Autoradiographic localization of 131I-labelled thyroxine in the tissues of rat

An attempt was made to visualize the sites of localization of 131I-labelled thyroxine in the tissues of the rat by autoradiographic dipping techniques. The maximal uptake of 131I-throxine in rats occurred at 12 hours in all the tissues examined. The radioactivity continued to decrease from 12 to 36 hours after the injection. In the liver and kidney, the decline after 12 hours was rather marked. The radioactivity decreased only slightly from 12 to 36 hours in the spleen. After 3 hours of injection, the radioactivity was consistently higher in the thyroid follicular epithelial cells than in the interfollicular connective tissue. A high concentration of radioactivity was found at the periphery of the colloid areas.