Correction of the causes of secondary forms of hypertension usually restores blood pressure to normal. Hypothyroidism is a potentially important but overlooked cause of hypertension and restoration of euthyroidism with thyroxine therapy usually results in a substantial reduction in both systolic and diastolic blood pressure, especially in younger subjects. The mechanism of hypertension in hypothyroidism is not completely understood: changes in circulating catecholamines, their receptors and the renin-angiotensin-aldosterone system have all been implicated. Effective treatment with thyroxine is readily available and inexpensive.