Fractional Excretion of Calcium < 2 % in Hypercalcemia: Diagnostic and Management Guidance

Diagnostic Interpretation of FeCa

A fractional excretion of calcium (FeCa) ≤ 2 % strongly favors familial hypocalciuric hypercalcemia (FHH) over primary hyperparathyroidism (PHPT); this distinction is critical because FHH is a benign genetic condition that does not benefit from parathyroidectomy. 1
FeCa > 2 % is diagnostic for PHPT and identifies patients who are appropriate candidates for surgical treatment, whereas FeCa ≤ 2 % indicates FHH and should preclude unnecessary parathyroidectomy. 1
FeCa is calculated as FeCa = (urine calcium × plasma creatinine) / (urine creatinine × plasma calcium) × 100; this ratio provides superior discrimination between PHPT and FHH compared with a simple 24‑hour urinary calcium measurement. 1

Vitamin D status must be verified; a 25‑hydroxyvitamin D level > 30 ng/mL maintained for at least six months is required because deficiency can elevate PTH and falsely lower FeCa. 1
Inadequate dietary calcium intake (less than ≈ 800–1000 mg per day) can cause secondary hyperparathyroidism with relative hypocalciuria, potentially mimicking FHH. 1
Serum creatinine and estimated glomerular filtration rate should be measured to exclude chronic kidney disease, which alters calcium handling and may reduce urinary calcium excretion, leading to a low FeCa. 1

Even when FeCa ≤ 2 %, 24‑hour urinary calcium may be normal or elevated in FHH; therefore, urinary calcium alone is insufficient for differentiation, and FeCa remains the preferred discriminator. 1
An oral calcium loading test can be employed when imaging identifies a parathyroid lesion but FeCa remains ≤ 2 %; a failure of PTH to suppress after calcium administration supports a diagnosis of PHPT rather than FHH. 1

Step 1 – Accurate Measurement: Calculate FeCa using simultaneous blood and urine specimens, preferably from a 24‑hour urine collection, to ensure reliable results. 1
Step 2 – Genetic Confirmation: If FeCa ≤ 2 %, order genetic testing for pathogenic variants in the calcium‑sensing receptor (CASR), GNA11, or AP2S1 to confirm FHH.
Step 3 – Family Screening: Screen first‑degree relatives for mild hypercalcemia and determine their FeCa, given the autosomal‑dominant inheritance and high penetrance of FHH.
Step 4 – Exclude Modifiable Causes: Correct any vitamin D deficiency, optimize dietary calcium intake, assess renal function, and review medications (e.g., thiazide diuretics) that could lower FeCa.
Step 5 – Management of Confirmed FHH: Reassure patients that FHH is benign and does not require surgery; monitor serum calcium annually, and consider cinacalcet only if calcium exceeds the upper normal limit by > 1 mg/dL (≈ 0.25 mmol/L) or if hypercalcemia‑related symptoms develop.
Step 6 – Persistent PHPT Suspicion: When genetic testing is negative or unavailable and PHPT remains likely, perform parathyroid imaging (ultrasound + 99mTc‑sestamibi or 18F‑choline PET/CT). If an adenoma is identified, confirm autonomous PTH secretion with an oral calcium loading test before referring for surgery. 1

Offer genetic counseling and cascade testing to first‑degree relatives once FHH is diagnosed, because the disorder follows an autosomal‑dominant inheritance pattern with high penetrance. Evidence: Expert consensus (moderate). 2
Confirm the diagnosis with molecular testing for pathogenic variants in CASR, GNA11, or AP2S1 when clinical suspicion is strong. Evidence: Expert consensus (moderate). 2 & 3

Calculate the calcium‑to‑creatinine clearance ratio (CCCR); a value below 0.01 strongly supports FHH over primary hyperparathyroidism. Evidence: Diagnostic guideline (high). 4
Perform a 24‑hour urinary calcium excretion measurement; while relative hypocalciuria is typical for FHH, overlapping ranges with primary hyperparathyroidism limit its standalone diagnostic utility. Evidence: Observational data (moderate). 3
Do not rely exclusively on urinary calcium levels to differentiate the two conditions, because the “hypocalciuric” label can be misleading—FHH patients often have normal urinary calcium compared with healthy individuals, whereas primary hyperparathyroidism usually shows elevated urinary calcium. Evidence: Expert consensus (moderate). 3

Consider cinacalcet in adult FHH patients when serum calcium is more than 1 mg/dL (≈0.25 mmol/L) above the upper limit of normal or when hypercalcemia‑related symptoms are present. Evidence: Expert opinion (low). 2
Recognize that calcimimetic efficacy is markedly reduced in patients with bi‑allelic CASR mutations compared with heterozygous carriers. Evidence: Case series (moderate). 2

Use cinacalcet as the first‑line therapy for infants with life‑threatening hypercalcemia due to homozygous or specific heterozygous CASR mutations, as it has largely supplanted surgical approaches. Evidence: Case series and expert consensus (moderate). 2 & 3
Reserve parathyroidectomy for neonates who fail to respond to medical therapy; it is now rarely required. Evidence: Expert consensus (low). 2
Bisphosphonates may be employed as an alternative, but their efficacy is variable and they can provoke rebound increases in PTH and calcium after discontinuation. Evidence: Case series (low). 2

During breastfeeding, monitor maternal calcium levels every 4–8 weeks to detect rises caused by combined effects of mammary PTHrP and circulating PTH. Evidence: Expert consensus (low). 4
In newborns of mothers with FHH, screen for low calcium levels (hypocalcemia) secondary to suppressed fetal parathyroid activity. Evidence: Expert consensus (low). 4