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Difficulty: Very hardEndocrinology and Metabolism

A 64-year-old woman is evaluated in the clinic following routine laboratory testing that revealed hypercalcemia. She has no symptoms of fatigue, abdominal pain, bone pain, or cognitive changes. Her medical history is significant for essential hypertension and osteopenia. Her medications include chlorthalidone and cholecalciferol. Blood pressure is 132/82 mm Hg, and pulse is 72/min. Physical examination is unremarkable.

Laboratory results are as follows:

TestResultReference Range
Calcium11.2 mg/dL11.2 \text{ mg/dL}8.4−10.2 mg/dL8.4-10.2 \text{ mg/dL}
Phosphorus2.6 mg/dL2.6 \text{ mg/dL}2.5−4.5 mg/dL2.5-4.5 \text{ mg/dL}
Intact parathyroid hormone78 pg/mL78 \text{ pg/mL}15−65 pg/mL15-65 \text{ pg/mL}
25-hydroxyvitamin D36 ng/mL36 \text{ ng/mL}20−50 ng/mL20-50 \text{ ng/mL}
Creatinine0.8 mg/dL0.8 \text{ mg/dL}0.6−1.2 mg/dL0.6-1.2 \text{ mg/dL}
24-hour urine calcium85 mg85 \text{ mg}100−300 mg100-300 \text{ mg}
24-hour urine creatinine1000 mg1000 \text{ mg}800−1800 mg800-1800 \text{ mg}

The calculated calcium-to-creatinine clearance ratio is 0.0060.006.

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

  1. Discontinue chlorthalidone and repeat laboratory testingAnswer
  2. B
    Perform genetic testing for CASR mutations
  3. C
    Order technetium-99m sestamibi scintigraphy
  4. D
    Refer for surgical parathyroidectomy
  5. E
    Measure serum 1,25-dihydroxyvitamin D levels

Answer

The most appropriate next step is to discontinue chlorthalidone and repeat the laboratory testing.
The most appropriate next step in management is to discontinue the thiazide diuretic (chlorthalidone) and repeat the laboratory testing in several weeks. This patient presents with PTH-dependent hypercalcemia (elevated serum calcium with an unsuppressed, elevated PTH). The primary differential for PTH-dependent hypercalcemia includes primary hyperparathyroidism, familial hypocalciuric hypercalcemia (FHH), and medication-induced hypercalcemia (e.g., lithium, thiazide diuretics). To differentiate between these, a 24-hour urine calcium and a calcium-to-creatinine clearance ratio (CCCR) are calculated. A CCCR of <0.01< 0.01 indicates profound hypocalciuria. While this is the classic biochemical hallmark of FHH, thiazide diuretics act on the distal convoluted tubule to increase calcium reabsorption, producing an identical laboratory profile. Therefore, before pursuing genetic testing for FHH or evaluating for primary hyperparathyroidism, the confounding medication must be stopped, and the calcium profile reassessed to determine the true underlying physiology.

Step-by-Step Solution

1
Analyze the serum calcium and intact PTH levels.
The patient has hypercalcemia (11.2 mg/dL11.2 \text{ mg/dL}) with an inappropriately elevated PTH (78 pg/mL78 \text{ pg/mL}).
This identifies the hypercalcemia as PTH-dependent, narrowing the differential to primary hyperparathyroidism, familial hypocalciuric hypercalcemia (FHH), and drug-induced causes.
2
Evaluate the 24-hour urine calcium and the calcium-to-creatinine clearance ratio (CCCR).
The 24-hour urine calcium is low (85 mg85 \text{ mg}) and the CCCR is 0.0060.006 (<0.01< 0.01).
A CCCR <0.01< 0.01 strongly suggests enhanced renal calcium reabsorption, which is characteristic of FHH or thiazide diuretic use, and makes primary hyperparathyroidism (typically CCCR >0.02> 0.02) highly unlikely.
3
Correlate the biochemical findings with the patient's medication list.
The patient is actively taking chlorthalidone, a thiazide diuretic.
Thiazides act on the distal convoluted tubule to decrease calcium excretion, producing an identical laboratory profile to FHH (mild hypercalcemia, high-normal or elevated PTH, and profound hypocalciuria).
4
Determine the most appropriate initial management step.
Discontinue the chlorthalidone and reassess.
The pharmacological confounder must be removed to establish an accurate baseline diagnosis before pursuing expensive genetic testing or invasive surgical evaluations.

Key Concept

Thiazide-induced hypercalcemia and its mimicry of Familial Hypocalciuric Hypercalcemia (FHH)
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Topics

Endocrinology and Metabolism

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