THE EXTRATHYROIDAL DISTRIBUTION OF 131I THYROXINE

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Abstract
The elimination of radioactivity from the blood stream has been examined in 6 subjects after intravenous administration of I131 -thyroxine. About 50% of the radioactivity left the blood within 60-90 min. and 75% within 6 hrs. In both in vivo and in vitro experiments there was a lipid linking of the labelled thyroxine to the serum proteins. The elimination of radioactivity from the blood and the excretion in urine and feces was followed in 12 subjects for 12 days. To calculate the distribution and exchange of the labelled thyroxine between the various extrathyroidal pools, a model comprising 4 pools was constructed. The working of the model was analysed by means of an analogue computer. There was a close agreement between the curves thus obtained and those yielded by empirical methods. According to the 4-pool model 6-7 times the intravascular thyroxine pool was exchanged with the extravascular pool in 24 hr.