THYROID FUNCTION IN SUPRAVENTRICULAR TACHYCARDIAS: TURNOVER OF INTRAVENOUSLY INFUSED I131-LABELED THYROXINE AND THE RED BLOOD CELL UPTAKE OF I131-LABELEDl-TRIIODOTHYRONINE*†

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Abstract
Thyroid function was studied in 28 euthyroid patients with paroxysmal or recent prolonged atrial fibrillation and in 11 patients with paroxysmal atrial tachycardia. The serum protein-bound iodine level, basal metabolic rate, and thyroidal I131 uptake were within normal limits. In 11 of these cases, the fate of intravenously administered I131 -thyroxine was studied. The half-time of disappearance of plasma radioactivity following infusion of I131-labeled thyroxine ranged from 4.1 to 6.8 days and averaged 5.74 days-signi-ficantly faster than the turnover in previous studies of euthyroid human subjects. The thyroxine distribution space ranged from 8.0 to 16.4 liters (average, 12.2 liters), in contrast to a range of 7 to 12 liters (average, 9.9 liters) in the control group. The daily degradation rate of thyroxine varied from 52 to 126 micrograms per day (average, 83.6) in the group under study but was only 36 to 61 micrograms in the control group. The two-hour in vitro red blood cell uptake of I131-triiodothyronine from whole blood in 18 male patients with atrial arrhythmias was 18.6 per cent per 100 hematocrit. In 8 of these cases, the values were in the range often seen in thyrotoxicosis. The red blood cell uptake of I131-triiodothyronine in 21 female patients similarly studied averaged 16.5 per cent. Seven of the 21 had uptakes greater than 18 per cent, the upper limit for euthyroid females. In the 28 patients with atrial fibrillation, the red blood cell uptake was abnormal in 12. In the 11 patients with paroxysmal atrial tachycardia, the red blood cell uptake was abnormal in 3. Five patients were treated with large doses of potassium iodide, although they were considered euthyroid. Significant improvement was noted in each.

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