Evaluation of Triiodothyronine (T3) Kinetics in Normal Subjects, in Hypothyroid, and Hyperthyroid Patients Using Specific Antiserum for the Determination of Labeled T3in Plasma

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Abstract
Triiodothyronine (T3) kinetics was evaluated using [I25I]T3 and the single injection technique; 5 hypothyroid, 6 hyperthyroid patients, and 10 euthyroid control subjects were studied. Plasma-labeled T3 concentration was measured by means of a new method based on extraction of the hormone on Sephadex G-25 columns followed by elution with the specific antiserum. This technique allows a far better separation of the hormonal radioactivity from the labeled iodide produced from T3 catabolism in comparison with the TCA-precipitation- butanol extraction method. The analysis of the experimental data has been performed using non-compartmental treatment (integral approach); results of mono-compartmental analysis of the same data are also reported for comparison. Average metabolic clearance was 15.3 ± 0.6 (mean + SEM) liters/day/m2 body surface in normal subjects; it was significantly decreased in hypothyroid patients (11.4 ± 1.1) and significantly increased (33.4 ± 4.0) in hyperthyroidism. The total plasma equivalent distribution volume was found significantly enlarged in hyperthyroid patients (22.6 ± 0.9 liters/m2) in respect to that measured in the control group (15.6 ± 0.4), whereas it was not different from normal value in hypothyroid patients (17 ± 1.7). Using plasma concentration of native Ta, absolute turnover rate and extrathyroidal pool were also estimated; their values were 6.5, 23.7, and 131.7 μwg/day/m2 and 10.1, 24.2, and 90.6 αxg/m2, respectively, in hypothyroid, normal, and hyperthyroid groups. (J. Clin Endocrinol Metab46: 203, 1977)

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