Altered muscle sympathetic nerve activity in hyperthyroidism and hypothyroidism.

To determine whether sympathetic nerve activity is altered in hyperthyroidism and hypothyroidism we microneurographically measured muscle sympathetic nerve activity in patients with these thyroid dysfunctions and compared the results with those of normal controls. Patients with hyperthyroidism tended to have less muscle sympathetic nerve activity than normal controls, and patients with hypothyroidism had significantly greater muscle sympathetic nerve activity than normal controls (P < 0.05). In all subjects, there was a significantly negative-correlation between the serum concentration of free triiodothyronine or free thyroxine and muscle sympathetic nerve activity, and there was a significantly positive correlation between the serum concentration of thyroid-stimulating hormone and the muscle sympathetic nerve activity. These results suggest an inverse relationship between thyroid function and sympathetic nerve activity.