Neonatal changes in the concentrations of thyrotropin, triiodothyronine, thyroxine and cortisol in the plasma of pre-term and full-term lambs.

The neonatal changes in the plasma concentration thyrotropin, T3, T4 and cortisol were followed in six lambs born 7 days before term, after injection of oestrogen into the ewes, and in six full-term animals. The plasma T3 concentrations were significantly lower in the preterm animals 1 h, 8 h, 36 h, 44 h, 5 days, 15 days, 20 days and 30 days after birth. The plasma T4 concentrations were significantly higher in the preterm lambs than in the controls during the first 16 h of life. The T3/T4 ratios were constantly lower in the preterm animals during the first 20 days of life. The plasma cortisol concentrations were significantly lower in the preterm lambs over the first 8 h of life. In contrast to the full-term animals, the thyroid of the preterm ones did not respond to an increase in the plasma concentration of thyrotropin by a rise in the plasma concentration of the iodinated hormones. Thyroid function seems affected by prematurity during the first month of life. The T4 to T3 conversion may be impaired and the plasma T3 concentration is depressed. Since the thyroid seemed to be relatively insensitive to a thyrotropin surge induced, for instance, by cold, thermoregulation could be impaired in preterm lambs. The low concentration of T3 and cortisol observed at birth may also contribute to a reduction in the synthesis of the pulmonary surfactant. These results can therefore, at least in part, explain the high mortality rate observed in premature mammals.