Thyrotoxicosis: Comparison of Effects of Thyroid Ablation and Beta-Adrenergic Blockade on Metabolic Rate and Ventilatory Control*

Abstract

To elucidate the role of the betasympathetic system in thyrotoxicosis (THY), we.examined cardiac sensitivity to infused betaagonist and compared the effect of betablockade with that of resolution of the hyperthyroid state. Beta–sympathetic (βSYM) sensitivity was measured as the heart rate response to isoproterenol in THY patients and in normal subjects. The patients with THY showed both lower threshold (P < 0.05) and increased slope (P < 0.05) of the heart rateisoproterenol response, suggesting βSYM hypersensitivity. The βSYM like features of THY were measured in 7 patients before and 7 days after βSYM blockade with propranolol (mean dose 411 ± 32 mg/day [SEM]) which was shown to block the heart rate response to isoproterenol. These results were compared with those in a similar group of thyrotoxic patients rendered euthyroid with ¹³¹I. During αSYM blockade, heart rate decreased from 101 ± 6.3 to 78 ± 4.6 (P< 0.01), but the elevated metabolic rate (VO2), resting ventilation (VE), and increased hypoxic and hypercapnic ventilatory responses were not significantly affected. In the group rendered euthyroid with ^I3II, heart rate decreased from 110 ± 3.5 to 76 ± 7.8 (P< 0.02), but in contrast to the result of αJαSYM blockade, a 28% decrease in VO2 (P < 0.01), 41% decrease in VE (P< 0.05), a 38% decrease in hypercapnic ventilatory response (P < 0.05), and a 66% decrease in hypoxic response (P < 0.03) occurred. During THY, αSYM mechanisms are responsible for part of the tachycardia, but the metabolic and ventilatory abnormalities are not αSYM mediated.