BIOLOGIC EFFECTS OF TRIIODOTHYRONINE IN HUMAN SUBJECTS*†

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Abstract
THE present report is virtually an extension of the observations on triiodothyronine by McConahey et al. (1) and Lerman (2), presented at the meeting of the American Goiter Association in May 1953. NMORMAL HUMAN SUBJECTS The remarkable potency of triiodothyronine1 when given by mouth to normal subjects, shown by its ability to reduce the thyroidal uptake of a second tracer dose of radioactive iodine (I131) as compared with the first has already been reported (3). This phenomenon is well demonstrated in Figure 1, in which it is noted that an oral daily dose of .0088 milligram of triiodothyronine for seven days caused a 75 per cent reduction in the uptake of the second tracer dose, as compared with the first tracer dose, in the same person. In this short space of time, and with this minute dose of triiodothyronine, there was a significant reduction in the concentration of protein-bound iodine (PBI) of blood drawn at the end of the seven days of therapy. An oral daily dose of triiodothyronine half this size, namely 0.0044 milligram, did not reduce the thyroidal I131 uptake of the second tracer dose. Approximately fifteen times the 0.0088-mg. dose, namely 0.140 milligram, did not produce any greater depression of the I131 uptake of the second tracer.

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