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Difficulty: HardEndocrinology and Metabolism

Match each clinical presentation of hypercalcemia to its most likely underlying pathophysiologic mechanism.

  • A 52-year-old woman with recurrent nephrolithiasis. Labs show serum calcium 11.2 mg/dL, phosphorus 2.2 mg/dL, intact PTH 85 pg/mL (normal 10-65), and 24-hour urine calcium 350 mg.Autonomous overproduction of parathyroid hormone
  • A 28-year-old asymptomatic man with no family history of kidney stones. Labs show serum calcium 11.0 mg/dL, phosphorus 3.0 mg/dL, intact PTH 58 pg/mL (normal 10-65), and 24-hour urine calcium 45 mg.Inactivating mutation of the calcium-sensing receptor
  • A 65-year-old man with a 40-pack-year smoking history and a central lung mass. Labs show serum calcium 13.5 mg/dL, phosphorus 2.0 mg/dL, and intact PTH 8 pg/mL (normal 10-65).Tumor production of a parathyroid hormone analog
  • A 45-year-old woman with a chronic dry cough and bilateral hilar lymphadenopathy. Labs show serum calcium 11.8 mg/dL, phosphorus 3.5 mg/dL, intact PTH 9 pg/mL (normal 10-65), and elevated 1,25-dihydroxyvitamin D.Macrophage-mediated extrarenal 1-alpha-hydroxylase activity

Answer

The correct matches pair the nephrolithiasis patient with autonomous parathyroid hormone overproduction, the asymptomatic man with low urine calcium to the calcium-sensing receptor mutation, the patient with a central lung mass to the parathyroid hormone analog (PTHrP) production, and the patient with hilar lymphadenopathy to macrophage-mediated 1-alpha-hydroxylase activity.
Each presentation strictly corresponds to a distinct pathophysiologic mechanism of hypercalcemia. Primary hyperparathyroidism exhibits autonomous PTH secretion resulting in hypercalciuria. FHH is characterized by a mutated calcium-sensing receptor mimicking hypocalcemia, causing renal calcium retention (hypocalciuria). Malignancy (squamous cell lung cancer) secretes PTHrP, an analog of PTH. Sarcoidosis macrophages express 1-alpha-hydroxylase, synthesizing excess active Vitamin D.

Step-by-Step Solution

1
Evaluate the intact PTH level in the setting of hypercalcemia to distinguish between PTH-dependent and PTH-independent causes.
The first two patients have high or inappropriately normal PTH (PTH-dependent), while the last two patients have suppressed PTH (PTH-independent).
Intact PTH is the primary branch point in the diagnostic algorithm for hypercalcemia.
2
Analyze the 24-hour urine calcium for the PTH-dependent cases.
The 52-year-old woman has high urine calcium (primary hyperparathyroidism), whereas the 28-year-old man has low urine calcium (familial hypocalciuric hypercalcemia).
Both conditions can present with hypercalcemia and normal/elevated PTH, but FHH features increased renal calcium reabsorption due to a defective calcium-sensing receptor, leading to hypocalciuria.
3
Examine the clinical context and secondary labs for the PTH-independent cases.
The man with the lung mass and hypophosphatemia has PTHrP-mediated hypercalcemia (humoral hypercalcemia of malignancy), while the woman with hilar lymphadenopathy has 1,25-dihydroxyvitamin D-mediated hypercalcemia (sarcoidosis).
Squamous cell carcinomas secrete PTHrP, which mimics PTH, causing hypophosphatemia. Granulomatous diseases involve macrophages that autonomously convert 25-hydroxyvitamin D to its active form, leading to increased intestinal calcium absorption.

Key Concept

The diagnostic workup of hypercalcemia relies on initial stratification by intact PTH, followed by 24-hour urine calcium for PTH-dependent causes, and specific markers (PTHrP, 1,25-OH Vitamin D) for PTH-independent causes.
Estimated Time:2m 0s
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Topics

Endocrinology and Metabolism

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