Thiazide diuretics reduce urinary excretion of calcium by about 40% in patients with intact parathyroid glands (normal parathyroid function or primary hyperparathyroidism). Despite equivalent sodium losses, thiazide diuretics do not lower calcium excretion in patients with hypoparathyroidism. Thiazide diuretics diminish hydroxyproline excretion by about 22%, and they do not affect 3′5′-cyclic AMP excretion in subjects with intact parathyroid glands. These findings, together with additional data in the literature, suggest that thiazides may potentiate the renal action of parathyroid hormone, resulting in retention of calcium and partial suppression of parathyroid hormone secretion.