Thomas H. Oddie

Thyroid function at birth in infants of 30 to 45 weeks' gestation

Serum T4 was measured in cord blood of 2,683 infants of gestational ages 30 to 45 weeks and birth weights of 1,460 to 5,250 gm. In addition, serum TBG was measured in 180, and serum TSH in 125 of these infants. Mean serum T4 increased with gestational age from 9.4 microng/dl at 30 weeks to 11.7 at 45 weeks. TBG varied from 3.1 to 11.4 mg/dl but showed no correlation with gestational age. There was a positive correlation between T4 and TBG. Serum TSH levels showed a significant negative correlation with gestational age, decreasing from 15 micronU/ml at 30 weeks to 7 micronU/ml at 45 weeks, while the serum T4/TSH ratio increased from 0.6 to 1.6. The results indicate that cord, T4 varies mainly with TBG, and to a lesser extent with gestational age.

Catecholamine physiology in the ovine fetus. I. Gestational age variation in basal plasma concentrations.

Fifty-five ewes with chronically catheterized singleton gestations were studied to assess changes in basal concentrations of fetal catecholamines with increasing gestational age. All pregnancies were time dated, and measurements of catecholamines were conducted at least 5 days after placement of fetal catheters when fetal metabolic parameters had normalized. Plasma concentrations of catecholamines were measured by radioenzymatic assay. Additionally, fetal heart rate (FHR) and corrected mean blood pressure were analyzed in 32 of the fetuses for correlation with plasma levels of catecholamines. Multiple regression analysis revealed significant inverse correlations of fetal plasma concentrations of catecholamines with gestational age, as follows: norepinephrine (p less than 0.001), epinephrine (p less than 0.05), and dopamine (p less than 0.01). FHR correlated inversely with gestational age (p less than 0.001) and positively with circulating levels of norepinephrine (p less than 0.001).

305 AMNIOTIC FLUID TRIIODOTHYRONINE AND CORTISOL CORRELATIONS FROM 10 TO 43 WEEKS GESTATION

Thyroxine (T4), triiodothyronine (T3) and reverse T3 (rT3), concentrations by RIA and cortisol (C) by radiotransinassay were measured in amniotic fluid (AF) samples from human pregnancies between 10 and 43 weeks gestation. T4 increased with gestational age (GA) between 10 and 30 weeks (r = +0.54, p<0.01) reaching a mean of 1.2 μg/dl (95% confidence limits [CL] 0.81 to 1.8 μg/dl) between 25 and 30 weeks. T4 decreased with GA between 20 and 43 weeks (r = -0.34, p<0.01) to a mean term level of 0.61 μg/dl (CL 0.33 to 1.13 μg/dl). rT3 increased with GA between 10 and 20 weeks (r = +0.66, p<0.001) to a mean level of 474 ng/dl. After 20 weeks, rT3 decreased with GA (r = -0.77, p<0.001) to a mean concentration of 62 ng/dl at term. Mean T3 increased from 5.4 ng/dl at 16-20 weeks to 12.1 ng/dl at 39-42 weeks. Mean C in AF increased between 30 and 43 weeks (13.5 ng/ml to 27.8 ng/ml). AF-C correlated directly with T3 (r = +0.51, p<0.01) and indirectly with rT3 (r = -0.43, p<0.01) between 10 and 43 weeks. Although the source of the AF iodothyronines is unknown, the high rT3 and low T3 concentrations reflect fetal serum levels more than maternal. The significant correlation between T3 and C are consistant with recent data indicating cortisol dependent increases in serum T3 concentrations and in vitro tissue conversion of T4 to T3 prior to the onset of labor in fetal sheep.

188 THE PRENATAL TRIIODOTHYRONINE (T3) SURGE IN THE FETAL SHEEP

Thomas, et al, reported an increase in fetal T3 concentrations during cortisol induced premature labor in sheep. The present study characterizes changes in Thyroxine (T4), T3, and reverse T3 (rT3) concentrations during the 2 week period prior to spontaneous vaginal delivery (PTD) and during the immediate neonatal period in the sheep. Serial blood samples were collected from 5 chronically catheterized fetal sheep during the 2 weeks PTD. All 5 fetuses were live born. Serum was analyzed for T4, T3, and rT3 by radioimmunoassay. Serum T3 began to increase 4-6 days PTD from a stable geometric mean of 30 ng/dl (upper 95% confidence limit = 39 ng/dl) to a geometric mean of 131 ng/dl (lower 95% confidence limit = 100 ng/dl) on the day of delivery (Day 0). Serum T4 decreased in 4 of the animals and remained unchanged in 1 during this time interval. Serum rT3 decreased from a stable mean of 472 ng/dl to 249 ng/dl on Day 0. The fetal serum T3/T4 ratio increased markedly during the week PTD. Serum T3 levels increased further after delivery; a mean level of 487 ng/dl was measured in 3 lambs at 2-4 hours of age. Conclusions: a) fetal serum T3 levels gradually increase while T4 and rT3 concentrations decrease during the 4-6 days PTD in sheep; b) the increase in serum T3 and fall in T4 and rT3 before birth probably are due to augmented T4 to T3 conversion; c) the perinatal T3 surge has 2 phases: a slow prenatal phase and an abrupt postnatal phase. The prenatal phase may be cortisol induced.

306 ONTOGENESIS OF IODOTHYRONINE PRODUCTION AND CLEARANCE IN SHEEP

Production rates (PR, μg/M2/d) and metabolic clearance rates (MCR, L/M2/d) for thyroxine (T4), triiodothyronine (T3), and reverse T3 (rT3) were measured in 4 newborn sheep 7 to 14 days of age using single injection non-compartmental methods. Results were related to data in fetal and adult sheep. Thyroid secretion of T3 and rT3 were calculated from T3/T4 and rT3/T4 ratios measured in adult and fetal thyroid glands:Newborn T4 MCR>fetal (p<.05) and adult (p<.01); newborn T4 PR> fetal (p<.05) and adult (p<.01). Fetal T3 MCR>newborn (p<.01) = adult; newborn T3 PR>fetal (p<.01)>adult (p<.001). Fetal rT3 MCR = newborn<adult (p<.05); fetal rT3 PR>newborn (p<.01) = adult. The T3 secreted from the thyroid is minimal in fetus, approximates 54% in newborn, and 51% in adult animals. Percent rT3 secretion approximates 3% in fetus, 18% in newborn, and 3% in adult animals. Conclusions: In the newborn a) T4 secretion is increased; b) T3 production is increased as a result of increased secretion and augmented T4-T3 conversion; c) rT3 production from T4 is decreased, and rT3 secretion is minimal; d) thyroid sensitivity to TSH or pituitary-thyroid feedback appears to be altered during this period.