FAMILIAL HYPOCALCIURIC HYPERCALCEMIA

Familial Hypocalciuric Hypercalcemia — a benign condition that mimics primary hyperparathyroidism, where molecular CASR diagnosis prevents the unnecessary parathyroid surgery that FHH patients are frequently subjected to.

Whole genome sequencing identifies all CASR variants — distinguishing FHH (no surgery needed) from primary hyperparathyroidism (surgery curative) — the single test that prevents unnecessary operations in patients with hypercalcemia.

CASR

About this condition

Familial Hypocalciuric Hypercalcemia

Familial hypocalciuric hypercalcemia (FHH) is an autosomal dominant condition caused by heterozygous inactivating variants in CASR (calcium-sensing receptor, chromosome 3q13.33) — the receptor that detects serum calcium levels and regulates PTH secretion and renal calcium excretion. CASR haploinsufficiency shifts the calcium setpoint upward: the parathyroid glands interpret normal calcium as 'low' and maintain PTH secretion, producing mild-to-moderate hypercalcemia with inappropriately normal or mildly elevated PTH — a biochemical profile indistinguishable from primary hyperparathyroidism (PHPT).

FHH affects approximately 1 in 10,000-50,000 people and is almost always asymptomatic — the hypercalcemia is benign and does not cause the nephrolithiasis, osteoporosis, or neurocognitive symptoms seen in PHPT. The critical diagnostic feature is relative hypocalciuria: FHH patients have calcium-to-creatinine clearance ratio (CCCR) <0.01, while PHPT patients typically have CCCR >0.02. However, there is significant overlap in the 0.01-0.02 range, and many patients with FHH are misdiagnosed as PHPT and referred for parathyroidectomy.

Parathyroidectomy does NOT correct hypercalcemia in FHH — because the abnormality is in the calcium-sensing receptor, not the parathyroid gland. FHH patients who undergo parathyroidectomy remain hypercalcemic, may undergo re-exploration (with additional surgical risk), and may eventually have total parathyroidectomy causing permanent hypoparathyroidism requiring lifelong calcium and calcitriol replacement. This entirely preventable surgical harm occurs when FHH is not considered in the differential of hypercalcemia with elevated PTH. Molecular CASR genotyping provides the definitive distinction.

Homozygous CASR inactivation causes neonatal severe hyperparathyroidism (NSHPT) — a neonatal emergency requiring urgent total parathyroidectomy. FHH parents should be identified before pregnancy to assess NSHPT risk in offspring of two carriers.

Gene locus: CASR (3q13.33)

FHH is misdiagnosed as primary hyperparathyroidism in up to 10% of surgical series. Molecular CASR testing prevents unnecessary parathyroidectomy—one of the clearest examples of genotype-guided surgical decision-making.

Parathyroidectomy does not fix FHH — molecular diagnosis prevents surgery that cannot help and may cause hypoparathyroidism

In surgical series, approximately 5-10% of patients referred for parathyroidectomy actually have FHH rather than PHPT. These patients undergo surgery that does not correct their hypercalcemia — because the calcium setpoint abnormality is in CASR, not the parathyroid glands. Failed first surgery may lead to re-exploration, and ultimately total parathyroidectomy causing permanent hypoparathyroidism. CASR genotyping before surgery identifies FHH patients, preventing this entirely avoidable iatrogenic harm. Whole genome sequencing provides CASR evaluation alongside comprehensive genomic analysis.

Calcium-to-creatinine clearance ratio has a gray zone of 0.01-0.02 — only molecular testing provides a definitive answer in equivocal cases

The CCCR cutoff of <0.01 for FHH and >0.02 for PHPT leaves a substantial gray zone where biochemistry cannot distinguish the two conditions. Factors affecting urinary calcium (vitamin D status, dietary calcium, renal function, diuretic use) further confound the CCCR calculation. In a patient with mild hypercalcemia, borderline PTH, and a CCCR in the equivocal range, molecular CASR genotyping provides the definitive diagnosis — FHH (no surgery) vs. PHPT (surgery curative). This single test resolves diagnostic ambiguity that would otherwise lead to either inappropriate surgery or unnecessary surveillance.

Familial Hypocalciuric Hypercalcemia — a benign condition that mimics primary hyperparathyroidism, where molecular CASR diagnosis prevents the unnecessary parathyroid surgery that FHH patients are frequently subjected to.

Familial Hypocalciuric Hypercalcemia

FHH is misdiagnosed as primary hyperparathyroidism in up to 10% of surgical series. Molecular CASR testing prevents unnecessary parathyroidectomy—one of the clearest examples of genotype-guided surgical decision-making.

Parathyroidectomy does not fix FHH — molecular diagnosis prevents surgery that cannot help and may cause hypoparathyroidism

Calcium-to-creatinine clearance ratio has a gray zone of 0.01-0.02 — only molecular testing provides a definitive answer in equivocal cases

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