Plasma tri iodothyronine concentration in the foetal and newborn lamb

Fetal plasma was obtained by the use of chronic indwelling catheters, and neonatal plasma was obtained by jugular vein puncture. Tri iodothyronine was measured by radioimmunoassay. In the assay system the coefficient of variation was 16% and the cross reactivity with thyroxine less than 0.1%. Recovery of added tri iodothyronine was 105%. In four fetal lambs the mean plasma tri iodothyronine concentration in the last fetal sample, obtained 5-7 h before birth, was less than 0.7 ng/ml; within 3 h of birth it had risen to a mean of 1.8 ng/ml. On the first day of neonatal life plasma tri iodothyronine measured 3.3 ng/ml in four other non catheterized lambs. There was a rise in concentration after birth. Similarly, in twin lambs, (gestational age 145 days at birth), plasma tri iodothyronine continued to rise in the early postnatal period. It is clear that plasma tri iodothyronine concentrations do not follow thyroxine concentration. The tri iodothyronine concentration continues to rise during the period in which thyroxine concentrations fall. The sudden increase in plasma tri iodothyronine after birth does not necessarily reflect an increased conversion of thyroxine to tri iodothyronine but could possibly be due to the absence of placental clearance and metabolism of tri iodothyronine after delivery. Further work is required to assess the role of tri iodothyronine in the fetus and newborn animal.