A 56-year-old postmenopausal woman is evaluated in the clinic after a routine metabolic pa… — USMLE Step 2 CK

A $56$ -year-old postmenopausal woman is evaluated in the clinic after a routine metabolic panel showed an elevated serum calcium level. She has no history of nephrolithiasis, bone fractures, abdominal pain, or psychiatric symptoms. She takes no medications and has no family history of significant medical conditions. Her vital signs are within normal limits. Physical examination reveals no palpable neck masses or other abnormalities.

Laboratory results are as follows:
Serum calcium: $11.2\text{ mg/dL}$ (normal: $8.4-10.2\text{ mg/dL}$ )
Serum albumin: $4.0\text{ g/dL}$ (normal: $3.5-5.0\text{ g/dL}$ )
Serum phosphorus: $2.3\text{ mg/dL}$ (normal: $2.5-4.5\text{ mg/dL}$ )
Intact parathyroid hormone: $92\text{ pg/mL}$ (normal: $10-65\text{ pg/mL}$ )
Serum creatinine: $0.9\text{ mg/dL}$ (normal: $0.6-1.2\text{ mg/dL}$ )
$25$ -hydroxyvitamin D: $35\text{ ng/mL}$ (normal: $20-50\text{ ng/mL}$ )

A dual-energy x-ray absorptiometry (DEXA) scan reveals a T-score of $-2.6$ at the lumbar spine and $-1.8$ at the femoral neck.

Which of the following is the most appropriate next step in the management of this patient?

A
Administration of intravenous normal saline
B
Sestamibi parathyroid scintigraphy
Measurement of 24-hour urinary calcium excretionAnswer
D
Measurement of serum parathyroid hormone-related peptide
E
Surgical referral for bilateral neck exploration

Answer

Measurement of 24-hour urinary calcium excretion is required to differentiate primary hyperparathyroidism from familial hypocalciuric hypercalcemia before proceeding to surgical planning.

The correct answer is measurement of 24-hour urinary calcium excretion. This patient has PTH-dependent hypercalcemia. While primary hyperparathyroidism is the most common cause, familial hypocalciuric hypercalcemia (FHH) can present identically on standard blood tests. A 24-hour urine calcium test (or urine calcium to creatinine clearance ratio) is required to differentiate the two. This step is mandatory before considering surgery because parathyroidectomy is curative for primary hyperparathyroidism (and indicated here due to her osteoporosis) but is strictly contraindicated and ineffective for FHH.

Step-by-Step Solution

Interpret the basic laboratory findings.

The patient has hypercalcemia (

11.2\text{ mg/dL}

) with an elevated intact parathyroid hormone (

92\text{ pg/mL}

) and low-normal phosphorus (

2.3\text{ mg/dL}

Establishing that the hypercalcemia is parathyroid hormone (PTH)-dependent.

Identify the differential diagnosis for PTH-dependent hypercalcemia.

The primary differential includes primary hyperparathyroidism and familial hypocalciuric hypercalcemia (FHH).

Both conditions present with elevated serum calcium and inappropriately normal or elevated PTH levels.

Determine the necessary test to distinguish between these conditions.

A 24-hour urinary calcium excretion test is indicated.

Primary hyperparathyroidism typically presents with high or normal urinary calcium, whereas FHH is characterized by low urinary calcium (urine calcium to creatinine clearance ratio

< 0.01

). This distinction is critical because primary hyperparathyroidism is treated surgically (especially with a T-score

\le -2.5

), whereas FHH is a benign condition that contraindicates parathyroidectomy.

Key Concept

In patients presenting with PTH-dependent hypercalcemia, a 24-hour urinary calcium excretion test must be performed to rule out familial hypocalciuric hypercalcemia (FHH) before firmly diagnosing primary hyperparathyroidism and planning surgical intervention.

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