Comparison of plasma and urinary methods for the direct measurement of the thyroxine to 3,5,3' - triiodothyronine conversion rate in man.

A further development of a new method recently proposed for the direct measurement of the conversion ratio (CR) of T4 to T3 in man is presented. [125I]T4 and [131I]T3 are injected simultaneously, and Sephadex chromatography is performed on urine samples to determine [125I]T3 formed in vivo, while plasma samples are used to measure the injected tracers. CR is calculated with the assumption that urinary [125I]T3 closely reflects [125I]T3 appearing in plasma after the injection of precursor [125I]T4. Four normal subjects and six patients with various thyroid disorders were studied using this method. The experimental data were also analyzed by our previous method based on plasma sampling only and by two recently described methods based on urinary measurements. These comparisons were made in an attempt to ascertain whether there is any systematic difference between the conversion values derived from plasma data and those derived from urinary data. Using plasma data alone, the CR was 28.6 +/- 3.4% (mean +/- SEM) in a group of four normal subjects, 37%, in two untreated hypothyroid patients, 40.2% in one hypothyroid subject receiving T4 treatment, 30.9% in one hyperthyroid patient, 24.9% in one patient with selective hyperthyroxinemia due to amiodarone treatment, and 10.7% in one normal subject after iopanoic acid administration. These values were in excellent agreement with those obtained by the modified procedure described here, in which both urinary and plasma measurements are used. Of the methods using urinary data alone, however, one gave similar results, while the other systematically overestimated the CR, possibly due to delayed excretion of labeled T4 metabolites into the urine. We conclude that 1) the analytical procedure to separate the labeled tracers and metabolites in urine or plasma is critical for the accurate estimation of CR; 2) when an adequate separation procedure is available, plasma and urinary methods for measuring CR yield comparable results; and 3) the plasma method should be used when, in addition to CR, other kinetic (distribution and turnover) parameters of T4 and T3 metabolism are to be estimated.