 9.86 t 1.27 pg/l, 0.977, P > 0.421. There was no change in T,, during the respectively). Similarly, serum cortisol values obtained control SCAT, but it decreased significantly during the before the SCAT were not different in the control and 24- and 44-wk SCAT [F (4, 56) = 10.93, P < O.OOOOl]. 24 and 44 wk pre-SCAT measurement. The serum total Neither hyperventilation nor increased anxiety was protein concentrations measured before the SCAT were noted during the 24- and 44wk SCAT, minimizing their not different before or after the 24- and 44-wk periods potential effects on T,, and MAP. The subjects appeared (85.6 t 5.2, 92.7 t 7.7, 77.0 t 4.5 g/l, respectively). more relaxed during the 24- and 44-wk SCAT and shivPhysiological measurements (Table 4). Initial body ered less noticeably than during the control SCAT. No weight of 81.3 t 2.7 kg was unchanged after 24 and 44 quantitative measurements of shivering or respiration wk AR (84.0 t 2.7 and 83.2 t 2.8 kg, respectively). MAP rate are available. was increased over control with 24 and 44 wk AR [ F(2, Prolactin and cortisol measurements. The serum pro- 28) = 13.58, P < 0.0002]. After 24 and 44 wk T,, was lactin value decreased equally to near the lower limit of significantly lower than control [F( 2, 28) = 8.17, P < assay detection after 60 min of the control (4.12 t 0.92 0.002], analyzed by two-way ANOVA with date as a pg/l), 24-wk (4.45 t 1.01 pg/l), and 44-wk (4.41 t 0.80 treatment. T,, was weakly correlated with FT3 over the ,ug/l) SCAT. Th e pre-SCAT prolactin values were not course of the study (r = 0.193, P = 0.048), and no different in the control, 24-wk, or 44-wk periods. Serum association was found between T,, and FT4 for the same cortisol also decreased after 60 min of the control, 24- period. FT4, pmol/l
TABLE
2. %FT4 during a SCAT administered before and after 24 and 44 wk Subj No.
1 2 3 4 5 6 7 6 9 10 11 12 13 14 15 Mean
& SE
Values are means Percent free fraction
Control
of Antarctic
residence
24 wk
44 wk
Pre
Post
Pre
Post
Pre
Post
0.049
0.030 0.032 0.031 0.033 0.034 0.032 0.031 0.031 0.027 0.031 0.034 NA 0.029 0.040 0.031
0.028 0.028 0.024 0.030 0.031 0.035
0.029
0.035 0.031 0.027 0.031 0.034 0.028 0.027 0.028 0.026 0.028
0.033 0.030
0.035 0.033 0.031 0.032 0.028 0.031 0.032 0.028 0.037 0.030
0.029 0.031 0.048 0.028 0.0334zk0.0017 t SE of 15 men before of Tq (%FT,) decreased
0.029 NA 0.030 0.031 0.030 0.026 0.027 0.031 0.032
0.030 0.030 0.031 0.031 0.026 0.030 0.030 0.028 0.030 0.028
0.029 0.032 0.037 0.030
0.0318t0.0007 and after a 60-min SCAT carried out in California [F(2, 53) = 7.39, P < 0.002]. Value did not change
0.029
0.029
0.028 0.032 0.028 NA 0.032 0.028 0.031 0.032
0.028 0.028 0.030 0.033
0.030 0.035 0.032
0.029
0.0306t0.0005 and after significantly
24 and 44 wk of Antarctic residence. with SCAT. NA, not available.
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1470 TABLE
THYROID
FUNCTION
IN
ANTARCTICA
3. Percent free fraction of T3 during a SCAT before and after 24 and 44 wk of Antarctic Subj No.
Control
24 wk
residence 44 wk
Pre
Post
Pre
Post
Pre
Post
1
0.379
2 3 4 5 6 7 8 9
0.272 0.246 0.307 0.280 0.280 0.251 0.268 0.243 0.310 0.248 0.238 0.263 0.401 0.233
0.266 0.252 0.248 0.268 0.274 0.239 0.276 0.275 0.215 0.276 0.247 NA 0.236 0.304 0.209
0.292 0.222 0.188 0.232 0.256 0.273 0.208 NA 0.216 0.264 0.283 0.213 0.239 0.253 0.259
0.209 0.233 0.223 0.316 0.258 0.228 0.234 0.210 0.250 0.280 0.250 0.224 0.268 0.293 0.220
0.237 0.229 0.225 0.312 0.261 0.214 0.240 0.252 0.227 0.262 0.216 0.231 0.255 0.282 0.245
0.301 0.252 0.218 0.269 0.286 0.237 0.230 0.248 0.262 0.212 0.185 0.188 0.242 0.282 0.254
10 11 12 13 14 15
0.2444kO.0087 Mean rf~SE 0.2812t,O.O128 0.2560t0.0069 0.2427t0.0082 0.2464t0.0081 0.2458t0.0067 Values are means t SE of 15 men before and after a 60-min SCAT carried out in California and after 24 and 44 wk of Antarctic residence. Percent free fraction of T, (%FT,) decreased [F(2, 54) = 6.12, P < 0.0051. Value did not change significantly with SCAT. TABLE
4. MAP, heart rate, and TOl.before and during a SCAT before and after 24 and 44 wk of Antarctic 0°C
22°C 0 min
residence
15 min
30 min
45 min
60 min
MAP, mmHg 94.5zk3.4 Control 85.4t2.7 89.622.4 89.5zk2.9 94.0rt3.5 87.0t2.7 88.0t2.6 24 wk 90.8t2.6 91.2t2.2 90.8t2.2 102.0t3.7 42 wk 100.6t2.6 103.4t3.3 101.0t3.5 102.lt3.5 Heart rate, beats/min 66.4A2.7 Control 74.8t3.3 71.4t2.6 68.5t3.0 66.8A2.4 24 wk 76.0t1.3 72.1t1.4 70.421.9 68.121.6 66.5tl.9 74.4k2.3 44 wk 75.7t2.8 74.5k2.3 72.7t2.8 73.0t2.1 To,, “C Control 36.80t0.07 36.76t0.18 36.94&O. 13 36.92t0.14 36.7020.16 24 wk 36.66t0.09 36.5OkO.16 36.21t0.17 36.00&O. 12 35.9920.17 44 wk 36.50t0.13 36.31t0.20 36.2720.21 36.15t0.21 36.03kO.21 Values are means t SE of 15 men in California before and after 24 and 44 wk of Antarctic residence (AR). These measurements were before and during exposure to 0°C at each testing period. Mean arterial pressure (MAP) increased after 24 and 44 wk AR [F(2,28) = 13.58, P < 0.0002] compared with control and failed to increase further with SCAT [F(4,56) = 0.977, P > 0.4271. Sublingual oral temperature (To,) fell after 24 and 44 wk AR [F(2, 28) = 8.17, P < 0.0021 compared with control. T,, also decreased during SCAT after 24 and 44 wk AR [F(4,56) = 10.93, P < 0.00001].
DISCUSSION
Changes in human thyroid hormone binding to serum proteins in response to an acute cold air challenge are not clearly understood (31). We now report that a 60min exposure to cold air before or after extended AR fails to change significantly thyroid hormone values or the percent free fraction of the hormones. However, AR is associated with a fall in the free fraction of both T4 and T3, adding further characterization to the recently described “polar T3 syndrome” (27). Many reports confirm that thyroid hormone levels are altered by cold exposure, although the response depends on the duration, type, and severity of exposure. Recently, this topic has been reviewed (8). Serum T3 levels are increased during the winter months in Japanese natives living with poor housing conditions and thus exposed to cold climates (18). Ingbar et al. (10) have reported a near doubling in the thyroid clearance of 13’1 and a similar increased production of organic iodide in euthyroid men living in an environmental chamber at 13°C for 12 days. Recently, Solter et al. (31) showed that serum total T3
0.400 g ; ii
0.400
T
0.360 1
0.320 0.280 0.240
%FT4
%FT3
LT iz *Pi 3 $ aI E t+
0.200
FIG. 1. Dialyzable free fractions of T4 (%FTd) and T3 (%FT& in California before Antarctic residence (m) and after 24 (a) and 44 wk (I@ of Antarctic residence (means * SE). %FT, declined (P c 0.002), as did %FT3 (P c 0.005) with AR. %FT4 values have been multiplied by 10 to make the graph more legible.
and T4 are decreased in a large group of male and female factory workers dressed warmly and exposed chronically to air temperatures of -10 to 8OC. This exposure for 8 h produces a decrease in total T3 and increases in both FT* and FT3. However, shorter exposures of adults to cold
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THYROID
FUNCTION
air (*OOC) failed to elicit major changes in thyroid hormone status (13, 21, 35). The serum values of FT3 from paired subjects vary only slightly with polar residence (24, 25). When a small number of nonpaired subjects are grouped together and compared, the differences may not be distinguishable (27). Thus the mechanisms for these changes are likely multifactorial and dependent on subject variability. Possible contributions to a decreasing FT3 value during extended AR include a new steady-state condition with 1) increased hormone clearance (25), 2) increased tissue distribution (27), and 3) altered binding to serum proteins (22). The values of %FT, and %FTs we present are comparable to normal values in previous reports (7, 19, 38). This fall in the free fraction of both T3 and T4 in this study suggests the involvement of new or altered serum proteins that results in a net higher binding capacity or an affinity for thyroid hormones. Because of the difference in T4 and T3 binding constants and serum pools, small fluctuations in serum values of T4 may obscure differences in the %FT4 compared with similar changes in the %FTs. Seventy percent of the receptor sites on TBG, a major T3 serum carrier protein, are not occupied. Thus, measurement of the serum concentration of this carrier protein yields minimal information unless its concentration is dramatically changed. However, measurement of the maximum binding of [1251]T3 would help determine the total number of receptors. We have reported that the electrophoretic pattern of maximum [1251]T3 binding to serum proteins, including TBG, in a similar group of Antarctic residents studied in 1986-87 was decreased by -50% after 42 wk (22). The serum value of TBG in that study tended to decrease after 42 wk compared with basal studies in California but was not statistically lower. Albumin remained constant over the same period of Antarctic residence. This observation, combined with unchanged serum TT4 .and TT3 values in the present study, favors an increase in receptor affinity rather than a 10% increase in receptor number. Our observations do not allow us to identify the cause or to select a mechanism to explain these observations. However, overfeeding (6, 38), fasting (20, 37), dietary iodine deficiency or excess (27), social isolation with possible depression (15), freezer storage at -30°C (7), and changes in photoperiodicity do not alter thyroid hormone concentrations and binding in the manner we describe. The mechanisms responsible for altered thyroid hormone binding associated with cold adaptation must explain decreased FT3 values and decreased free fractions of both T4 and T3. An enriched serum pool of thyroidbinding proteins with slightly increased binding affinities is a reasonable explanation. The hormonal response to acute cold air exposure has been shown to include decreased values of serum prolactin (21) and cortisol (11) and variable changes in TSH (13, 21, 35). In agreement with these reports, we find that serum prolactin and cortisol decline in response to an acute 60-min cold air exposure and that this fall does not change with AR. Exposure of seminude men to cold for 30-60 min elevates MAP by -16% and does not markedly change rectal or oral temperature (17, 23, 26). Repeated cold air
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1471
or water exposures will attenuate this pressor response to cold (17). The blood pressure response to cold in our subjects after 24 and 44 wk AR was decreased to a degree similar to that described for men exposed to a 5-wk cold water adaptation program (17). The attenuation of this cold pressor response may involve adaptation of the cutaneous circulation (2). Repeated cold air or water exposures over several weeks decrease body temperature and the temperature that initiates shivering when an individual is exposed to a cold air challenge (2, 23). Subjects clothed in traditional polar garments experience significant cooling of the face and torso when exposed to -32°C (14). Our findings of a lower oral temperature after 24 and 44 wk AR are in agreement with Guenter et al. (9), Bittel et al. (3), and Budd and Warhaft (5). The fall in oral temperature with cold exposure that we describe is in agreement with Bittel et al. (2, 3) and Briick et al. (4) and is in contrast to Budd and Warhaft (5), who did not perform their studies in a climatic chamber in Antarctica. Our findings, supported by these earlier reports, are consistent with a hypothermic type of cold adaptation characterized by a “greater cooling of body temperature with less metabolic compensation” (3) or “low metabolic response and a more pronounced cooling of the body” (23). The mechanisms relating the changes in human thyroid hormones and hypothermic adaptation to cold after AR are unknown. Existing evidence suggests that much of the human T3 pool is located in muscle (19). Muscle cells with an increased content of T3 have an increased number of mitochondria, increased oxygen utilization, and decreased contractile strength and are thought to have an uncoupling of oxidative phosphorylation (1). The increased [ 1251]T3 pool found with AR is likely distributed in part in skeletal muscles that are involved in shivering. If the set point of thermal equilibrium was shifted to favor heat generated from meaningful muscle contraction, as opposed to shivering, then a subject sitting quietly and exposed to cold may demonstrate a fall in core temperature and thus an apparent hypothermic response to cold. This same subject would require increased energy intake to carry out normal muscle function and would show a fall in the shivering threshold. The role of T4 as a prohormone and T3 as the active agent in bringing about this shift in thermal equilibrium is speculative and needs further investigation. A fall in the %FT3 and %FT, after AR suggests a significant modification of human thyroid hormone transport proteins after extended polar residence. These changes in protein binding may facilitate hormone delivery to specific tissues. The simultaneous development of hypothermic cold adaptation in these individuals provides an area of research that links changing thyroid hormone economy and cold adaptation. We thank the 1983-84 winter-over crew for participation in the study, the National Science Foundation, and Naval Support Force Antarctica for their support. We also personally thank Dr. Louis Homer for model fitting to derive estimates of changes in binding affinity and capacity of serum proteins, Dr. Stephen Lewis and Kathleen Kowalski for review of the manuscript, and Patricia Mu&nix for manuscript preparation. This work was supported in part by Naval Medical Research and Development Command work unit 62233N.MM33C30.004.
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