Differences in Thyroid Function Between Triiodothyronine-Treated and Hypophysectomized Rats: Binding Glands

Abstract

The fate of I131 during the first 16 minutes after its subcutaneous injection was observed in the binding thyroids of hypo-physectomized or triiodothyronine (T3)-treated rats which had or had not been given thyrotropin (TSH). The thyroids of T3-treated rats (compared with those of hypophysectomized rats) concentrated radio-iodide poorly but, especially early after the injection of I131, converted it more efficiently to organic I131. The fractional rate of binding of thyroidal radioiodide (Kb) changed with time after I131 injection. An elevation of Kb by TSH was clear-cut throughout the experimental period in T3-treated rats, but only toward its end in hypophysectomized animals. Thyroidal clearance of serum I131 by organic binding (Co/m) was more efficient in hypophysectomized rats than in T3-treated animals; when TSH was given, Co/m in the latter was raised to levels which, during the first 12 minutes after I131 injection, exceeded those seen in TSH-treated hypophysectomized rats. The results emphasize not only differences in thyroid function after T3-treatment vs. hypo-physectomy but also the heterogeneity of iodide which has entered the thyroid recently.