SIMULTANEOUS COMPARATIVE STUDIES OF THYROXINE AND TRI-IODOTHYRONINE DISTRIBUTION AND DISPOSAL RATES

Abstract

SUMMARY Determinations of the total serum tri-iodothyronine (T 3 ) and total serum thyroxine (T 4 ) were performed in healthy persons, and in patients with various thyroid diseases. In addition, tracer doses of 131 I-labelled T 3 and 125 I-labelled T 4 were given intravenously, and the disappearance rates of each hormone were plotted graphically. From this single injection slope-intercept technique, the volumes of distribution, half-times, fractional rates of disappearance, and hormonal production rates were calculated by standard techniques. In normal persons, the average T 3 production was 44·75 μg daily, and that for T 4 97·30 μg/day. In most hyperthyroid patients, the production of both hormones was increased in equal proportions to the normal group. In four patients with 'T 3 -thyrotoxicosis', the total serum T 4 and T 4 production rates were within normal limits, and only T 3 production was increased. In patients with primary hypothyroidism, the production of both hormones was reduced: however, T 4 production was more markedly reduced than T 3 production. Two patients with Hashimoto's disease who were clinically euthyroid showed low T 4 production rates, but T 3 production was almost within normal limits. These findings suggest that in primary thyroid deficiency there appears to be a sequential failure of T 4 production and then of T 3 production, before hypothyroidism develops clinically. This may be due to increased endogenous thyrotrophin, and perhaps a low intrathyroidal iodine content. In contrast, patients with secondary hypothyroidism due to hypopituitarism had much higher ratios of T 4 :T 3 production rates than patients with primary hypothyroidism.