DYNAMIC STUDY OF POST-NATAL THYROID FUNCTION IN THE RAT

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Abstract
The thyroid function in development was investigated in postnatal rats. The thyroid I content rapidly increased from birth (137 .+-. 26 ng I/mg thyroid) to day 10 (338 .+-. 42 ng I/mg thyroid) and increased more slowly up to day 30 (425 .+-. 34 ng I/mg thyroid). The maximal plasma concentration of thyroxine [T4] was observed on day 16 (56.9 .+-. 3.5 ng T4/ml) and of I- on day 10 (110.2 .+-. 12.6 ng I-/ml). The turnover rate constant of extrathyroidal T4 was higher at birth (8.0 .+-. 2.3%/h) than at any older age studied (average 6%/h). T4 secretion by the thyroid was more intense before weaning (37 ng hormonal I/h per 100 g body wt on days 10 and 20) than after weaning (22 .+-. 6 ng hormonal I/h per 100 g body wt in 30 day old rats). The peripheral deiodination rate of T4 represented about 90% T4 secretion rate in newborn and 10 day old rats and only 40% in adult females. In pre-weaning rats after a single injection of both [131I]L-T4 and [125I]Na, extrathyroidal radioactivity disappeared more slowly than in 30 day old rats and adult animals. This suggests that I- concentrations of extrathyroidal tissues are higher before than after weaning.