Baroreflex-governed sympathetic outflow to muscle vasculature is increased in hypothyroidism.

Microelectrode recording of muscle nerve sympathetic activity (MSA), which is baroreflex-governed and involved in cardiovascular homeostasis, was made in five patients with hypothyroidism and in seven patients with hyperthyroidism prior to treatment and after recovery to euthyroidism. Patients with hypothyroidism had a higher level of MSA when hypothyroid than after recovery (mean +/- SEM 39.3 +/- 7.1 and 26.8 +/- 7.9 bursts/min, respectively, P less than 0.05), whereas hyperthyroidism was not accompanied by a change in the number of sympathetic bursts/min. The response of MSA to manouvres known to influence the neural outflow via baroreceptors and other receptor inputs was not changed with altered thyroid function. The findings provide direct evidence of an increased sympathetic activity in hypothyroidism, but suggest that other physiological properties of MSA are intact in thyroid disease.