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ALAN H. KLEIN, T H. ODDIE, DELBERT A. FISHER, Effect of Parturition on Serum Iodothyronine Concentrations in Fetal Sheep, Endocrinology, Volume 103, Issue 4, 1 October 1978, Pages 1453–1457, https://doi.org/10.1210/endo-103-4-1453
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T4, T3) and rT3 were determined by RIA on daily serum samples collected from five catheterized fetal sheep during the 2-week period before spontaneous vaginal delivery (PTD). Samples were also available from three newborn lambs during the first 4 h of life. T3 concentrations began to increase 4–6 days before spontaneous vaginal delivery, from a stable geometric mean concentration of 30 ng/dl to a mean concentration of 131 ng/dl on the day of delivery (day 0). T4 concentrations decreased in four of the animals and remained unchanged in one during this time interval. rT3concentrations decreased from a stable mean of 472 to 249 ng/ dl on day 0. The prenatal increase in serum T3 levels correlated temporally with increasing fetal serum cortisol concentrations. The mean T3 concentration increased further after delivery to a level of 487 ng/dl at 1 h of age without significant change in T4 or rT3 levels.
The following conclusions were reached: 1) fetal serum T3 concentrations increase while T4 and rT3 concentrations decrease during the 4–6 days preceding spontaneous labor, and 2) T3 concentrations increase further after delivery. Available evidence suggests that the prenatal increase in fetal serum T3 levels is mediated by a cortisol-induced increase in T4 to T3 conversion in fetal tissues. The mechanism(s) of the postnatal increase in serum T3 concentrations in the newborn is not clear.
