THE MODULATORY EFFECT OF ENDOGENOUS PARATHYROID-HORMONE ON THE ACTIONOF HYDROCHLOROTHIAZIDE IN PSEUDOHYPOPARATHYROIDISM TYPE-I

We compared the effect of orally administered 100 mg of hydrochlorothi azide (HCTZ) among eight patients with pseudohypoparathyroidism (PHP) type I, 11 patients with idiopathic hypoparathyroidism (IHP), and 12 p atients with primary hyperparathyroidism (1 degrees HPT). Patients wit h PHP type I or with IHP were studied during the treatment with 1 alph a-hydroxylated metabolites of vitamin D,. HCTZ raised serum levels of calcium (Ca) in 1 degrees HPT (P < 0.001) and PHP type I (P < 0.01) bu t 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 degrees HPT (P < 0.05) but not in PHP type I. Urinary e xcretion of cAMP did not change after HCTZ administration in PHP type I, IHP, or 1 degrees 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 degrees HPT with h igh serum levels of PTH. The inhibitory effect of HCTZ on renal tubula r reabsorption of P (probably from proximal tubules) was independent o f PTH. The resistance to this inhibitory effect of HCTZ on P reabsorpt ion in PHP type I suggested a proximal tubular dysfunction in this dis order.