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Difficulty: MediumEndocrinology and Metabolism

Match the following patient laboratory profiles to the most likely underlying pathophysiologic mechanism of their calcium homeostasis abnormality.

Reference ranges:
Calcium: 8.4−10.28.4-10.2 mg/dL
Intact PTH: 10−6510-65 pg/mL
Phosphorus: 2.5−4.52.5-4.5 mg/dL
24-hour urine calcium: 100−300100-300 mg/day

  • Calcium 11.211.2 mg/dL, PTH 8585 pg/mL, 24-hour urine calcium 350350 mg/dayAutonomous overproduction of parathyroid hormone from a single adenoma
  • Calcium 11.111.1 mg/dL, PTH 7575 pg/mL, 24-hour urine calcium 4545 mg/dayInactivating mutation of the calcium-sensing receptor (CaSR)
  • Calcium 12.512.5 mg/dL, PTH 55 pg/mL, Phosphorus 2.12.1 mg/dLEctopic secretion of a peptide that activates the PTH receptor
  • Calcium 7.87.8 mg/dL, PTH 9090 pg/mL, 24-hour urine calcium 6060 mg/dayDecreased intestinal absorption of calcium leading to compensatory parathyroid hyperplasia

Answer

Profile 1 matches autonomous overproduction of PTH (Primary hyperparathyroidism). Profile 2 matches an inactivating mutation of the CaSR (Familial hypocalciuric hypercalcemia). Profile 3 matches ectopic secretion of a peptide that activates the PTH receptor (Humoral hypercalcemia of malignancy). Profile 4 matches decreased intestinal absorption of calcium (Secondary hyperparathyroidism).
Each laboratory profile distinctly matches a specific pathophysiologic mechanism of calcium imbalance. Primary hyperparathyroidism is characterized by autonomous PTH secretion causing hypercalcemia and hypercalciuria. FHH is caused by an inactivating CaSR mutation leading to hypercalcemia with paradoxical hypocalciuria. Humoral hypercalcemia of malignancy is driven by ectopic PTHrP, which suppresses endogenous PTH and causes hypophosphatemia. Finally, states of decreased intestinal calcium absorption (like vitamin D deficiency) lead to hypocalcemia and an appropriate compensatory rise in PTH (secondary hyperparathyroidism).

Step-by-Step Solution

1
Analyze the first laboratory profile (Calcium 11.2, PTH 85, Urine Ca 350).
The profile shows hypercalcemia, high PTH, and hypercalciuria.
This confirms PTH-dependent hypercalcemia. The high urine calcium differentiates primary hyperparathyroidism from familial hypocalciuric hypercalcemia (FHH).
2
Analyze the second laboratory profile (Calcium 11.1, PTH 75, Urine Ca 45).
The profile shows hypercalcemia, high PTH, and significant hypocalciuria.
Hypercalcemia with an inappropriately unsuppressed PTH and very low urine calcium (<100 mg/day) strongly suggests FHH, caused by an inactivating CaSR mutation.
3
Analyze the third laboratory profile (Calcium 12.5, PTH 5, Phos 2.1).
The profile shows severe hypercalcemia, suppressed PTH, and hypophosphatemia.
The suppressed intact PTH indicates a PTH-independent cause of hypercalcemia. The low phosphorus suggests a PTH-like effect, characteristic of PTHrP secretion in humoral hypercalcemia of malignancy.
4
Analyze the fourth laboratory profile (Calcium 7.8, PTH 90, Urine Ca 60).
The profile shows hypocalcemia, high PTH, and hypocalciuria.
Hypocalcemia stimulates a compensatory rise in PTH (secondary hyperparathyroidism). This is most often driven by poor intestinal calcium absorption due to vitamin D deficiency.

Key Concept

Differential diagnosis of calcium homeostasis disorders based on serum calcium, intact parathyroid hormone, and 24-hour urinary calcium excretion.
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Topics

Endocrinology and Metabolism

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