Hyperphosphatemic familial tumoral calcinosis: Response to acetazolamide and postulated mechanisms

Hyperphosphatemic familial tumoral calcinosis (HFTC) is characterized by enhanced renal phosphate absorption, hyperphosphatemia, and tumor‐like extraosseous calcifications due to inactivating mutations in FGF23 or associated proteins. Surgical excision is needed when low phosphate diet and phosphate binders are ineffective. Sporadic reports have supported acetazolamide use. We report on a 7‐year‐old African American boy who presented with severe HFTC requiring numerous surgical excisions. Tumors continued to appear and others reoccurred despite phosphate restriction and sevelamer carbonate. At the age of 9.5 years, acetazolamide (40 mg/kg/day) was added and resulted in mild metabolic acidosis (bicarbonate 25.3 mEq/L vs. 21.4 mEq/L, P