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

Abstract

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.