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Difficulty: MediumEndocrinology and Metabolism

For each patient presenting with an abnormality in calcium homeostasis, select the most likely underlying diagnosis based on their specific clinical presentation and laboratory results.

  • A 52-year-old man with recurrent nephrolithiasis. Serum calcium 11.4 mg/dL, phosphorus 2.1 mg/dL, intact PTH 115 pg/mL (Normal: 10-65 pg/mL).Primary hyperparathyroidism
  • A 68-year-old man with progressive fatigue and an estimated glomerular filtration rate of 22 mL/min/1.73 m². Serum calcium 7.6 mg/dL, phosphorus 6.8 mg/dL, intact PTH 450 pg/mL.Secondary hyperparathyroidism (renal origin)
  • A 35-year-old woman with perioral numbness and muscle cramping 48 hours after undergoing a total thyroidectomy. Serum calcium 7.2 mg/dL, phosphorus 5.5 mg/dL, intact PTH 8 pg/mL.Hypoparathyroidism
  • A 45-year-old woman with chronic malabsorptive diarrhea and generalized bone pain. Serum calcium 8.0 mg/dL, phosphorus 2.2 mg/dL, intact PTH 180 pg/mL.Osteomalacia

Answer

The presentations map to their respective physiologic disruptions in the calcium-PTH-vitamin D axis. Primary hyperparathyroidism features autonomous hypercalcemia. Secondary hyperparathyroidism (renal) features hypocalcemia with hyperphosphatemia. Post-surgical hypoparathyroidism features primary hypocalcemia with absent PTH. Osteomalacia features hypocalcemia with severe hypophosphatemia.
Each diagnosis can be derived by reading the calcium, PTH, and phosphorus axis in sequence. Primary hyperparathyroidism is the only presentation with hypercalcemia. Post-surgical hypoparathyroidism is the only presentation with a suppressed PTH. The two remaining hypocalcemic states are distinguished by phosphorus: elevated in renal failure and decreased in vitamin D deficiency/osteomalacia.

Step-by-Step Solution

1
Evaluate the serum calcium level first.
Identifies whether the primary state is hypercalcemic (left_1) or hypocalcemic (left_2, left_3, left_4).
Calcium is the primary driver of the regulatory axis. Its derangement points toward the central pathology.
2
Assess the intact PTH level in response to the calcium derangement.
Determines if the PTH is the primary problem or acting as a compensatory mechanism.
An elevated PTH in hypercalcemia is inappropriate (primary hyperparathyroidism). An elevated PTH in hypocalcemia is an appropriate compensatory response (secondary hyperparathyroidism). A low PTH in hypocalcemia is inappropriate (hypoparathyroidism).
3
Analyze the serum phosphorus level.
Differentiates between the causes of secondary hyperparathyroidism.
In renal failure, impaired excretion causes hyperphosphatemia. In vitamin D deficiency/osteomalacia, low intestinal absorption combined with the phosphaturic effect of the compensatory high PTH results in severe hypophosphatemia.

Key Concept

Disorders of Calcium and Phosphorus Metabolism
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Topics

Endocrinology and Metabolism

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