Enhanced Conversion of Thyroxine to Triiodothyronine by the Neonatal Rat Pituitary*

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Abstract
Serum concentrations of T4 and T3 are markedly decreased without an appropriate increase in TSH concentration in the neonatal rat. Since TRH does not appear to regulate TSH secretion in the perinatal rat, and intrapituitary 5′-monodeiodination of T4 to T3 is a major modulator of TSH secretion, it seemed possible that enhanced pituitary generation of T3 from T4 might play a role in the inappropriately low serum TSH level in the neonatal rat. The conversion of T4 to T3 in pituitary homogenates was studied in rats varying in age from 1 day to 1 yr and compared to values obtained in 10-week-old male rats. Pools of neonatal and individual adult pituitaries were homogenized in 1 ml Trisbuffer, pH 7.4, containing 0.5 μg T4 and 20 mm dithiothreitol, and the T3 generated was measured in ethanolic extracts by RIA. Pituitary T3 generation from T4 in the 1-day-old neonates was similar to that obtained from 10-week-old males. However, by 5 days of age, pituitary T3 generation from T4 was markedly increased (226 ± 16.3%) compared to values obtained in 10-week-old rats. This enhanced T3 generation persisted through 30 days of age and then decreased to adult values by 45 days of age. These findings in the neonatal pituitary are in contrast to the decreased hepatic T3 generation from T4 in early neonatal life. In old male rats, pituitary and hepatic T3 generation from T4 was similar to that observed in the 10-week-old rats. No sexrelated difference in pituitary conversion of T4 to T3 was observed, in contrast to the increased hepatic T3 generation in male compared to female adult rats. The present study demonstrates enhanced 5′-monodeiodination of T4 to T3 in neonatal pituitary homogenates. In addition, striking differences exist between pituitary and hepatic conversion of T4 to T3 during the neonatal period and in adult male and female rats. (Endocrinology106: 1735, 1980)

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