Effect of Exogenous Thyroxine on Thyroid Accumulation and Secretion in Euthyroid Subjects

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Abstract
Euthyroid volunteer subjects from an Arkansas population were studied during administration of exogenous Na-L-thyroxine (T4) in doses of 0, 100 and 200 jug daily. Indices measured by chemical and I131 thyroidal and whole body counting methods were the daily thyroidal accumulation of iodide and the daily thyroxine degradation rate; the latter was derived from estimated absorption of the oral dose and total turnover measurements. Accumulation and proteinbound iodine (PBI) only were measured in 54 subjects, and both accumulation and degradation in each of a further 20 subjects. The results show that with increasing exogenous thyroxine dosage a progressive decrease in thyroidal uptake rate occurs. This approaches zero for an oral dose of about 280 ug of thyroxine daily. This dosage is associated with a small but definite increase in PBI. Total thyroxine turnover rate increased approximately in proportion to the PBI while the fractional thyroxine degradation rate remained relatively constant at these physiological dose levels.