DIVERGENT PHASES OF RESPONSIVENESS OF THYROID HORMONE BIOSYNTHESIS TO EXOGENOUS THYROID-STIMULATING HORMONE IN THE POSTNATAL RAT

We have found that the thyroids of postnatal rats respond to administration of thyroid-stimulating hormone (TSH) by significant changes in iodine metabolism and hormone biosynthesis. The specific activity of iodine in thyroxine (T4) and 3,5,3′-tri-iodothyronine (T3) was always higher than that in monoiodotyrosine (MIT) and di-iodotyrosine (DIT) in the TSH-treated rats. The highest sensitivity to TSH in this regard was observed during week 2 of life and again at sexual maturity. The lowest thyroid response to TSH with respect to biosynthesis of MIT, DIT, T4and T3was observed in weeks 3 and 4 of life rather than in the younger or older rats. However, the specific activities of MIT and DIT were not changed after TSH administration to 2-day-old pups.Increase in thyroid weight and in the specific activity of iodine in the thyroid occurred in almost all of the TSH-treated rats. In contrast, the thyroid iodine content decreased after TSH administration, except in the 2-day-old pups. The specific activity of iodine in the thyroid was highest in the 2-day-old pups and then abruptly declined during week 1 of life and continued to do so until maturity. The thyroid protein content decreased during the first 2 weeks of life but thereafter it was increased in the rats receiving TSH.We conclude that thyroid responsiveness to TSH in terms of thyroid hormone metabolism has several distinct phases in postnatal rats: the first phase, during week 1 of life is characterized by the highest specific activity of iodinated compounds in the thyroid and by the highest percentage representation of T4and T3; the second, during week 2 of life, by the highest thyroid response to TSH; the third, during weeks 3 and 4 of life, by the lowest thyroid response to TSH; and the last phase in mature rats is characterized by a relatively low specific activity of iodinated compounds in the thyroid but by a good response to TSH stimulation