Unusual features of neonatal thyroid function in small-for-gestational-age lambs. Origin of plasma T4 and T3 deficiencies.

Thyroid function was studied in small for gestational age (SGA) or control newborn lambs. Neonatal changes in plasma concentrations of TSH, T3, rT3, total and free T4 were monitored, and thyroid scintigraphs were performed. Responsiveness of the hypothalamic-pituitary-thyroid axis to cold exposure and TRH or TSH administration was assessed. In addition, T4 and T3 kinetic studies were performed. In agreement with results obtained in babies, plasma T3, total T4 and free T4 concentrations were depressed in low birth weight animals, whereas TSH and rT3 levels were not affected. Thyroid size expressed relatively to the body weight was higher in SGA animals, thus suggesting that a partial compensation for low thyroid hormone levels had occurred during the fetal life. Plasma TSH and T4 concentrations increased by a same extent after exposure to cold and TRH or TSH administration in SGA and control lambs; however, the rise in T3 levels was depressed in the former in all stimulation tests. T3 and T4 production rates were similar in the two experimental groups. In SGA lambs, the metabolic clearance rate and the total distribution space of these two hormones were significantly increased; the fast T3 pool was higher, and the slow T3 pool lower than in control animals. All these results demonstrate that, despite low circulating thyroid hormone concentrations, SGA lambs are not hypothyroid. An increased T4 and T3 storage in the extravascular compartment is probably the major factor involved in the occurrence of this plasma deficiency.(ABSTRACT TRUNCATED AT 250 WORDS)