Influence of short-time application of a low sodium diet on blood pressure in patients with hyperthyroidism or hypothyroidism during therapy.

Hyperthyroidism or hypothyroidism are commonly associated with altered blood pressure (BP). Restriction of sodium in the diet produces a decrease in BP in some individuals. It is also well known that hormones other than thyroid affect BP. The present study was designed to evaluate the influence of a low sodium diet on BP in patients with hyperthyroidism or hypothyroidism during therapy. The occurrence of salt-sensitive or salt-nonsensitive BP was compared with hormonal levels (plasma renin activity, aldosterone, atrial natriuretic peptide, and arginine vasopressin). Patients with hyperthyroidism (75 subjects) were investigated before the initiation of treatment, 2 weeks after the treatment, and after the attainment of euthyroid state. Patients with hypothyroidism (31 subjects) were studied before the treatment and in the euthyroid state. Control values were obtained from 37 healthy individuals. Blood pressure, changes of plasma volume, serum aldosterone, atrial natriuretic peptide, vasopressin levels, and plasma renin activity were measured in all investigated subjects after application of a normal sodium diet and after 3 days on a low sodium diet. Elevated systolic BP was found in patients with hyperthyroidism and hypothyroidism. Mean arterial BP was higher only in the untreated hypothyroid patients. The high incidence of salt-sensitive BP was found only in untreated hypothyroid patients. Also in hypothyroid patients the application of a low sodium diet led to a lower increase in plasma renin activity in subjects with salt-sensitive BP than in individuals with salt-resistant BP. Therefore, different mechanisms are responsible for BP elevation in patients with hyperthyroidism or hypothyroidism.