Peripheral metabolism of thyroid hormones in man. I. Direct measurement of the conversion rate of thyroxine to 3,5,3'-triiodothyronine (T3) and determination of the peripheral and thyroidal production of T3.

We describe here a new method for the direct measurement of the conversion rate of T4 to T3 in man. The metabolic study was performed in 23 subjects: 13 healthy controls, 7 T4-treated hypothyroids, and 3 sick euthyroid patients. The experimental protocol involved the simultaneous iv bolus injection of [125I]T4 and [131I]T3, and the use of Sephadex G-25 column chromatography to determine the plasma concentrations of [125I]T4, [131I]T3, and [125I]T3 newly formed through 5'-monodeiodination of labeled T4 in the peripheral tissues. The T4 and T3 kinetic parameters were determined by noncompartmental analysis. The conversion rate of T4 to T3 was computed by a method based on the precursor-product relationship, using the [131I]T3 disappearance curve for correcting the concentrations of newly formed [125I]T3 (convolution method). The conversion rate of T4 to T3 was 0.2541 +/- 0.0125 (mean +/- SEM) in the control group and was significantly reduced (0.1283 +/- 0.0204; P less than 0.001) in the sick euthyroid patients, while it was slightly, though not significantly, increased in the T4-treated patients (0.2932 +/- 0.0220). A close agreement was found between the values for the conversion rate obtained by the convolution approach and those derived from the ratio between the serum concentrations of [125I]T3 and [125I]T4 at equilibrium. The conversion rates obtained by the convolution approach were also in good agreement with the values estimated from the molar ratio between the turnover rates of T3 and T4. In the control group, 72.0 +/- 3.6% of the circulating T3 was produced by 5'-monodeiodination of T4 in the peripheral tissues, and 28.0 +/- 3.6% of the circulating T3 derived from direct thyroidal secretion. The sick euthyroid patients showed a significantly smaller proportion of circulating T3 deriving from peripheral conversion of T4 (52.5 +/- 3.9%; P less than 0.025).