A Hypercalcemic Deception: Uncovering an Unusual Case of Familial Hypocalciuric Hypercalcemia

Although a rare cause of hypercalcemia, familial hypocalciuric hypercalcemia (FHH) is an inherited condition most often due to a missense mutation in the calcium-sensing receptor (CASR) gene, giving rise to increased calcium levels with elevated parathyroid hormone (PTH) levels and hypocalciuria. Many clinical features of FHH show a high degree of overlap with the much more common disorder of primary hyperparathyroidism (PHPT), making the correct diagnosis a challenge since surgery should be avoided in FHH. In the current case, although PHPT was initially suspected based on urine biochemistry and imaging, FHH-1 and metastatic micropapillary thyroid cancer were diagnosed following thyroid surgery. Moreover, an underlying novel missense CASR mutation shared with the patient's biological father was uncovered. This case highlights the challenge of making the correct hypercalcemic diagnosis without genetic tools. The concurrence with thyroid cancer has been reported once before, albeit not with this novel CASR mutation.