Thyrotropin Secretion and Metabolism in Rats during Propylthiouracil Treatment

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Abstract
The action of propylthiouracil (PTU; 0.01% in drinking water) on the development of goiter, thyroid hormone synthesis, plasma and pituitary TSH levels determined by radioimmunoassay, and on pituitary TSH secretion rate, has been studied in the normal rat. It has been found that PTU acts first on the peripheral metabolism of thyroxine and later on the thyroid hormone synthesis, both actions inducing a rapid increase in the plasma TSH concentration during the first week of treatment after which time the plasma TSH level returns to the normal range in all animals treated for 17 days. Then, it increases regularly up to the end of treatment (6 months). The pituitary TSH concentration diminishes immediately (24 hr) after the beginning of PTU treatment and remains low for three weeks. It reaches the initial level only after one month. In spite of these important pituitary and plasma TSH variations, the thyroid gland increases regularly. Study of the pituitary TSH secretion rate has shown that it increases during the first week of PTU treatment and returns to normal between the 17th day and one month. After two months of treatment, the secretion rate of TSH is four times higher than normal. The intrathyroid iodine concentration is decreased by PTU treatment and is about 30 times lower after one month. It is possible that this low iodine content may play a role on the TSH regulation of the thyroid. The half—time of disappearance of the injected 131I—bTSH is prolonged in all groups of PTU—treated rats. (Endocrinology92: 166, 1973)

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