Quantitative study of extrathyroid enzymatic conversion of T4 to T3 in man

The authors present a quantitative study of the extrathyroid conversion process of T4 to T3 in 3 athyroid patients, on the basis of an original biochemical model and a rigorous mathematical treatment, utilizing the kinetic isotope method. The values of thor the constant of the reaction rate of conversion of T4 to T3 and of the T4 fraction that undergoes this process--serum concentrations of T4 and T2--were determined in two completely different ways: 1) the radiochromatographic method, and 2) radioimmunoassay of T3 and competitive protein binding of T4. The results thus obtained supported each other and revealed that more than 1/3 of the body total thyroxine is turned into triiodothyronine by the extrathyroid conversion process. The maximum concentration of the triiodothyronine thus formed is reached after 5-6 days, if the reference level of this hormone is zero. However, the higher the T3 reference concentration in the body, up to the normal physiologic level, the shorter the time interval for reaching the T3 maximum concentration following a single moderate dose of T4.