Endocrine Abstracts

JOINT3872

Introduction: Primary hyperparathyroidism is a common pathology, with diagnosis based mainly on biological parameters alone. Localization should only be performed if surgery is indicated. We report a case of MAX syndrome mistaken for primary hyperparathyroidism in the presence of a parathyroid adenoma.

Case report: A 64-year-old postmenopausal woman with 7 years’ postmenopausal history, no personal or family history of lithium or thiazide diuretic use, consulted an endocrinologist for a parathyroid incidentaloma. The initial work-up revealed a biological profile of elevated calcemia (108 mg/l), elevated PTH (2 times normal) and hypophosphatemia, raising the initial suspicion of primary hyperparathyroidism without prior measurement of 24-hour calciuria. Given the presence of osteoporosis at bone mineral density, surgery was indicated. Subsequently, a 24 h calciuria was performed, which came back collapsed with a urinary calcium fraction lower than 0.01, thus rectifying the diagnosis of familial hypocalciuric hypercalcemia.

Discussion and conclusion: Marx syndrome or familial hypocalciuric hypercalcemia (FHH) is a genetic disorder caused by mutation of the calcium-sensitive receptor (CASR) gene, resulting in decreased receptor activity in response to serum calcium levels. This pathology is the first differential diagnosis to be ruled out before considering primary hyperthyroidism. The decisive biological element between these two entities is urinary calcium excretion over 24 h, with a ratio of calcium clearance to creatinine clearance of less than 0.01, indicating FHH. The coexistence of FHH and a parathyroid incidentaloma is possible, as in our case, and this is why imaging should always come last after establishing a diagnosis of primary hyperpathyroidism.