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Difficulty: HardEndocrinology and Metabolism

Match each of the following patient laboratory profiles to the most likely underlying pathophysiologic mechanism.

(Reference ranges: Serum Calcium: 8.4-10.2 mg/dL; Intact PTH: 10-65 pg/mL; Phosphorus: 2.5-4.5 mg/dL; 24-hour urine calcium: 100-300 mg/day).

  • Serum Calcium: 11.2 mg/dL, Intact PTH: 55 pg/mL, 24-hr urine calcium: 45 mg/dayInactivating mutation of the calcium-sensing receptor (CaSR)
  • Serum Calcium: 11.8 mg/dL, Intact PTH: 8 pg/mL, 1,25-dihydroxyvitamin D: elevated, Phosphorus: 3.8 mg/dLEctopic extra-renal 1-alpha-hydroxylase activity
  • Serum Calcium: 12.5 mg/dL, Intact PTH: 5 pg/mL, Phosphorus: 2.1 mg/dL, PTHrP: elevatedTumor secretion of a peptide homologous to the amino-terminal end of PTH
  • Serum Calcium: 11.5 mg/dL, Intact PTH: 120 pg/mL, Phosphorus: 2.2 mg/dL, 24-hr urine calcium: 350 mg/dayAutonomous overproduction of parathyroid hormone by a single hypercellular gland
  • Serum Calcium: 7.5 mg/dL, Intact PTH: 110 pg/mL, Phosphorus: 2.1 mg/dL, Alkaline phosphatase: 250 U/LDefective intestinal calcium absorption leading to secondary parathyroid hyperplasia

Answer

The accurate diagnostic match depends on differentiating PTH-dependent states (primary hyperparathyroidism, FHH) from PTH-independent states (malignancy, granulomatous disease) and identifying secondary responses to hypocalcemia (vitamin D deficiency).
Each laboratory profile distinctly isolates a core mechanism of calcium homeostasis disruption: a shifted CaSR set-point (FHH), unregulated endogenous hormone production (Primary HPT), ectopic vitamin D activation (Sarcoidosis), ectopic peptide secretion (HHM), or appropriate secondary response to nutrient deficiency (Osteomalacia).

Step-by-Step Solution

1
Evaluate the intact PTH level relative to serum calcium.
Identifies if the condition is PTH-mediated (high/normal PTH with high calcium) or PTH-independent (suppressed PTH with high calcium).
Normal physiological response to hypercalcemia is complete suppression of PTH. Unsuppressed PTH indicates the parathyroid gland is the source or the set-point is altered.
2
For PTH-dependent hypercalcemia, check the 24-hour urine calcium.
Differentiates Primary Hyperparathyroidism (high/normal urine calcium) from Familial Hypocalciuric Hypercalcemia (very low urine calcium).
Both present with high calcium and unsuppressed PTH, but FHH features a mutant CaSR that increases renal calcium reabsorption.
3
For PTH-independent hypercalcemia, evaluate PTHrP and vitamin D metabolites.
Differentiates humoral hypercalcemia of malignancy (high PTHrP) from granulomatous diseases (high 1,25-dihydroxyvitamin D).
Different ectopic mechanisms cause the hypercalcemia, requiring distinct therapeutic interventions.

Key Concept

Differential diagnosis of calcium and phosphorus metabolic disorders using intact PTH, urine calcium, and specific systemic markers.
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Topics

Endocrinology and Metabolism

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