Early Human Development 90 (2014) 621–624
Contents lists available at ScienceDirect
Early Human Development journal homepage: www.elsevier.com/locate/earlhumdev
Establishing a reference range for triiodothyronine levels in preterm infants Ki Won Oh a, Mi Sung Koo b, Hye Won Park c, Mi Lim Chung d, Min-ho Kim e, Gina Lim a,⁎ a
Department of Pediatrics, Ulsan University Hospital, University of Ulsan College of Medicine, Ulsan, South Korea Department of Pediatrics, Maryknoll Medical Center, Busan, South Korea Department of Pediatrics, Konkuk University Medical Center, Konkuk University School of Medicine, Seoul, South Korea d Department of Pediatrics, Haeundae Paik Hospital, College of Medicine, Inje University, Busan, South Korea e Biomedical Research Center, Ulsan University Hospital, University of Ulsan College of Medicine, Ulsan, South Korea b c
a r t i c l e
i n f o
Article history: Received 21 April 2014 Received in revised form 16 July 2014 Accepted 29 July 2014 Available online xxxx Keywords: Premature infants Thyroid function test Triiodothyronine
a b s t r a c t Objectives: Thyroid dysfunction affects clinical complications in preterm infants and older children. However, thyroid hormone replacement in preterm infants has no proven benefits, possibly owing to the lack of an appropriate reference range for thyroid hormone levels. We aimed to establish a reference range for triiodothyronine (T3) levels at 1-month postnatal age (PNA) in preterm infants. Methods: This retrospective study included preterm infants born at a tertiary referral neonatal center at gestational age (GA) b 35 weeks with no apparent thyroid dysfunction, for 6 consecutive years, with follow-up from PNA 2 weeks to 16 weeks. Using thyroid function tests (TFT), the relationships between T3 levels and thyrotropin (TSH) and free thyroxine (fT4) levels, birth weight, GA, postmenstrual age (PMA), and PNA were examined. The conversion trend for fT4 to T3 was analyzed using the T3/fT4 ratio. Results: Overall, 464 TFTs from 266 infants were analyzed, after excluding 65 infants with thyroid dysfunction. T3 levels increased with fT4 levels, birth weight, GA, PMA, and PNA but not with TSH levels. The T3/fT4 ratio also increased with GA, PNA, and PMA. The average T3 level at 1 month PNA was 72.56 ± 27.83 ng/dL, with significant stratifications by GA. Conclusions: Relatively low T3 and fT4 levels in preterm infants were considered normal, with T3 levels and conversion trends increasing with GA, PMA, and PNA. Further studies are required to confirm the role of the present reference range in thyroid hormone replacement therapy. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Thyroid dysfunction is common in preterm infants, and low thyroxine (T4) levels in preterm infants have been linked with neurodevelopmental deficits in motor and cognitive function in later life [1–3]. Further, low plasma triiodothyronine (T3) levels have been linked with reductions in IQ at 8 years of age [4]. T3 represents the active form of the thyroid hormone; it increases cardiac output, cardiac motility, and heart rate and decreases systemic vascular resistance [5]. Low T3 levels are associated with high mortality in children following cardiopulmonary bypass surgery [6]. In preterm infants, atypical hypothyroidism is common, with features such as elevated thyrotropin (TSH), hypothyroxinemia, and a Abbreviations: GA, gestational age; fT4, free thyroxine; PMA, postmenstrual age; PNA, postnatal age; T3, triiodothyronine; T4, thyroxine; TFT, thyroid function test; TSH, thyroid stimulating hormone. ⁎ Corresponding author at: Department of Pediatrics, Ulsan University Hospital, University of Ulsan College of Medicine, 877 Bangeojinsunhwan-doro, Dong-gu, Ulsan 682-714, South Korea. Tel.: +82 52 250 8640; fax: +82 52 250 8071. E-mail address: [email protected] (G. Lim).
http://dx.doi.org/10.1016/j.earlhumdev.2014.07.012 0378-3782/© 2014 Elsevier Ltd. All rights reserved.
delay in TSH elevation. However, no consensus exists regarding the age at which thyroid tests should be performed, normal values, or the need for retesting in preterm infants. Longitudinal measurements of thyroid hormone levels in preterm infants have shown inconsistent results. While some authors found little or no correlation of thyroid hormones with gestation age (GA) or postnatal age (PNA) [7–10], others have reported a positive correlation of free T4 levels with GA in the first week of life [11–13]. In the present study, we investigated the relationship between T3 levels and the T3/fT4 ratio in preterm infants with GA, PNA, and postmenstrual age (PMA); further, we attempted to establish a reference range for T3 levels at approximately 1 month of PNA for determining the need for thyroid hormone replacement therapy. 2. Materials and methods 2.1. Thyroid function tests in preterm infants This retrospective cohort study was conducted at a tertiary care teaching institution. We included infants with gestational age
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K.W. Oh et al. / Early Human Development 90 (2014) 621–624
(GA) b 35 weeks born at Chungbuk National University Hospital between January 2007 and December 2012. Their demographic data included GA, birth weight, and sex. GA was calculated from the menstrual history and, in most instances, was confirmed by ultrasound examination in the first trimester. On the basis of the TSH results of newborn screening tests (NST), we conducted thyroid function tests (TFT), comprising T3, fT4, and TSH levels, at 2–4 weeks of PNA for screening of thyroid dysfunction. Infants with abnormal NST underwent TFT at 2 weeks of age, while those with normal NST underwent TFT at 4 weeks of PNA. TSH, fT4, and T3 were measured at the hospital laboratory by immune radiometric assay (Beckman Coulter Company, Brea, CA, USA), radioimmune assay (Beckman Coulter), and radioimmune assay (Cisbio International, Bangnois/Ceze, France), respectively. The reference range in adults was considered as 0.17–5.05 mU/L for TSH, 0.8–1.8 ng/dL for fT4, and 78–180 ng/dL for T3. If the TFT results were abnormal (beyond the reference range for either TSH or fT4), we repeated TFT or started levothyroxine (L-T4) treatment. We excluded infants treated with L-T4 as well as infants with abnormal TFT that persisted until 8 weeks of PNA. Infants who might have been exposed to iodine excluded. Blood sampling for TSH, fT4, and T3 levels was carried out in the morning and usually timed to coincide with sampling for other tests. The study protocol was reviewed and approved by the institutional review boards of Chungbuk National University Hospital. 2.2. Relationship of T3 with various parameters The TFT of infants with GA b 35 weeks conducted from 2 weeks of PNA to 16 weeks of PNA and/or PMA 44 weeks. The relationships between T3 levels and fT4 and TSH levels, GA, birth weight, and sex were analyzed. The PNA and PMA when TFT was conducted were recorded and their relationships with T3 levels were analyzed. To identify the conversion rate of fT4 to T3, we analyzed the T3/fT4 ratio. 2.3. Average T3 levels at 1 month of PNA In preterm infants, TFT was repeated at 1 month of PNA in many neonatal intensive care units (NICUs). Thus, we attempted to establish a reference range for TFT at 1 month of PNA. T3, fT4, and TSH values from TFT conducted at 25–35 (28.2 ± 3.2) days of PNA were selected as the average reference range at 1 month of PNA. Infants were grouped according to the GA, as 25–28, 29–31, and 32–34 weeks. The T3, fT4, and TSH levels for percentiles were calculated.
thyroid function were included and analyzed. The gestational age and birth weight of these infants were 30.55 ± 2.46 (25–34) weeks and 1468.5 ± 415.8 (580–2760) g, and 138 infants (51.9%) were male. 3.1. Relationship of T3 with various parameters In the 464 TFT results analyzed, T3 was observed to be correlated with GA (r = 0.436, P b 0.001), birth weight (r = 0.347, P b 0.001), PNA (r = 0.344, P b 0.001), calculated PMA (r = 0.602, P b 0.001), and fT4 levels (r = 0.460, P b 0.001) but not with TSH levels (r = 0.059, P b 0.206). We have representatively described the association between T3 and PMA in Fig. 1. Further, sex-related differences in T3 levels were observed; after adjustment for GA and birth weight, female infants had significantly higher T3 levels as compared to male infants (male vs. female; 75.2 ± 31.5 vs. 83.0 ± 26.4, β = 5.235, 95% CI = 0.254–10.217, P = 0.039). The T3/fT4 ratio was correlated with GA (r = 0.259, P b 0.001), birth weight (r = 0.242, P b 0.001), PNA (r = 0.380, P b 0.001), calculated PMA (r = 0.602, P b 0.001), and TSH levels (r = 0.095, P = 0.040) but not with fT4 levels ((r = − 0.075, P = 0.107) or gender (P = 0.065). 3.2. Average T3 levels at approximately 1 month of PNA Of the 266 infants included in the study, TFT was conducted at approximately 1 month of PNA in 248 infants. We observed that the T3, fT4, and TSH levels as well as the T3/fT4 ratio were significantly increased with higher GA. The reference ranges at 1 month of PNA for T3, fT4, and TSH levels at each GA group are shown in Table 1 and Fig. 2. 4. Discussion Normal thyroid function is important for neurodevelopment, metabolic activity, and pulmonary function in infants [14]. In particular, in preterm infants, many factors can lead to thyroid dysfunction, such as medical illness [15], medication [16–19], and immaturity of the hypothalamic–pituitary–thyroid axis. Recent studies have attempted to clarify the effects of thyroid hormone replacement therapy; however, the benefits of thyroid hormone replacement therapy in preterm infants remain unknown [20,21]. These studies evaluated all preterm infants, with and without thyroid dysfunction, rather than specifically those with thyroid dysfunction, because no reference range for TFT levels is
2.4. Statistical analysis Statistical analysis was performed using SPSS 18.0 (SPSS, Inc., Chicago, IL, U.S.A.) and R package (V3.0.2, R Foundation for Statistical Computing, Vienna, Austria) was used to calculate reference range by LMS method that using gamlss package. The relationship between hormone levels and age was analyzed using linear regression with Pearson's correlation coefficient. Simple linear regression models were used to calculate the linear effect (β), 95% confidence interval, and Pvalue. To establish a reference range, we calculated percentile values in each group with LMS method, which summarizes the changing distribution by three curves representing the median, coefficient of variation and skewness. The analysis of variance (ANOVA) test was used to compare subtypes within the GA (as 25–28, 29–31, and 32–34 weeks). P b 0.05 was considered statistically significant. 3. Results Overall, single or repeat TFT was performed in 331 infants during the study period. Of these, the results from 65 infants were excluded owing to L-T4 treatment or prolonged thyroid dysfunction. There was no infant excluded owing to suspicion with excessive iodine exposure. Eventually, the results from 464 TFTs conducted in 266 infants with normal
Fig. 1. Triiodothyronine levels were positively correlated with postmenstrual age.
K.W. Oh et al. / Early Human Development 90 (2014) 621–624
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Table 1 Reference intervals for thyroid function tests at approximately 1 month of postnatal age in preterm infants.
T3 (ng/dL)
fT4 (ng/dL)
TSH (mU/L)
Gestational age
N
Mean ± SD
P valuea
5–95%
10–90%
25–75%
25–28 29–31 32–34 Total 25–28 29–31 32–34 Total 25–28 29–31 32–34 Total
57 98 93 248 57 98 93 248 57 98 93 248
51.05 69.61 88.87 72.56 1.05 1.20 1.30 1.20 5.01 5.98 6.84 6.08
a b c b0.001 a b c b0.001 a a, b b 0.019
19–83.7 33–107.9 51.3–138.8 32.7–123 0.69–1.44 0.81–1.56 0.93–1.68 0.81–1.59 1.5–10.3 1.7–12.6 2.0–15.7 1.9–13.1
26.1–76.8 39.8–98.3 57.9–125.7 41.8–105 0.77–1.36 0.89–1.48 1.01–1.60 0.9–1.5 2.0–8.7 2.2–10.5 2.5–12.7 2.3–11.3
38.4–65.1 52.1–83 70.5–106 53.2–89.2 0.91–1.22 1.03–1.34 1.15–1.46 1.05–1.39 3.0–6.5 3.4–7.6 3.8–8.8 3.4–8.0
± ± ± ± ± ± ± ± ± ± ± ±
18.75 24.54 25.81 27.83 0.23 0.24 0.23 0.25 2.76 3.89 4.33 3.89
a Statistical significances were tested by one way analysis of variance among groups. The same letters indicate a non-significant difference between groups based on Tukey's multiple comparison test.
available for preterm infants. Therefore, in this study, we aimed to define an appropriate reference range for T3 levels and the T3/fT4 ratio in preterm infants at 1 month of PNA. After the postnatal surge of thyroid hormone, the T3 value in preterm infants was observed to progressively increase with GA (r = 0.436), PNA (r = 0.344), and PMA (r = 0.602). T3 levels were significantly associated with PMA, which might be the sum of GA and PNA. T3 levels were higher in female infants than in male infants. A previous report demonstrated higher fT4 and TSH levels in female preterm infants as compared to male preterm infants [22,23]. It is considered that female infants have higher TSH and fT4, and they have a lower incidence of respiratory distress syndrome and other co-morbidities. We analyzed the conversion of T3 from fT4 using the T3/fT4 ratio. Thus far, no studies have examined the conversion of fT4 to T3 in preterm infants. We observed differences in the T3/fT4 ratio with GA, birth weight, PMA, and PNA, suggesting that deiodinase II function is abnormal in preterm infants. It is difficult to measure deiodinase activity directly in hepatic renal tissue, although the measurement of deiodinase activity is the best method of evaluating the conversion of fT4 to T3. There are only few reports concerning deiodinase activity in preterm animal studies. The T3/fT4 ratio has been reported to be low or normal in various thyroid diseases as well as after thyroid hormone replacement therapy [24]. In our study, the T3/fT4, in other words conversion of T3 from fT4, was also immature in preterm infants with lower GA and lower birth weight. Insufficient conversion of fT4 to T3 may
Fig. 2. Triiodothyronine levels at 1 month of postnatal age were significantly increased with higher gestational age.
contribute to relatively low T3 levels in preterm infants. Our result cannot be considered complete because we did not compare any of the parameters with those in healthy full term infants; further, the conversion ratio was not calculated between the same protein bound (T3/T4) or unbound (fT3/fT4) form. Further study needed to confirm deiodinase activity in preterm infants. At 1 month of age, T3 levels were noted to be higher for higher GA in this study, as reported previously [25–27]. Otherwise, other studies reported that T3 decreased after birth. It may be considered by inclusion of postnatal thyroid surge. In our study, we included TFT values only after PNA 2 weeks to eliminate the effects of the postnatal surge on TFT levels, with definite differences in the results. No study has evaluated TFT results at 1 month of PNA in preterm infants. In our study, T3 levels increased with increasing GA and PMA. This increase appears to be related to the physiological maturation of the thyroid gland and/or the hypothalamic–pituitary–thyroid axis rather than thyroid dysfunction, similar to the increase in cord blood T3 values over the gestation period [27]. We did not analyze the clinical complications of the preterm infants, because the incidence of clinical complications appeared to be similar for similar GA and PMA. Several reports have described the effects of medication and illness on thyroid function in preterm infants. Such illnesses and medication may be considered natural or unavoidable in preterm infants, with their effects reflecting the “normal” value of TFT in this group. Similarly, differing plasma protein levels and abnormal protein binding affects the methodology of fT4 measurement itself, which may be considered natural or unavoidable. We excluded many infants from this study owing to abnormal thyroid function on the basis of TFT results. These infants did not have typical congenital hypothyroidism, such as thyroid agenesis, ectopic thyroid, or dyshormogenesis. Mostly, they were cases of atypical hypothyroidism, such as transient hypothyroidism, hyperthyrotropinemia, hypothyroxinemia, and/or delayed TSH elevation. In our institution, we applied flexible criteria for L-T4 treatment, which was started earlier when infants were suspected to have thyroid dysfunction rather than waiting for longer follow-up. Further studies are needed for clarifying the reasons underlying the high incidence of thyroid dysfunction in preterm infants. We planned to exclude infants suspected to have previous iodine exposure; however, no infants were eventually excluded for this reason. There might have been some iodine exposure due to the disinfectant used for central line insertion and surgery (patent ductus ligation and surgical repair of bowel perforation) and by breast milk feeding, which could not be measured. This could be one of the reasons for the high incidence of thyroid dysfunction. In conclusion, our results suggest a potential reference range for normal T3 levels at approximately 1 month of age in preterm infants. The T3 and fT4 levels increased with GA and PMA. It is important to evaluate T3 levels in preterm infants to identify thyroid dysfunction since the conversion of T3 from fT4 is not accurately reflected, as
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shown by the near consistent T3/fT4 values. Further studies are required to confirm the utility of the reference range presented here in determining the need for thyroid hormone replacement therapy and its effects in preterm infants with low T3 or fT4 levels. Conflict of interest We have nothing to disclose. Acknowledgments We have nothing to declare. References [1] Meijer WJ, Verloove-Vanhorick SP, Brand R, van den Brande JL. Transient hypothyroxinaemia associated with developmental delay in very preterm infants. Arch Dis Child 1992;67:944–7. [2] Den Ouden AL, Kok JH, Verkerk PH, Brand R, Verloove-Vanhorick SP. The relation between neonatal thyroxine levels and neurodevelopmental outcome at age 5 and 9 years in a national cohort of very preterm and/or very low birth weight infants. Pediatr Res 1996;39:142–5. [3] Reuss ML, Paneth N, Pinto-Martin JA, Lorenz JM, Susser M. The relation of transient hypothyroxinemia in preterm infants to neurologic development at two years of age. N Engl J Med 1996;334:821–7. [4] Lucas A, Morley R, Fewtrell MS. Low triiodothyronine concentration in preterm infants and subsequent intelligence quotient (IQ) at 8 year follow up. BMJ 1996; 312:1132–3 [discussion 3–4]. [5] Toft AD. Thyroxine therapy. N Engl J Med 1994;331:174–80. [6] Portman MA, Slee A, Olson AK, Cohen G, Karl T, Tong E, et al. Triiodothyronine Supplementation in Infants and Children Undergoing Cardiopulmonary Bypass (TRICC): a multicenter placebo-controlled randomized trial: age analysis. Circulation 2010;122:S224–33. [7] Clark SJ, Deming DD, Emery JR, Adams LM, Carlton EI, Nelson JC. Reference ranges for thyroid function tests in premature infants beyond the first week of life. J Perinatol 2001;21:531–6. [8] Carrascosa A, Ruiz-Cuevas P, Potau N, Almar J, Salcedo S, Clemente M, et al. Thyroid function in seventy-five healthy preterm infants thirty to thirty-five weeks of gestational age: a prospective and longitudinal study during the first year of life. Thyroid 2004;14:435–42. [9] Kok JH, Hart G, Endert E, Koppe JG, de Vijlder JJ. Normal ranges of T4 screening values in low birthweight infants. Arch Dis Child 1983;58:190–4. [10] Dilli D, Oğuz SS, Andiran N, Dilmen U, Büyükkağnici U. Serum thyroid hormone levels in preterm infants born before 33 weeks of gestation and association of transient hypothyroxinemia with postnatal characteristics. J Pediatr Endocrinol Metab 2010;23:899–912.
[11] Adams LM, Emery JR, Clark SJ, Carlton EI, Nelson JC. Reference ranges for newer thyroid function tests in premature infants. J Pediatr 1995;126:122–7. [12] Reuss ML, Leviton A, Paneth N, Susser M. Thyroxine values from newborn screening of 919 infants born before 29 weeks' gestation. Am J Public Health 1997;87:1693–7. [13] Williams FL, Mires GJ, Barnett C, Ogston SA, van Toor H, Visser TJ, et al. Transient hypothyroxinemia in preterm infants: the role of cord sera thyroid hormone levels adjusted for prenatal and intrapartum factors. J Clin Endocrinol Metab 2005;90: 4599–606. [14] Simic N, Asztalos EV, Rovet J. Impact of neonatal thyroid hormone insufficiency and medical morbidity on infant neurodevelopment and attention following preterm birth. Thyroid 2009;19:395–401. [15] Paul DA, Mackley A, Yencha EM. Thyroid function in term and late preterm infants with respiratory distress in relation to severity of illness. Thyroid 2010;20:189–94. [16] Arai H, Goto R, Matsuda T, Takahashi T. Relationship between free T4 levels and postnatal steroid therapy in preterm infants. Pediatr Int 2009;51:800–3. [17] Carrascosa A, Ruiz-Cuevas P, Clemente M, Salcedo S, Almar J. Thyroid function in 76 sick preterm infants 30–36 weeks: results from a longitudinal study. J Pediatr Endocrinol Metab 2008;21:237–43. [18] Williams FL, Ogston SA, van Toor H, Visser TJ, Hume R. Serum thyroid hormones in preterm infants: associations with postnatal illnesses and drug usage. J Clin Endocrinol Metab 2005;90:5954–63. [19] Filippi L, Pezzati M, Cecchi A, Poggi C. Dopamine infusion: a possible cause of undiagnosed congenital hypothyroidism in preterm infants. Pediatr Crit Care Med 2006;7:249–51. [20] Ng SM, Turner MA, Gamble C, Didi M, Victor S, Manning D, et al. An explanatory randomised placebo controlled trial of levothyroxine supplementation for babies born b28 weeks' gestation: results of the TIPIT trial. Trials 2013;14:211. [21] La Gamma EF, Paneth N. Clinical importance of hypothyroxinemia in the preterm infant and a discussion of treatment concerns. Curr Opin Pediatr 2012;24:172–80. [22] Sun X, Lemyre B, Nan X, Harrold J, Perkins SL, Lawrence SE, et al. Free thyroxine and thyroid-stimulating hormone reference intervals in very low birth weight infants at 3–6 weeks of life with the Beckman Coulter Unicel DxI 800. Clin Biochem 2014;47: 16–8. [23] Korada M, Pearce MS, Avis E, Turner S, Cheetham T. TSH levels in relation to gestation, birth weight and sex. Horm Res 2009;72:120–3. [24] Mortoglou A, Candiloros H. The serum triiodothyronine to thyroxine (T3/T4) ratio in various thyroid disorders and after Levothyroxine replacement therapy. Hormones (Athens) 2006;3:120–6. [25] Cartault Grandmottet A, Cristini C, Tricoire J, Rolland M, Tauber MT, Salles JP. TSH, FT4 and T3T evaluation in normal and preterm hospitalized newborns. Arch Pediatr 2007;14:138–43. [26] Williams FL, Simpson J, Delahunty C, Ogston SA, Bongers-Schokking JJ, Murphy N, et al. Developmental trends in cord and postpartum serum thyroid hormones in preterm infants. J Clin Endocrinol Metab 2004;89:5314–20. [27] Hume R, Simpson J, Delahunty C, van Toor H, Wu SY, Williams FL, et al. Human fetal and cord serum thyroid hormones: developmental trends and interrelationships. J Clin Endocrinol Metab 2004;89:4097–103.
Contents lists available at ScienceDirect
Early Human Development journal homepage: www.elsevier.com/locate/earlhumdev
Establishing a reference range for triiodothyronine levels in preterm infants Ki Won Oh a, Mi Sung Koo b, Hye Won Park c, Mi Lim Chung d, Min-ho Kim e, Gina Lim a,⁎ a
Department of Pediatrics, Ulsan University Hospital, University of Ulsan College of Medicine, Ulsan, South Korea Department of Pediatrics, Maryknoll Medical Center, Busan, South Korea Department of Pediatrics, Konkuk University Medical Center, Konkuk University School of Medicine, Seoul, South Korea d Department of Pediatrics, Haeundae Paik Hospital, College of Medicine, Inje University, Busan, South Korea e Biomedical Research Center, Ulsan University Hospital, University of Ulsan College of Medicine, Ulsan, South Korea b c
a r t i c l e
i n f o
Article history: Received 21 April 2014 Received in revised form 16 July 2014 Accepted 29 July 2014 Available online xxxx Keywords: Premature infants Thyroid function test Triiodothyronine
a b s t r a c t Objectives: Thyroid dysfunction affects clinical complications in preterm infants and older children. However, thyroid hormone replacement in preterm infants has no proven benefits, possibly owing to the lack of an appropriate reference range for thyroid hormone levels. We aimed to establish a reference range for triiodothyronine (T3) levels at 1-month postnatal age (PNA) in preterm infants. Methods: This retrospective study included preterm infants born at a tertiary referral neonatal center at gestational age (GA) b 35 weeks with no apparent thyroid dysfunction, for 6 consecutive years, with follow-up from PNA 2 weeks to 16 weeks. Using thyroid function tests (TFT), the relationships between T3 levels and thyrotropin (TSH) and free thyroxine (fT4) levels, birth weight, GA, postmenstrual age (PMA), and PNA were examined. The conversion trend for fT4 to T3 was analyzed using the T3/fT4 ratio. Results: Overall, 464 TFTs from 266 infants were analyzed, after excluding 65 infants with thyroid dysfunction. T3 levels increased with fT4 levels, birth weight, GA, PMA, and PNA but not with TSH levels. The T3/fT4 ratio also increased with GA, PNA, and PMA. The average T3 level at 1 month PNA was 72.56 ± 27.83 ng/dL, with significant stratifications by GA. Conclusions: Relatively low T3 and fT4 levels in preterm infants were considered normal, with T3 levels and conversion trends increasing with GA, PMA, and PNA. Further studies are required to confirm the role of the present reference range in thyroid hormone replacement therapy. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction Thyroid dysfunction is common in preterm infants, and low thyroxine (T4) levels in preterm infants have been linked with neurodevelopmental deficits in motor and cognitive function in later life [1–3]. Further, low plasma triiodothyronine (T3) levels have been linked with reductions in IQ at 8 years of age [4]. T3 represents the active form of the thyroid hormone; it increases cardiac output, cardiac motility, and heart rate and decreases systemic vascular resistance [5]. Low T3 levels are associated with high mortality in children following cardiopulmonary bypass surgery [6]. In preterm infants, atypical hypothyroidism is common, with features such as elevated thyrotropin (TSH), hypothyroxinemia, and a Abbreviations: GA, gestational age; fT4, free thyroxine; PMA, postmenstrual age; PNA, postnatal age; T3, triiodothyronine; T4, thyroxine; TFT, thyroid function test; TSH, thyroid stimulating hormone. ⁎ Corresponding author at: Department of Pediatrics, Ulsan University Hospital, University of Ulsan College of Medicine, 877 Bangeojinsunhwan-doro, Dong-gu, Ulsan 682-714, South Korea. Tel.: +82 52 250 8640; fax: +82 52 250 8071. E-mail address: [email protected] (G. Lim).
http://dx.doi.org/10.1016/j.earlhumdev.2014.07.012 0378-3782/© 2014 Elsevier Ltd. All rights reserved.
delay in TSH elevation. However, no consensus exists regarding the age at which thyroid tests should be performed, normal values, or the need for retesting in preterm infants. Longitudinal measurements of thyroid hormone levels in preterm infants have shown inconsistent results. While some authors found little or no correlation of thyroid hormones with gestation age (GA) or postnatal age (PNA) [7–10], others have reported a positive correlation of free T4 levels with GA in the first week of life [11–13]. In the present study, we investigated the relationship between T3 levels and the T3/fT4 ratio in preterm infants with GA, PNA, and postmenstrual age (PMA); further, we attempted to establish a reference range for T3 levels at approximately 1 month of PNA for determining the need for thyroid hormone replacement therapy. 2. Materials and methods 2.1. Thyroid function tests in preterm infants This retrospective cohort study was conducted at a tertiary care teaching institution. We included infants with gestational age
622
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(GA) b 35 weeks born at Chungbuk National University Hospital between January 2007 and December 2012. Their demographic data included GA, birth weight, and sex. GA was calculated from the menstrual history and, in most instances, was confirmed by ultrasound examination in the first trimester. On the basis of the TSH results of newborn screening tests (NST), we conducted thyroid function tests (TFT), comprising T3, fT4, and TSH levels, at 2–4 weeks of PNA for screening of thyroid dysfunction. Infants with abnormal NST underwent TFT at 2 weeks of age, while those with normal NST underwent TFT at 4 weeks of PNA. TSH, fT4, and T3 were measured at the hospital laboratory by immune radiometric assay (Beckman Coulter Company, Brea, CA, USA), radioimmune assay (Beckman Coulter), and radioimmune assay (Cisbio International, Bangnois/Ceze, France), respectively. The reference range in adults was considered as 0.17–5.05 mU/L for TSH, 0.8–1.8 ng/dL for fT4, and 78–180 ng/dL for T3. If the TFT results were abnormal (beyond the reference range for either TSH or fT4), we repeated TFT or started levothyroxine (L-T4) treatment. We excluded infants treated with L-T4 as well as infants with abnormal TFT that persisted until 8 weeks of PNA. Infants who might have been exposed to iodine excluded. Blood sampling for TSH, fT4, and T3 levels was carried out in the morning and usually timed to coincide with sampling for other tests. The study protocol was reviewed and approved by the institutional review boards of Chungbuk National University Hospital. 2.2. Relationship of T3 with various parameters The TFT of infants with GA b 35 weeks conducted from 2 weeks of PNA to 16 weeks of PNA and/or PMA 44 weeks. The relationships between T3 levels and fT4 and TSH levels, GA, birth weight, and sex were analyzed. The PNA and PMA when TFT was conducted were recorded and their relationships with T3 levels were analyzed. To identify the conversion rate of fT4 to T3, we analyzed the T3/fT4 ratio. 2.3. Average T3 levels at 1 month of PNA In preterm infants, TFT was repeated at 1 month of PNA in many neonatal intensive care units (NICUs). Thus, we attempted to establish a reference range for TFT at 1 month of PNA. T3, fT4, and TSH values from TFT conducted at 25–35 (28.2 ± 3.2) days of PNA were selected as the average reference range at 1 month of PNA. Infants were grouped according to the GA, as 25–28, 29–31, and 32–34 weeks. The T3, fT4, and TSH levels for percentiles were calculated.
thyroid function were included and analyzed. The gestational age and birth weight of these infants were 30.55 ± 2.46 (25–34) weeks and 1468.5 ± 415.8 (580–2760) g, and 138 infants (51.9%) were male. 3.1. Relationship of T3 with various parameters In the 464 TFT results analyzed, T3 was observed to be correlated with GA (r = 0.436, P b 0.001), birth weight (r = 0.347, P b 0.001), PNA (r = 0.344, P b 0.001), calculated PMA (r = 0.602, P b 0.001), and fT4 levels (r = 0.460, P b 0.001) but not with TSH levels (r = 0.059, P b 0.206). We have representatively described the association between T3 and PMA in Fig. 1. Further, sex-related differences in T3 levels were observed; after adjustment for GA and birth weight, female infants had significantly higher T3 levels as compared to male infants (male vs. female; 75.2 ± 31.5 vs. 83.0 ± 26.4, β = 5.235, 95% CI = 0.254–10.217, P = 0.039). The T3/fT4 ratio was correlated with GA (r = 0.259, P b 0.001), birth weight (r = 0.242, P b 0.001), PNA (r = 0.380, P b 0.001), calculated PMA (r = 0.602, P b 0.001), and TSH levels (r = 0.095, P = 0.040) but not with fT4 levels ((r = − 0.075, P = 0.107) or gender (P = 0.065). 3.2. Average T3 levels at approximately 1 month of PNA Of the 266 infants included in the study, TFT was conducted at approximately 1 month of PNA in 248 infants. We observed that the T3, fT4, and TSH levels as well as the T3/fT4 ratio were significantly increased with higher GA. The reference ranges at 1 month of PNA for T3, fT4, and TSH levels at each GA group are shown in Table 1 and Fig. 2. 4. Discussion Normal thyroid function is important for neurodevelopment, metabolic activity, and pulmonary function in infants [14]. In particular, in preterm infants, many factors can lead to thyroid dysfunction, such as medical illness [15], medication [16–19], and immaturity of the hypothalamic–pituitary–thyroid axis. Recent studies have attempted to clarify the effects of thyroid hormone replacement therapy; however, the benefits of thyroid hormone replacement therapy in preterm infants remain unknown [20,21]. These studies evaluated all preterm infants, with and without thyroid dysfunction, rather than specifically those with thyroid dysfunction, because no reference range for TFT levels is
2.4. Statistical analysis Statistical analysis was performed using SPSS 18.0 (SPSS, Inc., Chicago, IL, U.S.A.) and R package (V3.0.2, R Foundation for Statistical Computing, Vienna, Austria) was used to calculate reference range by LMS method that using gamlss package. The relationship between hormone levels and age was analyzed using linear regression with Pearson's correlation coefficient. Simple linear regression models were used to calculate the linear effect (β), 95% confidence interval, and Pvalue. To establish a reference range, we calculated percentile values in each group with LMS method, which summarizes the changing distribution by three curves representing the median, coefficient of variation and skewness. The analysis of variance (ANOVA) test was used to compare subtypes within the GA (as 25–28, 29–31, and 32–34 weeks). P b 0.05 was considered statistically significant. 3. Results Overall, single or repeat TFT was performed in 331 infants during the study period. Of these, the results from 65 infants were excluded owing to L-T4 treatment or prolonged thyroid dysfunction. There was no infant excluded owing to suspicion with excessive iodine exposure. Eventually, the results from 464 TFTs conducted in 266 infants with normal
Fig. 1. Triiodothyronine levels were positively correlated with postmenstrual age.
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Table 1 Reference intervals for thyroid function tests at approximately 1 month of postnatal age in preterm infants.
T3 (ng/dL)
fT4 (ng/dL)
TSH (mU/L)
Gestational age
N
Mean ± SD
P valuea
5–95%
10–90%
25–75%
25–28 29–31 32–34 Total 25–28 29–31 32–34 Total 25–28 29–31 32–34 Total
57 98 93 248 57 98 93 248 57 98 93 248
51.05 69.61 88.87 72.56 1.05 1.20 1.30 1.20 5.01 5.98 6.84 6.08
a b c b0.001 a b c b0.001 a a, b b 0.019
19–83.7 33–107.9 51.3–138.8 32.7–123 0.69–1.44 0.81–1.56 0.93–1.68 0.81–1.59 1.5–10.3 1.7–12.6 2.0–15.7 1.9–13.1
26.1–76.8 39.8–98.3 57.9–125.7 41.8–105 0.77–1.36 0.89–1.48 1.01–1.60 0.9–1.5 2.0–8.7 2.2–10.5 2.5–12.7 2.3–11.3
38.4–65.1 52.1–83 70.5–106 53.2–89.2 0.91–1.22 1.03–1.34 1.15–1.46 1.05–1.39 3.0–6.5 3.4–7.6 3.8–8.8 3.4–8.0
± ± ± ± ± ± ± ± ± ± ± ±
18.75 24.54 25.81 27.83 0.23 0.24 0.23 0.25 2.76 3.89 4.33 3.89
a Statistical significances were tested by one way analysis of variance among groups. The same letters indicate a non-significant difference between groups based on Tukey's multiple comparison test.
available for preterm infants. Therefore, in this study, we aimed to define an appropriate reference range for T3 levels and the T3/fT4 ratio in preterm infants at 1 month of PNA. After the postnatal surge of thyroid hormone, the T3 value in preterm infants was observed to progressively increase with GA (r = 0.436), PNA (r = 0.344), and PMA (r = 0.602). T3 levels were significantly associated with PMA, which might be the sum of GA and PNA. T3 levels were higher in female infants than in male infants. A previous report demonstrated higher fT4 and TSH levels in female preterm infants as compared to male preterm infants [22,23]. It is considered that female infants have higher TSH and fT4, and they have a lower incidence of respiratory distress syndrome and other co-morbidities. We analyzed the conversion of T3 from fT4 using the T3/fT4 ratio. Thus far, no studies have examined the conversion of fT4 to T3 in preterm infants. We observed differences in the T3/fT4 ratio with GA, birth weight, PMA, and PNA, suggesting that deiodinase II function is abnormal in preterm infants. It is difficult to measure deiodinase activity directly in hepatic renal tissue, although the measurement of deiodinase activity is the best method of evaluating the conversion of fT4 to T3. There are only few reports concerning deiodinase activity in preterm animal studies. The T3/fT4 ratio has been reported to be low or normal in various thyroid diseases as well as after thyroid hormone replacement therapy [24]. In our study, the T3/fT4, in other words conversion of T3 from fT4, was also immature in preterm infants with lower GA and lower birth weight. Insufficient conversion of fT4 to T3 may
Fig. 2. Triiodothyronine levels at 1 month of postnatal age were significantly increased with higher gestational age.
contribute to relatively low T3 levels in preterm infants. Our result cannot be considered complete because we did not compare any of the parameters with those in healthy full term infants; further, the conversion ratio was not calculated between the same protein bound (T3/T4) or unbound (fT3/fT4) form. Further study needed to confirm deiodinase activity in preterm infants. At 1 month of age, T3 levels were noted to be higher for higher GA in this study, as reported previously [25–27]. Otherwise, other studies reported that T3 decreased after birth. It may be considered by inclusion of postnatal thyroid surge. In our study, we included TFT values only after PNA 2 weeks to eliminate the effects of the postnatal surge on TFT levels, with definite differences in the results. No study has evaluated TFT results at 1 month of PNA in preterm infants. In our study, T3 levels increased with increasing GA and PMA. This increase appears to be related to the physiological maturation of the thyroid gland and/or the hypothalamic–pituitary–thyroid axis rather than thyroid dysfunction, similar to the increase in cord blood T3 values over the gestation period [27]. We did not analyze the clinical complications of the preterm infants, because the incidence of clinical complications appeared to be similar for similar GA and PMA. Several reports have described the effects of medication and illness on thyroid function in preterm infants. Such illnesses and medication may be considered natural or unavoidable in preterm infants, with their effects reflecting the “normal” value of TFT in this group. Similarly, differing plasma protein levels and abnormal protein binding affects the methodology of fT4 measurement itself, which may be considered natural or unavoidable. We excluded many infants from this study owing to abnormal thyroid function on the basis of TFT results. These infants did not have typical congenital hypothyroidism, such as thyroid agenesis, ectopic thyroid, or dyshormogenesis. Mostly, they were cases of atypical hypothyroidism, such as transient hypothyroidism, hyperthyrotropinemia, hypothyroxinemia, and/or delayed TSH elevation. In our institution, we applied flexible criteria for L-T4 treatment, which was started earlier when infants were suspected to have thyroid dysfunction rather than waiting for longer follow-up. Further studies are needed for clarifying the reasons underlying the high incidence of thyroid dysfunction in preterm infants. We planned to exclude infants suspected to have previous iodine exposure; however, no infants were eventually excluded for this reason. There might have been some iodine exposure due to the disinfectant used for central line insertion and surgery (patent ductus ligation and surgical repair of bowel perforation) and by breast milk feeding, which could not be measured. This could be one of the reasons for the high incidence of thyroid dysfunction. In conclusion, our results suggest a potential reference range for normal T3 levels at approximately 1 month of age in preterm infants. The T3 and fT4 levels increased with GA and PMA. It is important to evaluate T3 levels in preterm infants to identify thyroid dysfunction since the conversion of T3 from fT4 is not accurately reflected, as
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shown by the near consistent T3/fT4 values. Further studies are required to confirm the utility of the reference range presented here in determining the need for thyroid hormone replacement therapy and its effects in preterm infants with low T3 or fT4 levels. Conflict of interest We have nothing to disclose. Acknowledgments We have nothing to declare. References [1] Meijer WJ, Verloove-Vanhorick SP, Brand R, van den Brande JL. Transient hypothyroxinaemia associated with developmental delay in very preterm infants. Arch Dis Child 1992;67:944–7. [2] Den Ouden AL, Kok JH, Verkerk PH, Brand R, Verloove-Vanhorick SP. The relation between neonatal thyroxine levels and neurodevelopmental outcome at age 5 and 9 years in a national cohort of very preterm and/or very low birth weight infants. Pediatr Res 1996;39:142–5. [3] Reuss ML, Paneth N, Pinto-Martin JA, Lorenz JM, Susser M. The relation of transient hypothyroxinemia in preterm infants to neurologic development at two years of age. N Engl J Med 1996;334:821–7. [4] Lucas A, Morley R, Fewtrell MS. Low triiodothyronine concentration in preterm infants and subsequent intelligence quotient (IQ) at 8 year follow up. BMJ 1996; 312:1132–3 [discussion 3–4]. [5] Toft AD. Thyroxine therapy. N Engl J Med 1994;331:174–80. [6] Portman MA, Slee A, Olson AK, Cohen G, Karl T, Tong E, et al. Triiodothyronine Supplementation in Infants and Children Undergoing Cardiopulmonary Bypass (TRICC): a multicenter placebo-controlled randomized trial: age analysis. Circulation 2010;122:S224–33. [7] Clark SJ, Deming DD, Emery JR, Adams LM, Carlton EI, Nelson JC. Reference ranges for thyroid function tests in premature infants beyond the first week of life. J Perinatol 2001;21:531–6. [8] Carrascosa A, Ruiz-Cuevas P, Potau N, Almar J, Salcedo S, Clemente M, et al. Thyroid function in seventy-five healthy preterm infants thirty to thirty-five weeks of gestational age: a prospective and longitudinal study during the first year of life. Thyroid 2004;14:435–42. [9] Kok JH, Hart G, Endert E, Koppe JG, de Vijlder JJ. Normal ranges of T4 screening values in low birthweight infants. Arch Dis Child 1983;58:190–4. [10] Dilli D, Oğuz SS, Andiran N, Dilmen U, Büyükkağnici U. Serum thyroid hormone levels in preterm infants born before 33 weeks of gestation and association of transient hypothyroxinemia with postnatal characteristics. J Pediatr Endocrinol Metab 2010;23:899–912.
[11] Adams LM, Emery JR, Clark SJ, Carlton EI, Nelson JC. Reference ranges for newer thyroid function tests in premature infants. J Pediatr 1995;126:122–7. [12] Reuss ML, Leviton A, Paneth N, Susser M. Thyroxine values from newborn screening of 919 infants born before 29 weeks' gestation. Am J Public Health 1997;87:1693–7. [13] Williams FL, Mires GJ, Barnett C, Ogston SA, van Toor H, Visser TJ, et al. Transient hypothyroxinemia in preterm infants: the role of cord sera thyroid hormone levels adjusted for prenatal and intrapartum factors. J Clin Endocrinol Metab 2005;90: 4599–606. [14] Simic N, Asztalos EV, Rovet J. Impact of neonatal thyroid hormone insufficiency and medical morbidity on infant neurodevelopment and attention following preterm birth. Thyroid 2009;19:395–401. [15] Paul DA, Mackley A, Yencha EM. Thyroid function in term and late preterm infants with respiratory distress in relation to severity of illness. Thyroid 2010;20:189–94. [16] Arai H, Goto R, Matsuda T, Takahashi T. Relationship between free T4 levels and postnatal steroid therapy in preterm infants. Pediatr Int 2009;51:800–3. [17] Carrascosa A, Ruiz-Cuevas P, Clemente M, Salcedo S, Almar J. Thyroid function in 76 sick preterm infants 30–36 weeks: results from a longitudinal study. J Pediatr Endocrinol Metab 2008;21:237–43. [18] Williams FL, Ogston SA, van Toor H, Visser TJ, Hume R. Serum thyroid hormones in preterm infants: associations with postnatal illnesses and drug usage. J Clin Endocrinol Metab 2005;90:5954–63. [19] Filippi L, Pezzati M, Cecchi A, Poggi C. Dopamine infusion: a possible cause of undiagnosed congenital hypothyroidism in preterm infants. Pediatr Crit Care Med 2006;7:249–51. [20] Ng SM, Turner MA, Gamble C, Didi M, Victor S, Manning D, et al. An explanatory randomised placebo controlled trial of levothyroxine supplementation for babies born b28 weeks' gestation: results of the TIPIT trial. Trials 2013;14:211. [21] La Gamma EF, Paneth N. Clinical importance of hypothyroxinemia in the preterm infant and a discussion of treatment concerns. Curr Opin Pediatr 2012;24:172–80. [22] Sun X, Lemyre B, Nan X, Harrold J, Perkins SL, Lawrence SE, et al. Free thyroxine and thyroid-stimulating hormone reference intervals in very low birth weight infants at 3–6 weeks of life with the Beckman Coulter Unicel DxI 800. Clin Biochem 2014;47: 16–8. [23] Korada M, Pearce MS, Avis E, Turner S, Cheetham T. TSH levels in relation to gestation, birth weight and sex. Horm Res 2009;72:120–3. [24] Mortoglou A, Candiloros H. The serum triiodothyronine to thyroxine (T3/T4) ratio in various thyroid disorders and after Levothyroxine replacement therapy. Hormones (Athens) 2006;3:120–6. [25] Cartault Grandmottet A, Cristini C, Tricoire J, Rolland M, Tauber MT, Salles JP. TSH, FT4 and T3T evaluation in normal and preterm hospitalized newborns. Arch Pediatr 2007;14:138–43. [26] Williams FL, Simpson J, Delahunty C, Ogston SA, Bongers-Schokking JJ, Murphy N, et al. Developmental trends in cord and postpartum serum thyroid hormones in preterm infants. J Clin Endocrinol Metab 2004;89:5314–20. [27] Hume R, Simpson J, Delahunty C, van Toor H, Wu SY, Williams FL, et al. Human fetal and cord serum thyroid hormones: developmental trends and interrelationships. J Clin Endocrinol Metab 2004;89:4097–103.
