24-Hour Urinary Calcium in Familial Hypocalciuric Hypercalcemia
In familial hypocalciuric hypercalcemia (FHH), the typical 24-hour urinary calcium excretion is within the normal range—essentially the same as healthy subjects—despite the misleading name "hypocalciuric." 1
Specific Values and Clinical Context
Normal Range Definition
- Median 24-hour urinary calcium in FHH: 2.8 mmol/24 hours (IQR: 1.9–4.0 mmol/24 hours), which translates to approximately 112–160 mg/24 hours 2
- Another study reported a median of 3.2 mmol/24 hours (IQR: 2.1–6.1 mmol/24 hours) in genetically proven FHH 3
- Individual case reports document values as low as 69.6 mg/24 hours 4
The "Hypocalciuric" Misnomer
The label "hypocalciuric" is not justified; FHH subjects have the same calciuria as normal subjects. 1 The original description emphasized lower calciuria compared to primary hyperparathyroidism (PHPT), but this difference exists only relative to PHPT—not relative to healthy individuals. 1
Critical Diagnostic Pitfall
Overlap with Primary Hyperparathyroidism
The range of calciuria values in FHH and PHPT overlaps substantially, making it very difficult to distinguish the two conditions on 24-hour urinary calcium alone. 1 For comparison:
- PHPT median: 8.3 mmol/24 hours (IQR: 5.6–11.2 mmol/24 hours) 3
- Normal upper limits: <7.5 mmol/24 hours in men (300 mg) and <6.25 mmol/24 hours in women (250 mg) 1
Why 24-Hour Urine Calcium Alone Is Insufficient
- Using a cut-off of <2.5 mmol/24 hours (<100 mg/24 hours) to diagnose FHH yields only 29.4% specificity for FHH 3
- This means that a significant proportion of PHPT patients would also fall below this threshold, particularly those with vitamin D deficiency, renal insufficiency, low calcium intake, or thiazide use 1
- The negative predictive value for FHH using 24-hour urinary calcium is only 95.2%, meaning it cannot reliably exclude FHH 5
The Superior Diagnostic Test: Fractional Excretion of Calcium (FeCa)
Calculate FeCa using the formula: FeCa = (urine Ca × plasma creatinine) / (urine creatinine × plasma Ca) × 100 6, 7, 3
Diagnostic Thresholds
- FeCa ≤ 2% strongly suggests FHH 6, 7, 3
- FeCa > 2% confirms PHPT 6, 7, 3
- At a cut-off of ≤0.015 (1.5%), the negative predictive value for FHH reaches 100% 5
Why FeCa Outperforms 24-Hour Urine Calcium
FeCa provides superior discrimination because it accounts for renal function and glomerular filtration rate, whereas absolute 24-hour urinary calcium does not. 6, 3, 5 The specificity of FeCa for FHH is 93% compared to only 29.4% for 24-hour urinary calcium 3, 5
Clinical Algorithm for Distinguishing FHH from PHPT
- Confirm hypercalcemia with elevated or inappropriately normal PTH 6, 7
- Calculate FeCa from a spot or 24-hour urine collection 6, 7, 3
- Obtain 24-hour urinary calcium only as a supplementary measure, recognizing its limited specificity 3, 5, 8
- Exclude secondary causes: vitamin D deficiency, renal insufficiency, thiazide diuretics, low calcium intake, and malabsorption 1
Additional Clinical Pearls
Atypical Presentations
Up to 43% of genetically confirmed FHH cases present with atypical features, including chondrocalcinosis (22%), bone fractures (14%), renal colic (11%), osteoporosis (16%), and kidney stones (28%) 2, 4 These findings underscore that FHH is not always benign and asymptomatic.
Surgical Implications
FHH does not require parathyroidectomy; surgery is ineffective and unnecessary. 6, 7, 5 However, 18% of FHH patients in one cohort underwent inappropriate parathyroid surgery before the correct diagnosis was established 2 This highlights the critical importance of calculating FeCa before referring any patient for parathyroidectomy.
Genetic Confirmation
When FeCa is ≤ 2%, genetic testing should be pursued to confirm FHH and prevent unnecessary surgery. 6, 5, 9 FHH1 (due to CASR mutations) accounts for the majority of cases, while FHH2 (GNA11) and FHH3 (AP2S1) are rarer 1, 10