Postnatal development of pituitary-thyroid function in male and female rats: comparison of plasma and thyroid T3 and T4 concentration.

Plasma T4, T3, and TSH, and total and 131l-labeled T4 and T3 in thyroid digests were determined in male and female rats from birth to 30--45 days postnatally. Mothers and offspring were fed a high-iodine Purina diet throughout. In both sexes, plasma T4 was less than 1 microgram/100ml at birth but rose linearly to a peak of 6--7 micrograms/100 ml at day 15. Plasma T3 remained low at 30--40 ng/100 ml until day 10, then rose steeply to 80--100 ng/100 ml at day 20--30. Plasma TSH had a bimodal distribution with an early peak in the intial 1--2 weeks of life and a second rise after day 30. Adult pattern sex differences in plasma TSH appeared by day 30. Labeled T3 was undetectable in thyroid digests until after day 7. Both labeled and stable T3 and T4 concentration and T3/T4 ratios rose progressively during the first 2 weeks of life. After day 7, stable and labeled intrathyroidal T3/T4 ratios were significantly correlated, but the latter was always higher than the former for any given age.