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

## TL;DR

A rare genetic condition causing high blood calcium was misdiagnosed as a more common disorder until genetic testing revealed a new mutation and thyroid cancer.

## Contribution

A novel missense mutation in the CASR gene was identified in a patient with FHH-1 and metastatic thyroid cancer.

## Key findings

- The patient was initially suspected of having primary hyperparathyroidism but was later diagnosed with FHH-1.
- A novel CASR mutation was found to be shared with the patient’s biological father.
- The case highlights the importance of genetic testing in distinguishing FHH from similar disorders.

## Abstract

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.