Hypertension in hypothyroidism: arterial pressure and hormone relationships

To investigate the regulation of arterial pressure and vasoactive hormones in hypothyroidism associated with hypertension, we measured intra-arterial pressure and hourly venous hormones (renin, angiotensin II, aldosterone, catecholamines and cortisol) for 24 hours in five hypertensive patients with primary hypothyroidism before commencing treatment, and again after three to six months of thyroxine replacement therapy. Arterial pressure fell significantly after thyroxine replacement in four patients. Thyroxine treatment was associated with a fall in plasma norepinephrine levels together with a decline in slopes of norepinephrine/arterial pressure regression lines which suggests that the sympathetic system may contribute to the hypertension in hypothyroidism. Variability of heart rate, blood pressure and plasma norepinephrine fell with thyroxine replacement consistent with impaired damping of swings in sympathetic activity in the untreated state. Reciprocal changes in arterial pressure and renin-angiotensin-aldosterone system activity suggested that this system was not the mediator of hypertension in hypothyroidism.