Plasma noradrenaline and blood pressure in hypothyroid patients: effect of gradual thyroxine treatment.

High plasma noradrenaline (PNA) levels have been reported in hypothyroid patients and hypothyroidism has been associated with hypertension. To explore the relationship between PNA and blood pressure (BP) in hypothyroid patients, and the effects of gradual thyroxine replacement, a prospective study was performed comparing BP, heart rate (HR) and PNA in a normotensive and a hypertensive group of hypothyroid patients before and during gradual thyroxine substitution. Thyroxine treatment reduced the BP; the reduction in supine BP was greater in the hypertensive than in the normotensive group. HR increased similarly in both groups during treatment. PNA was elevated in the normotensive group before treatment and decreased gradually during thyroxine treatment. The hypertensive group had normal PNA levels. The present study indicates that normotensive, in contrast to hypertensive, hypothyroid patients have increased sympathetic nervous activity. Although the mechanism is unclear, thyroid replacement therapy can reverse hypertension in hypothyroid patients.