ORIGINAL PAPER

Changes in thyroid function with age: results from the Pizarra population-based longitudinal study A. Mª Lago-Sampedro,1,2,a C. Guti
errez-Repiso,1,2,a S. Vald
es,1,2 C. Maldonado,1 N. Colomo,1,2 M. C. Almaraz,1,2 E. Rubio-Mart
ın,1 S. Morcillo,1,2 I. Esteva,1,2 M. S. Ruiz de Adana,1,2 V. Perez-Valero,3 F. Soriguer,1,2,4,b G. Rojo-Mart
ınez,1,2,4,b E. Garc
ıa-Fuentes1,4,b

SUMMARY

What’s known

Background: Results of studies examining the influence of age on thyroid function and TSH levels, in the absence of thyroid disease, remain controversial. The aim of this study was to determine the course of thyroid function over 11 years in a population with normal thyroid function. Methods: This is a population-based prospective study started in 1995–1997 (first phase), and reassessed 6 (second phase) and 11 years later (third phase). Results: The TSH and FT4 in the third phase were significantly increased (p = 0.001 and p = 0.001, respectively), with the values being higher particularly from the age of 50 years. In those persons with a baseline TSH ≥ 1.2 and < 3 lIU/mL, the OR of having a TSH of 3–5 lIU/ mL in the third phase was 6.10 (p = 0.004). In those with a baseline TSH ≥ 3 and ≤ 5 lIU/mL, the OR of having a TSH of 3–5 lIU/mL in the third phase was 20.8 (p < 0.0001). Similar results were found for FT4. Conclusion: In a population free of clinical thyroid disease, TSH and FT4 values rise over the years. This increase occurs in all age groups, but depends mainly on the basal concentrations of TSH and FT4.

Introduction Studies have shown that the intra-individual variation in thyroid hormones is less than the inter-individual variation (1,2). This intra-individual stability in thyroid hormone concentrations suggests the presence of a genetic base for their regulation, as shown by studies in twins (3,4). Nevertheless, various environmental factors, such as iodine intake (5,6) or smoking (7), can also influence the regulation of the hypothalamus–hypophysis–gonadal axis. The prevalence of autoimmune and nodular thyroid disorders increases with age (8). However, results of studies examining the influence of age on thyroid function and thyrotropin (TSH) levels, in the absence of thyroid disease, remain controversial. Some studies suggest that increasing age is associated with a reduction in the secretion of TSH (8,9). Other studies, though, including the National Health and Nutrition Examinations Survey III (NHANES III) (10) and others (11,12), have shown that TSH levels ª 2014 John Wiley & Sons Ltd Int J Clin Pract, May 2015, 69, 5, 577–587. doi: 10.1111/ijcp.12545

Results of studies examining the influence of age on thyroid function and TSH levels, in the absence of thyroid disease, remain controversial. However, most of these studies relating thyroid hormone concentrations with age in persons free of thyroid disease are cross-sectional, which limits the strength of their results.

What’s new TSH and FT4 values rise over the years in this population-based prospective study, with the values being higher particularly from the age of 50 years. In the absence of any clinical thyroid disorder, and within reference ranges considered normal, the increase in values of TSH and FT4 over time depends not so much on age but more on the baseline values of TSH and FT4. This finding may thus lead to the requirement to reconsider population-based reference values.

rise with age in persons with no clear thyroid disorder and in the absence of iodine deficiency. However, most of these studies relating thyroid hormone concentrations with age in persons free of thyroid disease are cross-sectional, which limits the strength of their results. Bremner et al. (13), in the Busselton Health Survey (mean age 58 years at follow-up), recently showed an increase in TSH and no change in free thyroxine (FT4) over a 13-year period. The largest TSH increase was in people with the lowest TSH at baseline. This suggests that the TSH increase arises from age-related alteration in the TSH set point or reduced TSH bioactivity rather than occult thyroid disease. Another study (the Cardiovascular Health Study All Stars Study) showed an increase in TSH over time in a cohort of elderly individuals who were not taking thyroid medication (14). However, Knudsen et al. (15), in an analysis of baseline data in a Danish cohort, found a negative association between TSH and age, an association that also remained true 7 years later (16). On the other hand,

1 Unidad de Gesti
on Cl
ınica de Endocrinolog
ıa y Nutrici
on, Instituto de Investigaciones Biom
edicas de M
alaga (IBIMA), Hospital Regional Universitario, M
alaga, Spain 2 CIBER de Diabetes y Enfermedades Metab
olicas Asociadas (CIBERDEM), Instituto de Salud Carlos IIII, M
alaga, Spain 3 Unidad de Gesti
on Cl
ınica de Laboratorio, Instituto de Investigaciones Biom
edicas de M
alaga (IBIMA), Hospital Regional Universitario, M
alaga, Spain 4 CIBER de Fisiopatolog
ıa Obesidad y Nutrici
on (CIBEROBN), Instituto de Salud Carlos III, M
alaga, Spain

Correspondence to: Eduardo Garc
ıa-Fuentes, Unidad de Gesti
on Cl
ınica de Endocrinolog
ıa y Nutrici
on, Hospital Regional Universitario, Plaza del Hospital Civil s/n, M
alaga 29009, Spain Tel.: + 34 9512 90346 Fax: + 34 9522 8670 Email: [email protected] Disclosures The authors declare that there is no conflict of interest associated with this study. a Lago-Sampedro A and Gutierrez-Repiso C contributed equally to this work. b

Soriguer F, Rojo-Martinez G and Garc
ıa-Fuentes E contributed equally to this work.

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another study found no significant age effect on TSH change (17). With this background, we analysed thyroid function in a population-based prospective study (Pizarra Study) started in 1995–1997, and reassessed 6 and 11 years later, about whom there is clinical, anthropometric and biochemical information, including data about iodine intake (18). The aims of this study were to: (i) determine the course of thyroid function over 11 years in a population from southern Spain that has experienced changes in iodine intake during the follow-up period, and (ii) evaluate the factors conditioning the changes in thyroid function over this time.

Material and methods Study subjects The study was carried out in a cohort of the Pizarra Study, a population-based prospective study undertaken in a population from Andalusia, southern Spain. Patients were excluded if they were hospitalised during the 4 weeks prior to the evaluation, in a geriatric institution, were pregnant, or had a severe disease or a condition that could affect the thyroid hormone levels (non-thyroidal illness). The study population (n = 1250) and the design of this survey have been described previously (19–22). A total of 225 subjects did not attend, had missing data or were excluded. The first phase of the study (1995–1997) included 1025 individuals, aged 18– 65 years. In this phase, studies were made of thyroid function (TSH, FT4, FT3 and antithyroid peroxidase (anti-TPO) antibodies) as well as urinary iodine excretion and intake of iodised salt. In this phase, 937 persons had a normal thyroid function at the baseline study (Figure 1); the remaining 88 either had positive anti-TPO antibodies (> 50 IU/ mL), TSH < 0.20 lIU/mL or > 5 lIU/mL, or were receiving antithyroid or thyroid hormone treatment. No other significant differences were found in the variables studied between those with and those without a normal thyroid function (data not shown). Six years after the first visit (second phase), 918 of the initial 1025 persons underwent repeat tests for thyroid function (TSH, FT4 and FT3) and urinary iodine concentration, and their iodised salt intake was again assessed. Of these, 75 had some sort of alteration in their thyroid function (TSH < 0.20 lIU/mL or > 5 lIU/mL) or were receiving antithyroid or thyroid hormone treatment. The final sample of persons with a normal thyroid function at this second phase was 843 (Figure 1). No other significant differences were found in the

variables studied between those with and those without a normal thyroid function (data not shown). Five years after the second visit (11 years after the first visit) (third phase), of the 918 persons assessed in the second phase, 640 underwent repeat testing of their thyroid function (TSH, FT4, FT3 and anti-TPO antibodies) and urinary iodine concentration, and their iodised salt intake was again assessed. In this phase, 88 persons had some sort of alteration in their thyroid function [positive anti-TPO antibodies (> 50 IU/mL), TSH < 0.20 lIU/mL or > 5 lIU/mL] or were receiving antithyroid or thyroid hormone treatment. Thus, the final sample of persons with a normal thyroid function in this third phase was 552 (Figure 1). No other significant differences were found in the variables studied between those with and those without a normal thyroid function (data not shown). Samples from subjects were processed and frozen immediately after their reception in the Regional University Hospital Biobank (Andalusian Public Health System Biobank). The research has been carried out in accordance with the Declaration of Helsinki (2008) of the Word Medical Association; all the participants gave their written informed consent and the study was reviewed and approved by the Ethics and Research Committee of Regional University Hospital, Malaga, Spain.

Procedures Measurements were made in all the participants of body mass index (BMI) (weight⁄height2). The subjects were classified into two groups according to their BMI: obese subjects, with a BMI ≥ 30 kg/m2, and non-obese subjects, with a BMI < 30 kg/m2. All the subjects completed a standardised survey on health habits (23). At the three study phases, blood samples were collected after a 12-h fast. The serum was separated and frozen at
80 °C. Serum thyroid hormones were analysed at the same time in an automated MODULAR ANALYTICS E170 analyser (Roche Diagnostics GmbH, Mannheim, Germany). The TSH, FT3 and FT4 were measured by chemoluminescence (Roche Diagnostics GmbH). For TSH, the reference values in adults were 0.2– 5.0 lIU/mL. For FT4, the reference values in adults were 10–22 pmol/L. For FT3, the reference values in adults were 3.10-6.8 pmol/L (24). The anti-TPO antibodies were measured by a radioimmunometric assay (Biocode S.A., Liege, Belgium). The iodine concentration in the first morning urine was measured by the modified Benotti and Benotti technique (25). ª 2014 John Wiley & Sons Ltd Int J Clin Pract, May 2015, 69, 5, 577–587

Thyroid function in a longitudinal study

Ramdon sample of the population in Pizarra (Spain) in 1995–1997 N = 1250. 225 subjects did not attend, with missing data or were excluded.

1st Phase Thyroid function assessed in 1025 subjects in 1995 –1997. Normal thyroid function (n = 937) TSH > 5 or using thyroid medication: with negative anti-TPO antibodies (n = 15) with positive anti-TPO antibodies (n = 35) TSH < 0.20 or using antithyroid medication (n = 17) Positive anti-TPO antibodies with normal TSH (≥ 0.20 and ≤ 5) (n = 21) 107 subjects did not attend, with missing data or were excluded. 2nd Phase Thyroid function assessed in 918 subjects in 2001–2003. Normal thyroid function (n = 843) TSH > 5 or using thyroid medication (57) TSH < 0.20 or using thyroid medication (n = 18) 278 subjects did not attend, with missing data or were excluded. 3rd Phase Thyroid function assessed in 640 subjects in 2006–2008. Normal thyroid function (n = 552) TSH > 5 or using thyroid medication: with negative anti-TPO antibodies (n = 17) with positive anti-TPO antibodies (n = 35) TSH < 0.20 or using thyroid medication (n = 15) Positive anti-TPO antibodies with normal TSH (≥ 0.20 and ≤ 5) (n = 21)

Figure 1 Pizarra study flow chart

Statistical study The deviation of the continuous variables from a normal distribution was tested with the Kolmogorov–Smirnov Z test. The hypothesis contrast of the qualitative variables (%) was made with the v2 test. For the multivariate statistical analysis, the variables that failed to adjust to a normal distribution received a log-normal transformation. The hypothesis contrast of the continuous variables was made with the Student’s t-test or a two-way or multi-way ANOVA. The hypothesis contrast between the same variables measured over time was performed with a repeatedmeasure ANOVA. The association between variables was measured by calculating the Spearman r correlation coefficient. Different models of multiple logistic regression were designed to calculate the risk (OR) of having TSH levels above a certain cut-off point. The OR and the 95% confidence intervals (CI) were calculated from the coefficients (beta) (eb) of the main variables in the models. In all cases, the level of rejection of a null hypothesis was a = 0.05 for two tails. The statistical analysis was carried out with SPSS (Version 11.5 for Windows; SPSS, Chicago, IL, USA). ª 2014 John Wiley & Sons Ltd Int J Clin Pract, May 2015, 69, 5, 577–587

Results Cross-sectional study in persons with normal thyroid function at each of the three study phases Thyroid hormones and gender In the first phase, the TSH was similar between men and women (1.85  0.95 vs. 1.94  0.89 lIU/mL, p = 0.16). However, significantly lower levels were found in women compared with men of FT4 (15.07  2.05 vs. 14.66  2.60 pmol/L, p = 0.006) and FT3 (5.25  0.61 vs. 4.88  0.71 pmol/L, p < 0.0001). After adjusting for age, obesity, smoking (yes/no) and urinary iodine concentration, the difference between men and women disappeared for FT4 (p = 0.93), but remained significant for FT3 (p < 0.0001). At the second study phase (after 6 years), the TSH was significantly higher in women compared with men (1.70  0.84 vs. 1.87  0.95 lIU/mL, p = 0.04). However, the FT4 (15.73  1.89 vs. 15.35  2.11 pmol/L, p = 0.04) and FT3 (5.20  0.60 vs. 4.80  0.63 pmol/L, p < 0.001) were significantly

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lower in women. After adjusting for age and obesity, only FT3 remained significantly different between men and women (p < 0.001). At the third study phase, the TSH (1.97  0.89 vs. 2.11  1.00 lIU/mL, p = 0.136) and FT4 (16.5  2.21 vs. 16.15.2.12 pmol/L, p = 0.108) were similar in men and women. The FT3 was significantly lower in women than in men (5.18  0.53 vs. 4.97  0.62 pmol/L, p = 0.003). This difference, though, lost its significance after adjusting for age, obesity and smoking (p = 0.31).

Thyroid hormones and iodine intake Only 7.1% of the population in the first phase consumed iodised salt. Half (54%) of the persons had urinary iodine excretion levels < 100 lg/L. Neither TSH, FT4 nor FT3 varied significantly depending on the intake of iodised salt (yes/no) nor were they associated with urinary iodine concentrations (data not shown). In the second and third phases, the intake of iodised salt rose (7.1% vs. 47.4% and 53.5%, respectively), as did the urinary iodine concentration (103.5  69.5 lg/L vs. 132.1  86.5 lg/L and 147.6  91.1 lg/L, respectively) (Table 1). No significant differences were seen in either the second or the third phases in TSH, FT4 or FT3 according to the intake of iodised salt (data not shown), nor were their levels associated with urinary iodine concentrations (data not shown).

Thyroid hormones and smoking The percentage of persons who did not smoke rose from 65.4% in the first phase to 71.2% in the second, and 74.9% in the third (Table 1). In the first

phase, the TSH levels were significantly lower in those who smoked, after adjusting for age, gender, obesity and urinary iodine (1.95  0.95 vs. 1.73  0.95 lIU/mL, p = 0.03). The levels of FT4 and FT3 were not affected by smoking (data not shown). In the second and third phases, no significant differences were seen in TSH, FT4 or FT3 according to whether the individuals smoked (data not shown).

Thyroid hormones and age In the first phase, age correlated negatively with TSH (r =
0.15, p < 0.0001). This association remained after adjusting in an ANOVA model for gender, obesity, urinary iodine and smoking (p = 0.001). Neither the FT4 nor the FT3 correlated significantly with age (data not shown). In the second phase, the TSH correlated negatively with age (r =
0.17, p < 0.0001). The significance remained after adjusting for gender, obesity, smoking and urinary iodine (p = 0.03). The FT3 correlated significantly with age (r =
0.166, p < 0.0001), and remained so after adjusting for gender, obesity, smoking and urinary iodine (p = 0.001). The FT4 did not correlate significantly with age (data not shown). In the third phase, the TSH correlated negatively with age (r =
0.11, p = 0.03). The significant association disappeared (p = 0.28) after eliminating those younger than 30 years of age, of whom there were only 12 persons in the third phase. The FT3 correlated significantly with age (r =
0.110, p = 0.009), remaining so after adjusting for gender, obesity, smoking and urinary iodine (p = 0.03). The FT4 did not correlate significantly with age (data not shown).

Table 1 Characteristics of the study population of those with normal thyroid function at all three phases of the study

N (Men/Women) Age (years) BMI (kg/m2) TSH (lIU/mL) FT4 (pmol/L) FT3 (pmol/L) Urinary iodine (lg/L) Intake of iodised salt (%) Smoker (%) No Yes

1st phase

2nd phase

3rd phase

552 (202/350) 41.7  12.8 27.6  7.2 1.79  0.81 14.9  2.0 5.08  0.68 103.5  69.5 7.1

552 (202/350) 47.9  13.6 28.7  5.2 1.77  0.78 15.5  1.9 4.98  0.52 132.1  86.5 47.8

552 (202/350) 52.4  13.3 29.1  5.9 1.95  0.86 16.3  2.3 5.07  0.60 147.6  91.1 53.5

65.4 34.6

71.2 28.8

74.9 25.1

p*

< 0.0001 < 0.0001 0.001 0.10 < 0.0001

*Adjusted for gender and obesity. Data are mean  standard deviation unless otherwise specified. BMI, body mass index; TSH, thyrotropin; FT4, free thyroxine; FT3, free triiodothyronine.

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Thyroid function in a longitudinal study

Thyroid function and age The TSH values in the third phase of the study were higher than those of the other two phases (p = 0.001) (Figure 2A.). This increase was not completely independent of age (p = 0.01), with the values being higher particularly from the age of 50 years (Figure 3A). The FT4 values were higher in the second and third phases (p < 0.0001) (Figure 2B), at whatever age (p = 0.23) (Figure 3B). The FT3 values did not differ for the three phases (p = 0.57) (Figure 2C), being significantly different according to age (p = 0.001) (Figure 3C). All these results were adjusted for gender and obesity.

Evolution of thyroid function according to baseline TSH At both the second phase (data not shown) and the third phase, those persons with a baseline TSH ≥ 3 and ≤ 5 lIU/mL experienced a lower increase in TSH (p < 0.0001) (Figure 4A). In those with a baseline TSH ≥ 1.2 (percentile-25) and < 3 lIU/mL (percentile-75), the OR of having in the third phase a TSH of 3–5 lIU/mL was 6.10 (95% CI: 1.81– 20.58). In those with a baseline TSH ≥ 3 and ≤ 5 lIU/mL, the OR of having in the third phase a TSH of 3–5 lIU/mL was 20.8 (95% CI: 5.83– 74.44), there being no association with age, gender, obesity, smoking or urinary iodine (Table 2, Model 1). Similar results for the FT4 and FT3 percentiles were found for FT4 (Figure 4B) and FT3 (Figure 4C). ª 2014 John Wiley & Sons Ltd Int J Clin Pract, May 2015, 69, 5, 577–587

TSH (µIU/ml) (95% CI)

The TSH, FT4 and FT3 levels at the three study phases are shown in Table 1. The association was studied between the values of the TSH, FT4 and FT3 in those persons with a normal thyroid function and who had measurements for all three study points. The TSH values correlated significantly (TSH1st Phase vs. TSH2nd Phase, r = 0.55; TSH1st Phase vs. TSH3rd Phase, r = 0.53; TSH2nd Phase vs. TSH3rd Phase, r = 0.61) (p < 0.001 for the three correlations). The FT4 and FT3 values also correlated significantly (FT41st Phase vs. FT42nd Phase, r = 0.37; FT41st Phase vs. FT43rd Phase, r = 0.41; FT42nd Phase vs. FT43rd Phase, r = 0.51; p < 0.0001 for the three correlations) (FT31st Phase vs. FT32nd Phase, r = 0.37; FT31st Phase vs. FT33rd Phase, r = 0.42; FT32nd Phase vs. FT33rd Phase, r = 0.37; p < 0.0001 for the three correlations).

2.1

2.0

1.9

1.8

1.7

1.6 1st phase 2nd phase 3rd phase (B)

17.5 17.0

FT4 (pmol/l) (95% CI)

Correlation between the levels of TSH, FT4 and FT3

(A)

16.5 16.0 15.5 15.0 14.5 1st phase 2nd phase 3rd phase

(C) 5.3

5.2

FT3 (pmol/l) (95% CI)

Follow-up study in those with normal thyroid function during all three study phases

5.1

5.0

4.9

4.8 1st phase 2nd phase 3rd phase Figure 2 Levels of TSH (lIU/mL) (A), FT4 (pmol/L)

(B) and FT3 (pmol/L) (C) in the follow-up study in those persons with normal thyroid function at all three study phases

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TSH ≥ 3 and ≤ 5 lIU/mL (OR = 7.11; 95% CI: 1.53–33.11; p = 0.01) (Table 2, Model 2). The likelihood of having TSH > 5 lIU/mL in the third phase was significant in those with a baseline TSH ≥ 3 and ≤ 5 lIU/mL (OR = 11.80; 95% CI: 4.50–30.93; p = 0.01) (Table 2, Model 3). All these results were adjusted for age, gender, obesity, smoking or urinary iodine in the first study phase.

(A)

2.6

TSH (µIU/ml) (95% CI)

2.4 2.2 2.0 1.8

Risk of hypothyroidism during the follow-up according to the TSH and anti-TPO antibodies in the first phase of the study

1.6 1.4 1.2 < 31

31–40 41–50 51–60

> 60

Age (years)

FT4 (pmol/l) (95% CI)

(B)

18

17

16

15

14

13

< 31

31–40 41–50 51–60

> 60

Including all the persons in the first phase (except those with hypothyroidism or hyperthyroidism under treatment), the likelihood of having TSH > 5 lIU/ mL in the third phase was not significantly associated with the presence of positive anti-TPO antibodies in the first phase, although it was with having a baseline TSH ≥ 3 and ≤ 5 lIU/mL (OR = 21.83; 95% CI = 5.15–90.5; p = 0.01). This significant association remained even after including in the model the presence of positive anti-TPO antibodies in the third phase. As expected, the presence of positive antiTPO antibodies in the third phase was significantly associated with TSH > 5 in the third phase (OR = 4.50; 95% CI = 1.15–10.9; p = 0.02) (Table 2, Model 4). All these results were adjusted for age, gender, obesity, smoking or urinary iodine in the first study phase.

Age (years)

FT3 (pmol/l) (95% CI)

(C)

5.6

Discussion

5.4 5.2 5.0 4.8 4.6 < 30

31–40 41–50 51–60

> 60

Age (years) Figure 3 Levels of TSH (lIU/mL) (A), FT4 (pmol/L) (B) and FT3 (pmol/L) (C)

), second ( ) and according to age (in decades) and the study phase [first ( )] in the follow-up study in those persons with normal thyroid third phases ( function at all three phases

Risk of hypothyroidism during the follow-up in persons with a normal thyroid function in the first phase of the study The likelihood of having TSH > 5 lIU/mL in the second phase was significant in those with a baseline

The results of this study show that in a population free of clinical thyroid disease, TSH and FT4 values rise over the years. This increase occurs in all age groups, but depends mainly on the basal concentrations of TSH and FT4. The clinical implications of age-associated changes in thyroid function are not well understood. It has been suggested that mild elevations in TSH may reflect the natural history of thyroid function with ageing, with recommendations to increase the upper limit of the reference range for TSH in older persons (10,12). However, the results found in our three cross-sectional studies differ slightly from those of other cross-sectional studies. The NHANES III crosssectional study and the Busselton Health Survey found slightly higher TSH levels in older persons (26–28). However, Tunbridge et al. (29) in the Whickham, UK survey found that serum TSH levels did not vary with age in men, but increased markedly in women aged over 45 years. In addition, a decrease in the median TSH has been reported in healthy centenarians, a population that may not be representative of the vast majority of elderly people ª 2014 John Wiley & Sons Ltd Int J Clin Pract, May 2015, 69, 5, 577–587

Thyroid function in a longitudinal study

(A)

TSH (µIU/ml) (95% CI) (3rd – 1st phase)

Figure 4 Increase in TSH, FT4 and FT3 in the follow-up study in those persons with normal thyroid function during the three study phases. The first phase values of TSH, FT4 and FT3 were categorised in three levels according to the percentile-25 (p25) and percentile-75 (p75) of the frequency distribution of TSH, FT4 and FT3, ), ≥ p25 and < p75 ( ), and respectively [< p25 ( )]. (A) Increase in TSH (lIU/mL) according to ≥ p75 ( age (p = 0.04) and TSH values in the first phase (< 1.2 lIU/mL (p25), ≥ 1.2 and < 3 lIU/mL (p75), and ≥ 3 lIU/mL) (p < 0.0001). (B) Increase in FT4 (pmol/L) according to age (p = 0.14) and FT4 values in the first phase (< 13.20 pmol/L (p25), ≥ 13.20 and < 16.40 pmol/L (p75), and ≥ 16.40 pmol/L) (p < 0.0001). (C) Increase in FT3 (pmol/L) according to age (p = 0.04) and FT3 values in the first phase (< 4.70 pmol/L (p25), ≥ 4.70 and < 5.40 pmol/L (p75), and ≥ 5.40 pmol/L) (p < 0.0001)

1.5 1.0 0.5 0.0 –0.5 –1.0 –1.5 –2.0 < 30

31–40

41–50

51–60

> 60

Age (years)

ª 2014 John Wiley & Sons Ltd Int J Clin Pract, May 2015, 69, 5, 577–587

FT4 (pmol/l) (95% CI) (3rd – 1st phase)

(B)

6 4 2 0 –2 –4 –6 < 30

31–40

41–50

51–60

> 60

Age (years) (C) 1.5

FT4 (pmol/l) (95% CI) (3rd – 1st phase)

(30). Considering the three phases of our study from a cross-sectional viewpoint, including only persons with a normal thyroid function, the TSH values fell slightly with age in the first and second phases of the study. Thus, other variables, such as obesity, iodine intake or smoking, may have contributed to the changes seen in TSH with age. The relationship between TSH and obesity has been extensively studied in this population in a previous study (18). Prior studies in this same population have shown that the changes in thyroid hormone levels could be the consequence, rather than the cause, of the increase in weight (18). Other cross-sectional population-based studies have shown a significant positive association between BMI and serum TSH (31). The association between thyroid hormone levels and obesity must be taken into account in this type of study, as obesity itself may also affect the serum TSH concentration, independently of the thyroid function (32). Thyroid function and obesity may affect each other. This has to be taken into account because of the possible association between TSH concentrations and an increased cardiovascular risk, which is closely associated with obesity. However, this has to be analysed more extensively in a prospective study. However, in this study, the systematic inclusion of the BMI did not modify the changes found in the TSH levels over the years of the follow-up. Another study found that the level of iodine intake was associated with considerable differences in the change in TSH at the 11year follow-up (17). In our case, however, urinary iodine failed to modify the relation between TSH levels and age. In a previous study, we found that the increase in urinary iodine excretion over the years was associated with an increase in iodised salt consumption and with higher iodine content in dairy products (33). The same was not seen with smoking,

1.0 0.5 0.0 –0.5 –1.0 –1.5 < 30

31–40

41–50

51–60

> 60

Age (years)

though, as the baseline TSH levels were lower in those who smoked. This effect of smoking on TSH is known (7,34). As obesity and smoking change with age, controlling for the effect of environmental

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Table 2 Different models of multiple regressions designed to calculate the risk (OR) of having TSH values above a

particular cutpoint

Model 1 *

Model 2 *

Model 3 *

Model 4 *

Dependent variable

Independent variable

OR (95% CI)

p

3rd phase TSH TSH < 3 lIU/mL (0) TSH ≥ 3 and ≤ 5 lIU/mL (1)

1st phase TSH < 1.2 lIU/mL (p25) ≥ 1.2 and < 3 lIU/mL (p75) ≥ 3 and ≤ 5 lIU/mL 1st phase TSH < 1.2 lIU/mL (p25) ≥ 1.2 and < 3 lIU/mL (p75) ≥ 3 and ≤ 5 lIU/mL 1st phase TSH < 1.2 lIU/mL (p25) ≥ 1.2 and < 3 lIU/mL (p75) ≥ 3 and ≤ 5 lIU/mL 1st phase TSH < 1.2 lIU/mL (p25) ≥ 1.2 and < 3 lIU/mL (p75) ≥ 3 and ≤ 5 lIU/mL TPO positive (1st phase) (0,1) TPO positive (3rd phase) (0,1)

Reference criterion 6.10 (1.81–20.58) 20.84 (5.83–74.44)

0.004 < 0.0001

Reference criterion 0.47 (0.06–3.42) 7.11 (1.53–33.11)

0.42 0.01

Reference criterion 0.87 (0.12–4.62) 11.80 (4.50–30.93)

0.34 0.01

Reference criterion 3.89 (0.46–32.34) 21.83 (5.15–90.50) 0.61 (0.23–1.60) 4.50 (1.15–10.90)

0.20 0.01 0.32 0.02

2nd phase TSH TSH ≤ 5 lIU/mL (0) TSH > 5 lIU/mL (1) 3rd phase TSH TSH ≤ 5 lIU/mL (0) TSH > 5 lIU/mL (1) 3rd phase TSH TSH ≤ 5 lIU/mL (0) TSH > 5 lIU/mL (1)

*Adjusted for age, gender, obesity, smoking and urinary iodine in the 1st study phase.

variables is very important when interpreting the importance of the TSH values in relation to age. A longitudinal analysis of our cohort, however, shows that TSH and FT4 levels rose during the 11year follow-up period, more so in older persons. How is it possible that these TSH levels are not higher in older persons in the cross-sectional studies, but they increase over the years in the longitudinal study? Some studies have shown the negative relation between TSH and FT4 and age in cross-sectional analyses (8,9), and other studies have shown a positive relation in longitudinal studies (13,14). However, we show that these two results are possible in the same longitudinal study according to the data considered. This apparent discrepancy in the association found between the longitudinal study and cross-sectional studies between TSH and FT4 levels and age confirms the difficulty drawing conclusions from cross-sectional studies. As in our study, others too have found an increase in TSH concentrations in longitudinal studies (13,14) and that the increase in TSH over time is greater in older persons (13). The reason for this increase in TSH is unknown, but it is possible that the consumption of medications commonly prescribed for older individuals may limit the efficiency of TSH on its receptor, modify the sensitivity of the hypothalamic–pituitary feedback system, or produce changes in the posttranslational processing of TSH within the thyrotroph (10,35). Addition-

ally, one of the metabolic explanations for the evolution of TSH and T4 with age could be a decrease in deiodinase activity. An age-dependent reduction in 50 -deiodinase activity, resulting in increased rT3 levels, has been observed in older subjects (36). Unfortunately, we have insufficient samples from all three study phases to analyse the rT3 and obtain reliable data. This, therefore, comprises a limitation of this study. This decrease in deiodinase activity might be age-dependent or linked to nonthyroidal illness, or as a result of a reduced stimulatory effect of TSH or an age-related increase in cytokines, which may induce an inhibitory action on the 50 -deiodination activity (36–38). However, in a longitudinal 4-year study, the authors found an ongoing decrease in TSH and increase in FT4 with age in a previously iodine-insufficient population, despite adequate iodine status at the time of the study (39). The authors suggest that low iodine intake at a young age leads to thyroid autonomy (and a tendency to hyperthyroidism) that persists despite normal iodine intake later in life. By contrast, in our case, the iodine intake was optimal in the second and third phases of the study. In populations with an optimal iodine intake, TSH is positively associated with age (28), as in our longitudinal study. Although not all studies have found an increase in TSH with age (17,39), it appears that the analysis of intra-individual changes over time is a ª 2014 John Wiley & Sons Ltd Int J Clin Pract, May 2015, 69, 5, 577–587

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more appropriate way to study the effects of ageing than comparing individuals who differ in age and other characteristics. In our longitudinal study carried out in persons with normal thyroid function, we found a greater increase in TSH in those persons with lower baseline TSH values, as have others (13). Different studies have found that, in the absence of any autoimmune disorders, thyroid function is very stable over time (1). Indeed, the TSH levels correlated positively and very significantly at all three points of the study, in the absence of any thyroid disorders. This positive association was also found for FT4 and FT3. It is not surprising, therefore, that it was the baseline levels of TSH, FT4 and FT3 that contributed most significantly to the explanation of their own levels over the years. Nonetheless, it does seem paradoxical that the increase in levels of TSH, FT3 and FT4 was greater in those persons who had lower levels of TSH, FT4 or FT3 in the first phase of the study. This may suggest the presence of a mechanism of adaptation that contributes to the maintenance of thyroid function within close limits (35), as well as over time (40). It may also be, though, that this finding is simply a consequence of the arbitrary TSH cut-off of 5 lIU/L used as an exclusion criterion in the three phases of the study. Person with a lower TSH at the first phase may have a higher increase of TSH than those with a higher TSH. In our longitudinal study, we excluded persons with altered TSH, those receiving antithyroid treatment or thyroid hormone therapy, and those with positive antithyroid antibodies. It is thus unlikely that the rise in TSH over time is because of any sort of subclinical thyroid disorder, although it is difficult to exclude completely the presence of an autoimmune disorder over time. In fact, in our longitudinal study, baseline levels of TSH within the normal range predicted very significantly the TSH values at 11 years, not only within the normal range, but also TSH values > 5 lIU/L at 6 and 11 years. Thus, those persons with high values within the normal range had a greater risk for hypothyroidism (TSH > 5 lIU/L), as has been seen in other studies (41). This greater risk was independent of the presence of positive antithyroid antibodies, which are known to increase the risk of hypothyroidism over the years (42). This is an important point, as for the same degree of thyroid function, the diagnosis of subclinical or clinical (overt) thyroid disease depends on the state of the TSH, FT4 and FT3 within the reference range of the relevant laboratory (1). In fact, the effect of having a thyroid function at the low or high limits of the reference range is a motive of controversy. Whilst some studies suggest that an optimal thyroid function could be represented by a low-normal TSH ª 2014 John Wiley & Sons Ltd Int J Clin Pract, May 2015, 69, 5, 577–587

concentration and a high-normal FT4 concentration (34), other studies associate this relative hyperfunction with an increased risk for atrial fibrillation (43), and have even related longevity with values of hypothyroid function within the normal laboratory ranges (40). For this reason, it is certainly reasonable to regard TSH concentrations at the upper limit of normality as a category of intermediate risk, especially with positive thyroid antibodies. Follow-up thyroid function testing for such individuals is appropriate, as already recommended (44,45). The possible narrowing of the reference range for TSH and thyroid hormones, when the selection of reference individuals is restricted by excluding those with antibodies and/or echographic abnormalities, is in accordance with the recommendation of The National Academy of Clinical Biochemists for the adoption of a TSH upper limit of 2.5 lIU/mL (46), or the study by Spencer et al. (47), in which the TSH upper limit (3 lIU/mL) is determined for populations with a low prevalence of thyroid autoimmunity (similar to our study, in which subjects with positive TPO were excluded). Additionally, the American Association of Clinical Endocrinologists (AACE) and the American Thyroid Association (ATA) suggest that while the normal TSH reference range (particularly for some subpopulations) may need to be narrowed, the normal reference range may widen with increasing age (27,48). The ATA/ AACE Guidelines for Hypothyroidism in Adults note that very mild TSH elevations in older individuals may not reflect subclinical thyroid dysfunction, but rather be a normal manifestation of ageing (48). This setting of the TSH upper limit is in light of the unsolved question of whether to treat subclinical hypothyroidism. Thus, not all patients who have mild TSH elevations are hypothyroid and not all would therefore require thyroid hormone therapy. Further studies are needed to determine the potential benefit of treating age-related changes in thyroid function. The main strengths of this study are that it is a population-based cohort study in which thyroid function was analysed at the same time at three different phases of the follow-up period, the organspecific immunological activity was assessed at two phases, and important confounding variables, such as iodine intake, obesity and smoking, were considered. As limitations, the use of drugs potentially affecting thyroid function and thyroid autoimmunity was not assessed. Nor can we exclude the possibility that an undefined proportion of patients was affected during the follow-up with chronic thyroiditis despite negative anti-TPO tests in the absence of ultrasonography data.

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In conclusion, the results of the study show that, in the absence of any clinical thyroid disorder, and within reference ranges considered normal, the increase in values of TSH and thyroid hormone over time depends not so much on age but more on the baseline values of TSH. This finding may thus lead to the requirement to reconsider population-based reference values.

Acknowledgements This work was supported in part by a grant from the Instituto de Salud Carlos III (PI11/02755). CIBER de Diabetes y Enfermedades Metab
olicas Asociadas and

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Author contribution GRM and FS designed research. CM, NC, MCA, IE and MSRA selected the subjects and collected serum samples; AMLS, ERM, SM, VPV, CGR and EGF analysed samples; FS, SV and EGF analysed data; FS, SV, GRM and EGF wrote the paper; all authors read and approved the final manuscript; EGF is the guarantor of this work and, as such, had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

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Paper received May 2014, accepted August 2014