The modulatory effect of endogenous parathyroid hormone on the action of hydrochlorothiazide in pseudohypoparathyroidism type I.

We compared the effect of orally administered 100 mg of hydrochlorothiazide (HCTZ) among eight patients with pseudohypoparathyroidism (PHP) type I, 11 patients with idiopathic hypoparathyroidism (IHP), and 12 patients with primary hyperparathyroidism (1'HPT). Patients with PHP type I or with IHP were studied during the treatment with 1 alpha-hydroxylated metabolites of vitamin D3. HCTZ raised serum levels of calcium (Ca) in 1'HPT (P < 0.001) and PHP type I (P < 0.01) but did not increase urinary excretion of Ca. Serum parathyroid hormone (PTH) in PHP type I decreased (P < 0.02) after HCTZ administration in response to the increase in serum Ca. HCTZ did not raise serum levels of Ca in IHP but increased urinary excretion of Ca in this group (P < 0.01). HCTZ suppressed tubular reabsorption of phosphate (P) in IHP (P < 0.01) and 1'HPT (P < 0.05) but not in PHP type I. Urinary excretion of cAMP did not change after HCTZ administration in PHP type I, IHP, or 1'HPT. Endogenous PTH modulated the effects of HCTZ on Ca mobilization from bone and renal reabsorption of Ca in PHP type I with normal or high serum levels of PTH and in 1'HPT with high serum levels of PTH. The inhibitory effect of HCTZ on renal tubular reabsorption of P (probably from proximal tubules) was independent of PTH. The resistance to this inhibitory effect of HCTZ on P reabsorption in PHP type I suggested a proximal tubular dysfunction in this disorder.