Effects of Variations of Thyroxine-Binding Globulin Capacity on the Disappearance of Triiodothyronine from the Plasma1

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Abstract
In a previous study from our laboratories alterations in TBG binding capacity were found not to alter the rate of egress of T3 from the circulation for the first hour after intravenous injection. It was considered of interest to study the fate of triiodothyronine after it reached equilibrium in its ultimate distribution space. The T3 half-time was obtained by extrapolating the rectilinear portion of the curve for blood radioactivity from the end of the second to seventh day after injection of a tracer dose. Data were obtained from 20 normal subjects, 15 estrogen-treated patients, 12 patients who were treated with androgens (5 of whom also received an anabolic agent), 7 patients treated with diphenylhydantoin. The doses used were sufficient to alter significantly the TBG binding capacity of the serum. In the estrogen-treated group the TBG capacity was increased; in the other 2 treated groups it was decreased. The mean T3 half-time determined from blood radioactivity was as follows: normal—1.60 days; estrogen-treated group—1.35 days; androgenanabolic agent group—2.09 days; diphenylhydantoin-treated group—2.11 days. These results are opposite to those that would be expected if plasma binding were the most important factor contributing to the fate of T3. Thus, other factors, e.g., tissue binding or cellular handling, must have a major influence on T3 turnover.

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