Triiodothyronine turnover in euthyroid subjects.

Abstract

Triiodothyronine (T3) kinetic studies and serum total and free T3 concentrations were measured in 22 euthyroid Arkansas subjects. T3 kinetics were assessed simultaneously using urine and plasma assay and whole body counting for three independent estimates of the kinetic parameters. The results were computer analyzed using a linearization method of nonlinear curve fitting with least squares and included correction for iodoprotein formation. Serum T3 concentration was measured using a double column chromatographic, protein binding method and by radioimmunoassay. Numerical values of the parameters chosen to represent the iodoprotein pool were found to have a marked effect on T3 degradation derived by whole body counting but only a small effect on the degradation rates computed from urine and plasma counting. The mean T3 distribution volume was 35 liters and the mean fractional T3 degradation rate 0.714 day−1; the mean serum T3 concentration was 97 ng/100 ml and the calculated mean T3 turnover 22.6 μg daily. Mean T4 turnover measured in the same subjects was 87 μg daily so that the T4/T3 turnover ratio was about 4/1.