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Difficulty: MediumInterpreting Laboratory Results

Match the following combinations of serum laboratory profiles to their most likely underlying pathophysiologic diagnosis. (Reference ranges: Calcium 8.4–10.2 mg/dL, Phosphorus 2.5–4.5 mg/dL, Intact PTH 10–65 pg/mL)

  • Ca: 11.2 mg/dL, PO4: 2.1 mg/dL, PTH: 145 pg/mLPrimary hyperparathyroidism
  • Ca: 7.6 mg/dL, PO4: 2.0 mg/dL, PTH: 180 pg/mLVitamin D deficiency
  • Ca: 7.8 mg/dL, PO4: 5.8 mg/dL, PTH: 350 pg/mLAdvanced chronic kidney disease
  • Ca: 7.5 mg/dL, PO4: 6.0 mg/dL, PTH: 8 pg/mLPost-surgical hypoparathyroidism

Answer

Profile 1 matches Primary hyperparathyroidism. Profile 2 matches Vitamin D deficiency. Profile 3 matches Advanced chronic kidney disease. Profile 4 matches Post-surgical hypoparathyroidism.
Each laboratory pattern specifically reflects the fundamental underlying pathophysiology of calcium and phosphate homeostasis. Intact PTH, calcium, and phosphorus must be interpreted simultaneously to differentiate primary parathyroid disorders from compensatory secondary responses to renal or nutritional deficits.

Step-by-Step Solution

1
Analyze the parathyroid hormone (PTH) level to categorize the disorder as primary PTH-driven or a secondary compensatory response.
Profile 4 has low PTH, while Profiles 1, 2, and 3 have elevated PTH.
An inappropriately low PTH in the setting of hypocalcemia indicates primary hypoparathyroidism, whereas high PTH can be primary (driving the disease) or secondary (reacting to hypocalcemia).
2
Evaluate the serum calcium level in relation to the elevated PTH profiles.
Profile 1 exhibits hypercalcemia with high PTH, indicating primary hyperparathyroidism. Profiles 2 and 3 show hypocalcemia with high PTH, indicating secondary hyperparathyroidism.
In primary hyperparathyroidism, the parathyroid gland is autonomously overproducing hormone, causing hypercalcemia. In secondary hyperparathyroidism, the gland is responding appropriately to a hypocalcemic state.
3
Differentiate the causes of secondary hyperparathyroidism (Profiles 2 and 3) by evaluating the serum phosphorus level.
Profile 2 shows low phosphorus (matching Vitamin D deficiency), while Profile 3 shows high phosphorus (matching advanced chronic kidney disease).
Vitamin D deficiency causes poor intestinal absorption of both calcium and phosphorus. Conversely, in chronic kidney disease, the failing kidneys are unable to excrete phosphorus, leading to marked hyperphosphatemia.

Key Concept

Diagnosis of calcium and phosphate metabolism disorders using intact PTH and basic metabolic panel laboratory results.
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Topics

Interpreting Laboratory Results

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