Hypertension is more common in hypothyroidic patients than in euthyroid controls 
in older age groups. Treatment of the thyroid deficiency alone lowers blood 
pressure in most patients. Hemodynamically, cardiac output is reduced and total 
peripheral resistance is elevated. The latter probably is secondary to an 
increase of sympathetic nervous tone and a relative increase in alpha-adrenergic 
response. In hyperthyroidism, elevation of diastolic blood pressure is uncommon. 
Systolic hypertension is more common in younger age groups. Treatment of the 
hyperthyroidism alone lowers systolic blood pressure in most patients. An 
increase in cardiac output and a decrease in total peripheral resistance 
accompany the hyperthyroidism. Potentiation of catecholamine action by an excess 
of thyroid hormone has been invoked as an explanation, because thyroid hormone 
excess is accompanied by increased beta-adrenergic receptors in some tissue, 
including heart.