During an annual comprehensive health screening, an asymptomatic 48-year-old man is found … — USMLE Step 3

During an annual comprehensive health screening, an asymptomatic $48$ -year-old man is found to have elevated serum calcium levels. He has no history of nephrolithiasis, bone pain, abdominal discomfort, or psychiatric symptoms. He takes no medications, including over-the-counter supplements. His family history is notable for a brother who was told he had 'calcium problems' but never required surgical intervention. Vital signs and physical examination are unremarkable.

Subsequent laboratory investigations reveal the following:

Test	Result	Reference Range
Calcium, total	$11.3\ mg/dL$	$8.4 - 10.2\ mg/dL$
Phosphorus	$2.2\ mg/dL$	$2.5 - 4.5\ mg/dL$
Albumin	$4.1\ g/dL$	$3.5 - 5.0\ g/dL$
Creatinine	$0.9\ mg/dL$	$0.7 - 1.3\ mg/dL$
Intact parathyroid hormone (PTH)	$78\ pg/mL$	$15 - 65\ pg/mL$
25-hydroxyvitamin D	$32\ ng/mL$	$20 - 50\ ng/mL$
24-hour urine calcium	$50\ mg/24h$	$100 - 300\ mg/24h$

Which of the following is the most likely diagnosis?

A
Humoral hypercalcemia of malignancy
B
Vitamin D toxicity
C
Primary hyperparathyroidism
Familial hypocalciuric hypercalcemiaAnswer
E
Secondary hyperparathyroidism

Answer

Familial hypocalciuric hypercalcemia

The patient has hypercalcemia, hypophosphatemia, and an inappropriately elevated parathyroid hormone (PTH) level, suggesting a PTH-dependent hypercalcemia. However, his 24-hour urine calcium excretion is markedly reduced. This combination of mild, asymptomatic hypercalcemia, normal or slightly elevated PTH, and prominent hypocalciuria is the hallmark of familial hypocalciuric hypercalcemia (FHH). FHH is an autosomal dominant disorder caused by an inactivating mutation in the calcium-sensing receptor (CaSR) in the parathyroid glands and kidneys. The defective receptors require higher serum calcium concentrations to suppress PTH release, and they simultaneously promote excessive renal tubular reabsorption of calcium. This leads to the characteristic hypocalciuria, which is the primary feature distinguishing FHH from primary hyperparathyroidism.

Step-by-Step Solution

Analyze the serum calcium, phosphorus, and parathyroid hormone (PTH) levels.

Serum calcium is elevated (

11.3\ mg/dL

), phosphorus is low (

2.2\ mg/dL

), and PTH is elevated (

78\ pg/mL

Establishing the dependency of hypercalcemia on PTH is the first diagnostic step. Elevated PTH in the setting of hypercalcemia indicates a PTH-dependent process, narrowing the differential diagnosis primarily to primary hyperparathyroidism and familial hypocalciuric hypercalcemia.

Evaluate the 24-hour urine calcium excretion.

The 24-hour urine calcium is abnormally low (

50\ mg/24h

Urinary calcium excretion is the key parameter differentiating the causes of PTH-dependent hypercalcemia. A low value strongly points toward altered renal calcium handling independent of the filtered load.

Synthesize the laboratory findings with the patient's history.

The combination of asymptomatic PTH-dependent hypercalcemia, significant hypocalciuria, and a family history of benign 'calcium problems' without nephrolithiasis points to a genetic etiology.

Familial hypocalciuric hypercalcemia is an autosomal dominant condition characterized by this exact laboratory pattern and does not require parathyroidectomy, making accurate lab interpretation critical to avoid unnecessary surgery.

Key Concept

Differentiating Familial Hypocalciuric Hypercalcemia (FHH) from Primary Hyperparathyroidism using 24-hour urine calcium levels.

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