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Difficulty: MediumEndocrinology and Metabolism

Match each presentation of calcium, phosphorus, and parathyroid hormone (PTH) laboratory abnormalities with its most likely underlying pathophysiologic diagnosis.

  • Elevated serum calcium, elevated PTH, and decreased serum phosphorusPrimary hyperparathyroidism
  • Decreased serum calcium, decreased PTH, and elevated serum phosphorusHypoparathyroidism
  • Decreased serum calcium, elevated PTH, elevated serum phosphorus, and normal serum creatininePseudohypoparathyroidism
  • Elevated serum calcium, decreased PTH, and decreased serum phosphorusHumoral hypercalcemia of malignancy
  • Decreased serum calcium, elevated PTH, elevated serum phosphorus, and elevated serum creatinineSecondary hyperparathyroidism (due to chronic kidney disease)

Answer

Primary hyperparathyroidism corresponds to high calcium/PTH and low phosphorus. Hypoparathyroidism matches low calcium/PTH and high phosphorus. Pseudohypoparathyroidism presents with low calcium, high phosphorus, high PTH, and normal creatinine. Malignancy (PTHrP) shows high calcium, low PTH, and low phosphorus. CKD shows low calcium, high phosphorus, high PTH, and elevated creatinine.
Each laboratory profile uniquely identifies a specific derangement in the calcium-PTH-vitamin D axis. Primary defects in PTH secretion cause calcium and PTH to move in the same direction. Secondary responses or ectopic mimics cause them to move in opposite directions. Phosphorus typically moves opposite to functional PTH activity on the kidney, except when overt renal failure physically prevents its excretion.

Step-by-Step Solution

1
Evaluate serum calcium and PTH directionality to determine primary versus secondary derangements.
If both move in the same direction, the defect is likely primary to the parathyroid gland (e.g., primary hyperparathyroidism or hypoparathyroidism). If they move in opposite directions, the parathyroid is responding appropriately to a systemic issue.
PTH regulates calcium; normally, low calcium stimulates PTH, and high calcium suppresses PTH.
2
Analyze serum phosphorus levels in relation to PTH.
PTH promotes phosphaturia. Elevated PTH should normally cause low phosphorus (as seen in primary hyperparathyroidism). If PTH is high but phosphorus is also high, suspect renal failure (CKD) or renal resistance to PTH (pseudohypoparathyroidism).
Kidneys are the primary site of phosphate regulation by PTH.
3
Differentiate between pseudohypoparathyroidism and secondary hyperparathyroidism.
Check renal function (creatinine). Elevated creatinine points to CKD, whereas normal creatinine points to pseudohypoparathyroidism.
CKD causes phosphate retention due to loss of glomerular filtration, whereas pseudohypoparathyroidism involves a specific receptor/signaling defect with normal overall renal filtration capacity.

Key Concept

Diagnosis of calcium and parathyroid disorders relies on recognizing the normal physiological feedback loops between serum calcium, serum phosphorus, intact parathyroid hormone, and renal function.
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Topics

Endocrinology and Metabolism

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