Serum Free Thyroxine and Thyroxine-Binding Protein Studies in Patients with Supraventricular Tachycardias1

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Abstract
Various indices of thyroid function have been studied in 35 patients with supraventricular tachycardias with and without demonstrable associated heart disease. These patients were euthyroid by clinical and laboratory evaluations. The resin uptake of 131I-labeled L-triiodothyronine was determined in 25 of these patients; the thyroxine binding pre-albumin (TBPA) and thyroxine-binding globulin (TBG) capacities and/or the serum free thyroxine were measured in all patients. The T3 resin uptake was elevated in 14 of 33 determinations carried out on 25 patients. The TBPA capacity was decreased in 12 of 14 patients having an increased T3 resin uptake, whereas the TBG capacity was normal. The free thyroxine values were increased in those patients with a low TBPA capacity. Twenty-one of 35 patients studied had either an increased free thyroxine or decreased TBPA capacity or both. The diminished TBPA capacity is presumably a result of decreased hepatic synthesis of pre-albumin and is seen in a number of thyroidal and nonthyroidal illnesses. The high T3 resin uptake and free thyroxine values are considered to be secondary to the fall in TBPA capacity. The relationship of thyroid hormone and catecholamines in the production of cardiac arrhythmias is discussed.