Collision Diagnoses: Primary Hyperparathyroidism Layered on Familial Hypocalciuric Hypercalcemia

Objective: Describe the presentation, work-up, and management of a rare co-occurrence of familial hypocalciuric hypercalcemia (FHH) and primary hyperparathyroidism (PHPT) in a single patient. Additionally, to explore the challenges in diagnosing in such patients, summarize the previous literature on this uncommon occurrence, and explore the possible etiologies for it. Methods: Case presentation from a tertiary care academic medical system. Results: Successful diagnosis and surgical management, returning the patient to his previously asymptomatic FHH baseline. Conclusion: FHH and PHPT are both causes of hypercalcemia. Very rarely, these two conditions can occur simultaneously in the same patient. The relationship between these two entities when they present concurrently is complex, likely the result of particular CASR mutations causing greater parathyroid cellular proliferation and an increased predisposition to developing PHPT. These rare patients can be difficult to diagnose, but when recognized appropriately and managed with surgery for the PHPT component, they can achieve an asymptomatic FHH state.