Cord Serum Thyroid-stimulating Hormone Research Articles

Mediating effect of endocrine hormones on association between per- and polyfluoroalkyl substances exposure and birth size: Findings from sheyang mini birth cohort study

BackgroundPrenatal exposure to per- and polyfluoroalkyl substances (PFAS) has been reported to affect fetus growth, but current results were inconsistent and their mechanism remained unclear. ObjectivesWe aimed to evaluate the associations of prenatal exposure to single and/or multiple PFAS with birth size and to elucidate whether thyroid hormones and reproductive hormones mediate these associations. MethodsA total of 1087 mother-newborn pairs from Sheyang Mini Birth Cohort Study were included in the present cross-sectional analysis. 12 PFAS, 5 thyroid hormones and 2 reproductive hormones were measured in cord serum. Multiple linear regression models and Bayesian kernel machine regression (BKMR) models were used to examine the associations of PFAS with either birth size or endocrine hormones. One-at-a-time pairwise mediating effect analysis was applied to estimate the mediating effect of single hormone in the association between individual chemical and birth size. High-dimensional mediation approach including elastic net regularization and Bayesian shrinkage estimation were further performed to reduce exposure dimension and figure out the global mediation effects of joint endocrine hormones. ResultsPerfluorononanoic acid (PFNA) exposure was positively associated to weight for length z score [WLZ, per log10-unit: regression coefficient (β) = 0.26, 95% confidence intervals (CI): 0.04, 0.47] and ponderal index (PI, β = 0.56, 95% CI: 0.09, 1.02), and PFAS mixture results fit by BKMR model showed consistent consequences. High-dimensional mediating analyses revealed that thyroid stimulating hormone (TSH) explained 6.7% of the positive association between PFAS mixtures exposure and PI [Total effect (TE) = 1.499 (0.565, 2.405); Indirect effect (IE) = 0.105 (0.015, 0.231)]. Besides, 7.3% of the PI variance was indirectly explained by 7 endocrine hormones jointly [TE = 0.810 (0.802, 0.819); IE = 0.040 (0.038, 0.041)]. ConclusionsPrenatal PFAS mixtures exposure, especially PFNA, was positively associated to birth size. Such associations were partly mediated by cord serum TSH.

Association of Cord Blood Thyroid-Stimulating Hormone Levels with Maternal, Delivery and Infant Factors

This study examined maternal, delivery and infant factors associated with cord thyroid-stimulating hormone (TSH) concentrations in an Asian population. The Growing Up in Singapore Towards healthy Outcomes (GUSTO) study is a mother-offspring birth cohort from 2 major hospitals in Singapore. Cord serum TSH was measured using the Abbott ARCHITECT TSH Chemiluminescent Microparticle Immunoassay and the ADVIA Centaur TSH-3 Immunoassay. After excluding infants with a maternal history of thyroid disease, screening cord TSH results from 604 infants were available for multivariable regression analysis in relation to the factors of interest. Babies born by vaginal delivery had significantly higher cord serum TSH concentrations than babies born by caesarean section. Cord serum TSH concentrations differed significantly by measurement method. There was no association of cord TSH concentrations with ethnicity, sex, birth weight, gestational age, maternal body mass index, gestational weight gain, gestational diabetes mellitus status and other maternal, delivery and infant factors studied. Interpretation of cord serum TSH results may need to take into account mode of delivery and measurement method.

Maternal serum perfluoroalkyl substance mixtures and thyroid hormone concentrations in maternal and cord sera: The HOME Study

BackgroundPer- and polyfluoroalkyl substances (PFAS) are ubiquitous. Previous studies have found associations between PFAS and thyroid hormones in maternal and cord sera, but the results are inconsistent. To further address this research question, we used mixture modeling to assess the associations with individual PFAS, interactions among PFAS chemicals, and the overall mixture. MethodsWe collected data through the Health Outcomes and Measures of the Environment (HOME) Study, a prospective cohort study that between 2003 and 2006 enrolled 468 pregnant women and their children in the greater Cincinnati, Ohio region. We assessed the associations of maternal serum PFAS concentrations measured during pregnancy with maternal (n = 185) and cord (n = 256) sera thyroid stimulating hormone (TSH), total thyroxine (TT4), total triiodothyronine (TT3), free thyroxine (FT4), and free triiodothyronine (FT3) using two mixture modeling approaches (Bayesian kernel machine regression (BKMR) and quantile g-computation) and multivariable linear regression. Additional models considered thyroid autoantibodies, other non-PFAS chemicals, and iodine deficiency as potential confounders or effect measure modifiers. ResultsPFAS, considered individually or as mixtures, were generally not associated with any thyroid hormones. A doubling of perfluorooctanesulfonic acid (PFOS) had a positive association with cord serum TSH in BKMR models but the 95% Credible Interval included the null (β = 0.09; 95% CrI: −0.08, 0.27). Using BKMR and multivariable models, we found that among children born to mothers with higher thyroid peroxidase antibody (TPOAb), perfluorooctanoic acid (PFOA), PFOS, and perfluorohexanesulfonic acid (PFHxS) were associated with decreased cord FT4 suggesting modification by maternal TPOAb status. ConclusionsThese findings suggest that maternal serum PFAS concentrations measured in the second trimester of pregnancy are not strongly associated with thyroid hormones in maternal and cord sera. Further analyses using robust mixture models in other cohorts are required to corroborate these findings.

Maternal urinary bisphenol A concentration and thyroid hormone levels of Chinese mothers and newborns by maternal body mass index.

Animal studies indicated that bisphenol A (BPA) exposure during pregnancy may disrupt thyroid function which is critical for fetal development. However, few epidemiological studies have examined this topic and the results were inconsistent. We aimed to evaluate whether prenatal BPA exposure is associated with thyroid hormone levels in Chinese mothers and newborns with stratification by maternal body mass index (BMI). BPA concentration were measured in urine samples collected from 555 women at late pregnancy. Maternal serum free thyroxin (FT4), thyroid-stimulating hormone (TSH) and thyroid peroxidase antibody (TPO-Ab) concentrations at the third trimester were abstracted from medical records. Cord serum-free triiodothyronine (FT3), FT4, TSH, and TPO-Ab levels were measured in 398 newborns. Prenatal urinary BPA was detected in 98.5% of mothers with a geometric mean of 1.32ng/mL (95% CI 1.17-1.49ng/mL). With each 10-fold increase in BPA concentrations, maternal log10_(TSH) mIU/L was 0.10 lowered (95% CI - 0.20, - 0.005, p < 0.05) among pre-pregnancy BMI>23kg/m2, with adjustment for maternal age, maternal education, gestation diabetes mellitus (GDM), husband smoking during pregnancy, parity, and gestational age at thyroid parameters measured, but no association was observed in pre-pregnancy BMI < 18.5, or 18.5-22.9kg/m2 stratum. No BPA-associated changes were observed in maternal FT4 level or odds of positive TPO-Ab in all BMI stratum. Also, no associations were observed between prenatal urinary BPA concentration and cord serum FT4, FT3, TSH levels, and odds of positive TPO-Ab in both male and female newborns among pre-pregnancy BMI < 18.5, 18.5-22.9 or>23kg/m2 stratum. In this study, prenatal urinary BPA concentration was associated with lower maternal TSH among women with overweight, but not associated with other maternal thyroid parameters or cord serum thyroid parameters across maternal BMI categories. More research on pregnant women and newborns cohort with BPA exposure are warranted.

Advanced Maternal Age, Mode of Delivery, and Thyroid Hormone Levels in Chinese Newborns.

Objective: Thyroid hormones are essential for fetal growth and neurodevelopment, however, data on cord blood thyroid hormones are sparse in China where maternal age at childbearing is increasing in recent decades. We aimed to assess cord blood levels of free triiodothyronine (FT3), free thyroxine (FT4), and thyroid stimulating hormone (TSH) in full-term Chinese newborns, and examine potential related perinatal factors.Methods: This study included 922 mother-newborn pairs from a prospective birth cohort enrolled in 2012–2013, Shanghai, China. Cord serum concentrations of FT3, FT4, TSH, and TPOAb were measured in newborns.Results: Newborns born via cesarean section had higher cord serum FT3 (mean ± SD: 1.90 ± 1.16 pmol/L) and lower cord serum TSH (5.15 ± 2.60 mIU/L) than those born via vaginal delivery (FT3: 1.62 ± 0.93 pmol/L; TSH: 9.27 ± 6.76 mIU/L). In cesarean section deliveries, the concentration of cord serum FT3 was 0.15 (95%CI: −0.03, 0.33; p = 0.10) pmol/L lower in infants of mothers aged 30–34 years, and 0.57 (95%CI: 0.22, 0.92; p = 0.002) pmol/L lower in infants of mothers ≥35 years compared to infants of mothers <30 years. Large-for-gestational-age (birth weight >90th percentile) was associated with higher TSH (p = 0.02). Similar results were also found in vaginal deliveries.Conclusions: In this Chinese term birth cohort, newborns born via cesarean section had higher cord serum FT3 and lower TSH than those born via vaginal delivery. Advanced maternal age was associated with lower fetal FT3. Further research is needed to understand whether this association may mediate the adverse impact of advanced maternal age on neurodevelopment in early life.

Associations of Exposure to Perfluoroalkyl Substances With Thyroid Hormone Concentrations and Birth Size.

Adequate thyroid function during pregnancy is essential for optimal fetal growth. Gestational exposure to perfluoroalkyl substances (PFAS) can negatively affect birth size and disrupt maternal and neonatal thyroid function, although the interrelationship is unclear. We aimed to quantify the associations between maternal serum-PFAS concentrations and birth weight, birth length, and cranial circumference. We also aimed to estimate associations between PFAS and thyroid hormone (TH) concentrations, thereby elucidating whether THs potentially mediate the associations between PFAS concentrations and birth size. We studied a population-based prospective cohort of 172 mother-singleton pairs from the Faroe Islands. Twelve PFAS were measured in maternal serum obtained at 34 weeks of gestation. THs were measured in maternal and cord serum. Associations between PFAS concentrations and birth size and TH concentrations were estimated using multivariable linear regressions. Sex-stratified analyses along with a mediation analysis were performed to estimate potential mediating effects of THs in the association between PFAS and birth outcomes. Several PFASs were negatively associated with birth weight, length, and head circumference, and a general positive association between maternal serum-PFASs and cord serum-thyroid-stimulating hormone (TSH; also known as thyrotropin) was found. For instance, a doubling in perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) was associated with a 53% (95% CI, 18%-99%) and 40% (95% CI, 8%-81%) increases in TSH concentrations, respectively. There was little evidence of sexually dimorphic associations. Overall, THs were not found to mediate associations between PFASs and birth size. In this study, several PFASs were negatively associated with birth size and increased THs; however, this did not explain lower birth weight among children exposed to PFAS.

Maternal urinary phthalate metabolites during pregnancy and thyroid hormone concentrations in maternal and cord sera: The HOME Study.

Phthalates, endocrine-disrupting chemicals that are commonly found in consumer products, may adversely affect thyroid hormones, but findings from prior epidemiologic studies are inconsistent. In a prospective cohort study, we investigated whether maternal urinary phthalate metabolite concentrations and phthalate mixtures measured during pregnancy were associated with thyroid hormones among pregnant women and newborns. We measured nine phthalate metabolites [monoethyl phthalate (MEP), mono-n-butyl phthalate, mono-isobutyl phthalate, monobenzyl phthalate (MBzP), and four monoesthers of di(2-ethylhexyl) phthalate] in urine collected at approximately 16 and 26 weeks' gestation among women in the Health Outcomes and Measures of the Environment Study (2003-2006, Cincinnati, Ohio). Thyroid stimulating hormone (TSH) and free and total thyroxine and triiodothyronine were measured in maternal serum at 16 weeks' gestation (n = 202) and cord serum at delivery (n = 276). We used multivariable linear regression to assess associations between individual urinary phthalate metabolites and concentrations of maternal or cord serum thyroid hormones. We used weighted quantile sum regression (WQS) to create a phthalate index describing combined concentrations of phthalate metabolites and to investigate associations of the phthalate index with individual thyroid hormones. With each 10-fold increase in 16-week maternal urinary MEP, maternal serum total thyroxine (TT4) decreased by 0.52 μg/dL (95% CI: -1.01, -0.03). For each 10-fold increase in average (16- and 26-week) maternal urinary MBzP, cord serum TSH decreased by 19% (95% CI: -33.1, -1.9). Among mothers, the phthalate index was inversely associated with maternal serum TT4 (WQS beta = -0.60; 95% CI: -1.01, -0.18). Among newborns, the phthalate index was inversely associated with both cord serum TSH (WQS beta = -0.11; 95% CI: -0.20, -0.03) and TT4 (WQS beta = -0.53; 95% CI: -0.90, -0.16). Our results suggest that co-exposure to multiple phthalates was inversely associated with certain thyroid hormones (TT4 in pregnant women and newborns, and TSH in newborns) in this birth cohort. These findings highlight the need to study chemical mixtures in environmental epidemiology.

Effects of maternal iodine supplementation during pregnancy and lactation on iodine status and neonatal thyroid-stimulating hormone.

To determine the iodine status in pregnant and lactating women, as well as neonatal thyroid-stimulating hormone (TSH) concentration. Pregnant women cared at our hospital, the University Hospital in Bangkok, had routinely received 200 μg iodine tablet daily since October 2010. Urinary iodine concentrations (UICs) of 1508 pregnant and 87 lactating women and 76 offspring and breast milk iodine concentration (BMIC) (n=57) were measured. Cord serum TSH levels from hypothyroidism screening were analyzed. Median UIC levels of pregnant and lactating women were 170.6 and 138.0 μg l(-1), respectively. Median BMIC and infants' UIC at 2-month postpartum in iodine-supplemented group were higher than the respective values of non-supplemented group. Median cord serum TSH level obtained before iodine supplementation (n=8332) was higher than that obtained after supplementation (n=5181; 7.3 vs 5.2 mU l(-1)). Maternal iodine supplementation improved iodine nutrition in their breast-fed offspring. A trend toward declining in cord serum TSH values after iodine supplementation indicates improvement of iodine status during pregnancy.

Cord blood thyroid‐stimulating hormone and free T4 levels in Turkish neonates: Is iodine deficiency still a continuing problem?

The objectives of this study were to determine the cord blood thyroid-stimulating hormone (TSH) and free T(4) (FT(4) ) levels in Turkish neonates and to determine whether these variables reveal iodine deficiency. We collected 818 cords from healthy mothers at parturition and measured levels of FT(4) and TSH. We also measured cord blood FT(4) and TSH levels in different stages of gestation and gender. We grouped the neonates according to cord serum TSH levels, either being less (Group A) or greater (Group B) than 10 mIU/L. Group A included 589 neonates (300 girls [51%] and 289 boys [49%]) and Group B included 229 neonates (105 girls [45%] and 124 boys [55%]). The percentage of subjects with cord blood TSH < 10 mIU/L and >10 mIU/L was 72% and 28%, respectively. Although cord TSH levels in Group B were greater than those in Group A (P < 0.001), cord blood FT(4) levels in Group B were lower than those in Group A (P < 0.05). There was no difference between both sex in terms of birthweight and maternal age. TSH and FT(4) levels did not vary according to neonate sex during gestation, except for from week 37 to 41. TSH levels of male neonates at the 41st week of gestation were higher than those of female neonates (P < 0.05). There were no effects of birthweight on TSH and FT(4) levels if the neonate was lighter than 2500 g at birth. TSH levels of male neonates were higher than those of female neonates when their birthweights were <2500 g (P < 0.05). There was no significant difference in TSH levels according to birthweights in male neonates. Our data provide the normative data for cord blood TSH and FT(4) levels in Turkish neonates and show that iodine deficiency is a still a public health problem in Turkey. These measurements can be useful for detection and verification of hypothyroidism in a screening program for congenital hypothyroidism as well as evaluation of the success of the iodination program.

Congenital hypothyroidism: increased incidence in Najran province, Saudi Arabia.

Neonatal screening for congenital hypothyroidism using cord serum thyroid-stimulating hormone (TSH) was initiated in Najran health region in September 1990. A total of 30810 newborn infants were screened by April 1995. Of the 24 infants with abnormal thyroid function tests on recall, 22 had permanent primary congenital hypothyroidism (incidence; 1:1400) and in two male siblings transient congenital hypothyroidism (incidence; 1:15400) was proved on follow-up. There was a significantly higher incidence of dyshormonogenesis. Eight (57 per cent) of the 14 infants who were adequately studied thyroid scan revealed ectopic glands with increased 99mTc uptake, while thyroid ectopy and aplasia were present only in three (22 per cent) infants each. Furthermore, goiter was evident clinically in two other patients.

Neonatal screening for congenital hypothyroidism in Saudi Arabia: results of screening the first 1 million newborns

Introduction: Since the initiation of pilot screening programs to detect congenital hypothyroidism (CH) in 1972, newborn screening has become routine in the developed world. A national screening program for CH was established in Saudi Arabia in November, 1989. Methods: The program utilizes cord serum thyroid stimulating hormone (TSH), tested by the Delfia method, supplemented when necessary with thyroxine (T4) assay, also by the Delfia method. TSH values above 60 mU/l alone were considered suggestive of CH and initiated recall of the infant. TSH values of 30–60 mU/l initiated T4 measurement. If the latter was below 80 nmol/l, the infant was also recalled. Results: Between November 1989 and April 1995, a total of 1007 350 infants were screened. The mean recall rate was 0.18% (range 0.05–0.3%). Of those, 306 infants were confirmed to have CH, indicating an incidence for CH in Saudi Arabia of 1:3292. However, a regional variation in incidence was noted. Of all the infants with congenital hypothyroidism who were adequately studied, 47.5% were found to have ectopic thyroid glands, 31.7% eutopic glands with increased 99mTc uptake and 20.8% athyreotic. The mean age at the time of recall was 19.4 days (range 2–130). The average cost of screening was US S3.20 per specimen. Discussion: Several of the organizational and administrative difficulties which were encountered during the operation were discussed and solved at regional levels and during the annual general meetings.

Neonatal Screening for Congenital Hypothyroidism in Riyadh: Analysis of Six Years' Experience

The regional screening program for congenital hypothyroidism (CH) in Riyadh Province was started as a pilot study in December 1988. By September 1990, the program covered all deliveries at the Ministry of Health (MOH) and King Saud University hospitals. The program utilizes cord serum thyroid-stimulating hormone (TSH) with a low recall rate of 0.1%. The average cost per specimen was SR 12 (US$ 3.20). Among 283,647 infants screened, 83 infants were confirmed to have CH (incidence 1:3417). In 17 infants, however, the diagnosis was not confirmed due to difficulties in recall. Eleven infants with cord serum TSH of more than 100 mU/L proved to be clinically and biochemically euthyroid at recall. Three of these were secondary to maternal proplthiouracil (PTU) therapy. The female to male ratio was 1.5:1. The majority of infants lacked clinical symptoms and signs of hypothyroidism. Thyroid scans showed the most common etiology to be thyroid ectopy (50%), followed by dyshormonogenesis (26%) and athyrosis (24%). Although there was no significant difference in the mean cord T4 values among the different groups, the mean T4 value at recall in the athyrotic group was significantly (P <0.001) lower than the cord results (14.8 versus 62.7) and that is of recall for the ectopic and dyshormonogenesis groups (14.8 versus 47.0 and 51.3 respectively0. There was bo significant difference in the mean TSH among the different groups in the cord and recall samples. Skeletal maturation was more delayed in the athyrotic group. The mean age at the time of recall was 16.4 days (range 4 to 64), and the mean age at the start of therapy was 17.8 days (range 5-64).

Congenital Hypothyroidism: Clinical and Laboratory Characteristics in Infants Detected by Neonatal Screening

A pilot cord serum thyroid-stimulating hormone (TSH) screening program for congenital hypothyroidism began in December 1988 at the Ministry of Health Maternity Hospitals in Riyadh. Out of 40,000 infants screened, 15 infants with congenital hypothyroidism have been detected. Of these, five had athyreosis, eight with ectopic thyroid glands, interestingly five of which had positive perchlorate discharge test suggestive of organification defects, and two had normally located glands with perchlorate discharge test suggestive of organification defects. The female to male ratio was 1.5:1. The clinical characteristics in our cases were not different from that previously reported. Although there was no significant difference in the mean cord T4 values among the different groups, the mean T4 values at recall in the aplastic group is significantly (P<0.001) lower than the cord results, (60.3 versus 15.3) and that is of recall for the ectopic group (15.3 versus 47.0). There was no significant difference in the mean TSH among the different groups in the cord and recall samples. The skeletal maturation was more delayed in the athyrotic group. The mean age at the time of recall was 17.8 days (range 6 to 64) and the mean age at the start of therapy was 20 days (range 8 to 64).

Influence of Mode of Delivery on Fetal Pituitary‐Thyroid Axis

We measured the cord serum levels of thyroid-stimulating hormone (TSH), thyroxine (T4) and triiodothyronine (T3) in 922 neonates delivered by mothers who had no thyroid disorders. The T4 and T3 levels increased progressively from the 27th to the 40th week of gestation. However, the TSH levels varied widely and had no correlation with gestational age, because they were affected by the mode of delivery. The mean cord serum TSH level in neonates delivered by vacuum extraction was 16.3 +/- 10.0 microU/ml (n = 30), which was significantly higher than the level following normal vaginal delivery (9.5 +/- 6.0 microU/ml, n = 622) (p less than 0.005). The mean cord serum TSH level following elective caesarean section was 6.5 +/- 3.1 microU/ml (n = 79), and this was significantly lower than after normal vaginal delivery (p less than 0.005). TSH levels in high-risk neonates were significantly higher than in neonates without risk factors. A significant positive correlation was found between the duration of the second stage of labor and the cord serum TSH level (r = 0.45, n = 412, p less than 0.01). However, there was no correlation between the cord serum TSH level and the congenital hypothyroidism screening TSH level (r = 0.01, n = 468). We conclude that the cord serum TSH level reflects delivery stress and that an elevated level does not influence the congenital hypothyroidism screening TSH test in which blood is obtained at five days of life.

Influence of Perinatal Factors and Sampling Methods on TSH and Thyroid Hormone Levels in Cord Blood.

To evaluate the effect of perinatal factors and sampling methods on thyroid stimulating hormone (TSH) and thyroid hormone levels in cord blood, serum TSH, free thyroxine (FT4) and free triiodothyronine (FT3) concentrations were measured in 124 healthy term neonates. Eighty-eight infants were born in normal vaginal deliveries, 25 were delivered by vacuum extractor and 11 by Cesarean section. There was no significant difference among the three infant groups in the mean TSH levels. Birth weight, the infant's sex, duration of labor and uterotonic agents had no effect on cord serum TSH and free thyroid hormone levels in the neonates born by normal vaginal delivery. To assess the adequacy of specimen collection, mixed cord blood samples, obtained by a direct application of cord on a filter paper, and venous blood withdrawn with a plastic syringe were collected in another 200 infants. There was a significant linear correlation in the TSH concentration in mixed cord blood and cord venous serum from the same individuals, while a poor correlation was found in T4 values from two specimens. Our results suggest that the TSH value in cord blood is less influenced by perinatal factors, including the sampling method, and the mixed cord blood collected by this technique might be a feasible alternative specimen for a TSH screening program with cord blood which is useful in countries where neonatal blood is not available.

Prediction of later development of thyrotoxicosis or central hypothyroidism from the cord serum thyroid-stimulating hormone level in neonates born to mothers with Graves disease

L'etude systematique de la TSH dans le sang du cordon par un dosage radio-immunologique tres sensible est precieuse pour predire une hyperthyroidie ou une hypothyroidie centrale chez les nouveau-nes de meres atteintes de maladie de Basedow

Cord serum thyroid-stimulating hormone and thyroglobulin levels decline with increasing birth weight in newborns.

Cord serum thyroglobulin (Tg) and TSH levels were related to birth weight in 3 groups of newborn infants composed of 101 infants. Serum free T3 index, free T4 index, and/or T4 also were determined. Group I consisted of normal term newborns (20 females and 19 males), whose mean +/- SD gestational ages (40.1 +/- 0.7 vs. 40.1 +/- 0.5 weeks) did not differ, but whose mean birth weights (3299 +/- 282 vs. 3757 +/- 447 g) differed significantly (P less than 0.005). In female infants, serum Tg levels (r = -0.401; P less than 0.05) and the log of TSH levels (r = -0.576; P less than 0.005) correlated negatively with birth weight, while Tg levels correlated positively with the log of TSH levels (r = 0.401; P less than 0.05). In contrast, none of these correlations was significant for male infants. However, T4 levels and birth weight correlated positively (r = 0.499; P less than 0.025) in male infants, but not in female infants. Group II consisted of newborns whose birth weights were less than 2500 g (19 females and 19 males). Mean birth weights of female (2032 +/- 301 g) and male (1850 +/- 413 g) infants did not differ significantly (P greater than 0.05). Both the Tg levels and the log of the TSH levels correlated negatively with birth weight in female (Tg, r = -0.891 and P less than 0.005; log TSH, r = 0.600 and P less than 0.005) and male (Tg, r = -0.849 and P less than 0.005; log TSH, r = -0.660; P less than 0.005) infants. Also, Tg levels correlated positively with the log of the TSH levels in female (r = 0.554; P less than 0.01) and male (r = 0.412; P less than 0.05) infants. Free T4 index levels correlated positively with free T3 index levels in female (r = 0.443; P less than 0.05) and male (r = 0.570; P less than 0.01) infants. Group III consisted of 12 normal female term newborns whose mean birth weight (3685 +/- 623 g) was not significantly (P less than 0.2) different from that of the males of group I, and 12 normal male term newborns whose mean birth weight (4104 +/- 248 g) was significantly (P less than 0.005) greater than that of the males of group I. Unlike in lower weight female or male infants, serum Tg levels did not correlate with birth weight or the log of TSH levels.(ABSTRACT TRUNCATED AT 400 WORDS)