DISAPPEARANCE FROM SERUM OF I131-LABELED L-THYROXINE AND L-TRIIODOTHYRONINE IN EUTHYROID SUBJECTS 1

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Abstract
Tracer amounts of l-thyroxine or l-triiodothyronine labeled with I131 were injected intravenously in euthyroid subjects. The subjects received non-radioactive iodine during the studies to minimize thyroid uptake and reutilization of radioiodine liberated on degradation of the administered compounds. Serum radioactivity was measured on blood samples drawn 10 minutes after injection, and daily for 2-weeks. There was a rapid initial fall, attributed to distribution of the compound. Within 2 days after injection of radiothyroxine, the serum radioactivity declined more gradually, forming a straight line when plotted semilogarithmi-cally. This slow exponential component of the disappearance curve was interpreted as indicating thyroxine turnover, or the rates of formation and degradation, which in a steady state should be identical. The mean half-time of thyroxine turnover in 6 subjects was 6.7 days. The disappearance curve observed after injection of labeled triiodothyronine was much more rapid, with a mean half-time of 2.7 days in 4 subjects. Successive studies with l-thyroxine and l-triiodothyronine in 1 individual clearly demonstrated the more rapid disappearance of the latter con.pound.