Thyroxine accumulation in thyroids of hypophysectomized rats.

Abstract

Rats were fed a low iodine (Remington) diet for 8 months, after which radioiodine (125I) was mixed with the diet for 130 days. The metabolism of iodine was studied continuously while isotopic equilibrium was approached for 130 days. The large thyroids of control animals contained few follicles and only 4.4% of their thyroidal iodine was in thyroxine (T4) and 5.9% was triiodothyronine (T3). Hypophysectomy after the 60th day of feeding the labeled diet resulted in a continuous increase of iodine and T4 in the thyroid. Following hypophysectomy, the goiters atrophied and the ng of T4 iodine in the thyroid increased to 7.9 times greater than in intact controls; the concentration of T4 iodine increased 58 times greater than in controls while the concentration of T3 iodine increased to 4.3 times greater than in controls. The results suggest that hypophysectomy inhibited the release of T4 to a greater extent than it inhibited the synthesis of T4 in these goiters; the result was that T4 accumulated during involuti...