Retrospectively diagnosed familial hypocalciuric hypercalcaemia following total parathyroidectomy in an asymptomatic patient.

Background: Familial hypocalciuric hypercalcemia (FHH) is a rare, benign condition that shares characteristics with primary hyperparathyroidism (PHPT), a more sinister condition that requires surgical intervention. This case report demonstrates misdiagnosis of FHH and highlights important learning points to prevent this in the future. Case Presentation: Hypercalcaemia was incidentally discovered in a 21-year-old patient who had no symptoms of hypercalcaemia and no significant family history. Clinical examination was normal. Biochemical investigations revealed hypercalcaemia of 2.84mmol/L (2.15 - 2.50mmol/L) and hypophosphataemia of 0.71mmol/L (0.78 - 1.42mmol/L). Parathyroid hormone (PTH) concentration was mildly and inappropriately elevated (10.3pmol/L [2.0 - 8.5pmol/L]) triggering a suspicion of PTH-mediated hypercalcaemia. Parathyroid scintigraphy reported an ill-defined area of focal uptake above the left thyroid lobe. Fractional excretion of calcium estimations on 24hour urine collections were borderline (0.01) for FHH on multiple occasions however, further investigations to exclude FHH were not performed before a diagnosis of primary hyperparathyroidism was made, and a total parathyroidectomy performed. Several months post-operatively, the patient still demonstrated persistent hypercalcaemia. Her siblings had since been diagnosed with FHH. The patient was then retrospectively diagnosed with FHH. Genetic testing for FHH is not available in South Africa which limited the opportunity to confirm the diagnosis. Conclusions: This case report provides a classical presentation of the rare, benign disorder of FHH. It highlights the negative outcomes that may result from misdiagnosis of this condition as PHPT. Biochemical investigations play an integral role in differentiating these conditions. Effective clinician-laboratory communication is crucial for optimal patient outcomes.