Effect of Propylthiouracil Upon Thyroidal I131Release in the Methimazole-Thyroxine Treated Rat1

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Abstract
Propylthiouracil (PTU) in doses of 0.05, .15, 1 or 10 mg/day quickly accelerated the rate of thyroidal I131 release in 15 of 21 adult rats whose recycling of I131 was prevented by methimazole and whose initial thyroidal I131 release was slowed by exogenous thyroxine. The magnitude of the response was independent of PTU dose. One or 2 rats failed to respond to each dose of PTU administered. Rats whose thyroidal I131 release rates were accelerated by PTU had significantly higher plasma levels of radioactivity than rats whose thyroidal I131 release slopes were unaffected by PTU. The percentages of protein-bound I131 and inorganic I131 in the plasma, however, were unchanged. These data confirm the thesis that PTU administration promptly causes an increased thyrotrophin secretion by the pituitary.