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The thyroid function in development was investigated in post-natal rats. The thyroid iodine content rapidly increased from birth (137 \ m=+-\ 26 ng iodine/mg thyroid) up to day 10 (338 \ m=+-\42 ng iodine/mg thyroid) then increased more slowly up to day 30 (425 \ m=+-\34 ng iodine/mg thyroid). The maximal plasma concentration of thyroxine was observed on day 16 (56.9 \ m=+-\3.5 ng T4/ml) and of iodide on day 10 (110.2 \m=+-\12.6 ng I\m=-\/ml ).The turnover rate constant of extrathyroidal thyroxine was higher at birth (8.0 \ m=+-\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 \ m=+-\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.In the rat, during the 2 first post-natal weeks, the number, size and colloid content of the thyroid follicles increase (Legait et al. 1954; Phillips 8c Gordon 1955;Mitro et al. 1969). Simultaneously and as a result of these changes, iodine and thyroxine accumulate in the thyroid gland (Florsheim et al. 1966;

Summary. Plasma thyroxine (T 4 ), triiodothyronine (T 3 ) and iodide levels were measured from birth until 30 days of age in 17 Holstein x Friesian (HF), 7 Salers and 7 Charolais calves born at term. The response of plasma T 4 and T 3 levels to bovine thyrotropin (TSH) injection was also compared in two groups of 9 HF calves 3hrs and 21 days, respectively, after birth.In the HF calves, plasma T 4 and T 3 levels increased from birth to 6 hrs (when the rectal temperature of these calves decreased slightly), then diminished until day 7 and remained stable until 30 days of age. In Salers calves, changes in the plasma T 4 and T 3 levels were not different from those observed in HF animals. However, in Charolais calves, there was no significant increase in plasma T 4 and T 3 levels after birth. Twelve and 24 hrs after birth, the plasma iodothyronine levels measured in these animals were lower than in HF calves. In Salers and Charolais calves a positive linear relationship was demonstrated between plasma T 4 (or T 3 ) levels at birth and the birth weight. In the three groups of calves, plasma iodide levels decreased from birth until 10 days of age, then remained stable until the end of the first neonatal month.Intravenous injection of the same dose of bovine TSH (5 mU/kg of body weight) induced a rise in the plasma T 4 and T 3 levels which was significantly more intense in 21-day old calves than in 3-hr old calves.Introduction.