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We evaluated the effect of hydrochlorothiazide in a sample of anuric patients on hemodialysis and found an increase in serum calcium, which occurred only in those with parathyroid hormone >300 pg/ml. This finding highlights the extra-renal effect of this diuretic and a possible role of parathyroid hormone in the mechanism. Thiazide diuretics are commonly used in patients with chronic kidney disease to treat hypertension. Their effects on calcium and bone metabolism are not well established, once calciuria may not fully explain levels of calcium and parathyroid hormone (PTH) in this population. A previous study has suggested that thiazides require the presence of PTH as a permissive condition for its renal action. In anuric patients, however, the role of PTH, if any, in the thiazide effect is unknown. To assess thiazide extra renal effect on serum calcium and whether such an effect is reliant on PTH, hydrochlorothiazide (HCTZ) 100 mg was given orally once a day to a sample of 19 anuric patients on hemodialysis for 2 weeks. Laboratories’ analyses were obtained in three phases: baseline, after diuretic use, and after a 2-week washout phase. We demonstrated that serum calcium (Ca) increased in ten patients (52.6%) after HCTZ use, returning to previous levels after the washout period. Out of the 19 patients, ten presented PTH ≥ 300 pg/ml, and Ca has increased in eight of them, whereas in the other nine patients with PTH