Dynamic study of post-natal thyroid function in the rat.
The thyroid function in development was investigated in post-natal rats. The thyroid iodine content rapidly increased from birth (137 +/- 26 ng iodine/mg thyroid) up to day 10 (338 +/- 42 ng iodine/mg thyroid) then increased more slowly up to day 30 (425 +/- 34 ng iodine/mg thyroid). The maximal plasma concentration of thyroxine was observed on day 16 (56.9 +/- 3.5 ng T4/ml) and of iodide on day 10 (110.2 +/- 12.6 ng I-/ml). The turnover rate constant of extrathyroidal thyroxine was higher at birth (8.0 +/- 2.3 %/h) than at any older age studied (average 6 %/h). Thyroxine secretion by the thyroid was more intense before weaning (37 ng hormonal iodine/h/100 g body weight on days 10 and 20) than after weaning (22 +/- 6 ng hormonal iodine/h/100 g body weight in 30 days old rats). The peripheral deiodination rate of thyroxine represented about 90 % thyroxine secretion rate in newborn and 10 days 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 days old rats and adult animals. This suggests that iodide concentrations of extrathyroidal tissues are higher before than after weaning.
D000367:Age Factors; D000818:Animals; D001835:Body Weight; D005260:Female; D006801:Humans; D007454:Iodides / Q000097:blood; D007455:Iodine / Q000097:blood / Q000378:metabolism*; D009929:Organ Size; D051381:Rats; D013961:Thyroid Gland / Q000254:growth & development / Q000378:metabolism / Q000502:physiology*; D013974:Thyroxine / Q000097:blood / Q000378:metabolism*
D007454:Iodides; D007455:Iodine; D013974:Thyroxine