Ontogenesis of Thyroid Function in the Neonatal Rat. Thyroxine (T4) and Triiodothyronine (T3) Production Rates

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Abstract
The ontogenesis of thyroxine (T4) and triiodothyronine (T3) production in the neonatal rat (from 5 days to adulthood) was examined by measuring [125I]T4 and [125I]T3 kinetic parameters by single-compartmental analysis. T4 and T3 serum concentrations were measured by specific radioimmunoassay. The volume of distribution (VD) and the metabolic clearance rate (MCR) of T4, high at 5 days, increase to a peak at 22-26 days and decline to adult values. The T4 production rate (PR) low at 5 days, increases to peak values at 14-32 days and then declines toward adult values. The T4 fractional removal rate (K) is similar from birth to adulthood. No difference is observed for K, VD and MCR of T3 during the life of the rat. The T3 PR rises from minimal values at 5 days to attain peak values at 26 days. There is a delay of about 12 days between the peak of T4 PR and the attainment of the peak T3 PR. The T4/T3 PR ratio decreases from a maximal value at 5 days to values approaching those seen in the adult at 12 days. There is a significant rise at 14 days due to the marked rise in the T4 PR before the ratio returns to adult values by 26 days as a result of the increase in the T3 PR. The increasing T3 PR following the peak in the T4 PR may represent increasing peripheral T4 monodeiodination to T3 and may be analogous to the marked rise in T3 following birth in the sheep and human. These data confirm that the ontogenesis of thyroid hormone secretion occurs after birth in the rat and suggest that the first 20 days of life in the rat may correspond to the last trimester of gestation of the human fetus.