FAILURE TO DEMONSTRATE EXTRATHYRO1DAL THYROXINE FORMATION IN THYROIDECTOMIZED RATS ON LOW AND MODERATE IODINE INTAKES1

Abstract

Young rats were thyroidcctomized and maintained on low or moderate iodine intakes for 50 to 60 days. They were injected with 500 μc of I¹³¹, and blood and liver were obtained 70 to 96 hours later. Completeness of thyroidectomy was checked by inspection of the thyroid region under the dissecting microscope, and by measurement of the I¹³¹-concentration ratio (relative to plasma) in several sections of tissue from the base of the tongue to the thorax. Body weights were determined weekly, and body length and standard metabolic rates were determined at the end of the experiment. Plasma samples were first dialyzed to remove radioiodide, after which concentrated butanol extracts of plasma and of pancreatin digests of plasma were chromatographed on filter paper. The major I¹³¹ components observed in the dialyzed plasma of thyroidectomized rats were: 1) I¹³¹-protein or peptidc which, on digestion with pancreatin, released I¹³¹-monoiodotyrosine, but no I¹³¹-diiodotyrosine; and 2) an unknown, high R₁ component (or components). No I¹³¹-thyroxine was detected in plasma, plasma digests, or liver digests of completely thyroidectomized rats injected with 500 μc. of I¹³¹. The same procedure readily detected I¹³¹-thyroxine in the plasma of normal rats injected with only 1 μc. of I¹³¹. The procedure used here for detection of I¹³¹-thyroxine in the plasma proved to be a more sensitive means of determining completeness of thyroidectomy than were body growth, standard metabolic rate, or pituitary acidophil degranulation. It is concluded that no significant extrathyroidal thyroxine formation occurs in rats under the conditions of these experiments.