THYROXINE METABOLISM IN MAN ESTIMATED BY MEANS OF I131-LABELLED L-THYROXINE

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Abstract
The fate of thyroxine was determined in 31 subjects after the intravenous injection of 1131 -labelled L-thyroxine. The biological half-life of thyroxine in plasma varied between 7-9 days in normal subjects and was the same in hypermetabolic and strumectomized patients. In patients with thyrotoxicosis and hepatitis, the half-life was decreased. About 25-30% of the administered thyroxine activity was localized immediately in the liver, from about 20-30% excreted in the urine, and about 10% excreted in the feces in the course of 10 days. In all individuals, only radioactive I, but no thyroxine, was found in the urine.