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Difficulty: MediumEndocrinology and Metabolism

Match the following laboratory profiles of calcium, phosphorus, and parathyroid hormone (PTH) to the most likely underlying pathophysiologic diagnosis.

  • Elevated serum calcium, elevated intact PTH, decreased serum phosphorus, elevated 24-hour urine calciumPrimary hyperparathyroidism
  • Elevated serum calcium, suppressed intact PTH, decreased serum phosphorus, elevated PTHrPHumoral hypercalcemia of malignancy
  • Decreased serum calcium, elevated intact PTH, increased serum phosphorus, increased BUN and creatinineSecondary hyperparathyroidism due to chronic kidney disease
  • Elevated serum calcium, normal or mildly elevated intact PTH, low 24-hour urine calciumFamilial hypocalciuric hypercalcemia
  • Decreased serum calcium, suppressed intact PTH, increased serum phosphorus, normal BUN and creatininePrimary hypoparathyroidism

Answer

1 -> Primary hyperparathyroidism; 2 -> Humoral hypercalcemia of malignancy; 3 -> Secondary hyperparathyroidism due to chronic kidney disease; 4 -> Familial hypocalciuric hypercalcemia; 5 -> Primary hypoparathyroidism.
Calcium homeostasis is maintained primarily by PTH and Vitamin D. Understanding the negative feedback loops and the specific physiologic effects of PTH (bone resorption, renal calcium reabsorption, and renal phosphate excretion) allows for the logical deduction of the underlying pathology from any given laboratory profile.

Step-by-Step Solution

1
Analyze the serum calcium and PTH levels to determine if the defect is primarily parathyroid-driven or PTH-independent.
High calcium with high PTH indicates a parathyroid-driven process (or altered calcium sensing). High calcium with low PTH indicates a PTH-independent process (e.g., malignancy). Low calcium with high PTH indicates a secondary response. Low calcium with low PTH indicates primary gland failure.
PTH and calcium normally have an inverse relationship due to negative feedback; parallel changes suggest primary parathyroid pathology.
2
Evaluate the phosphorus and renal function markers to further differentiate the etiologies.
Low phosphorus is expected with high PTH or PTHrP activity. High phosphorus with low calcium suggests either renal failure (if BUN/creatinine are high) or hypoparathyroidism (if renal function is normal).
PTH promotes renal phosphate excretion. In chronic kidney disease, the failing kidneys cannot excrete phosphate, overriding the typical PTH effect.
3
Assess urine calcium to distinguish between primary hyperparathyroidism and familial hypocalciuric hypercalcemia (FHH).
Elevated urine calcium points to primary hyperparathyroidism, whereas low urine calcium points to FHH.
FHH involves a defective calcium-sensing receptor in the kidneys, leading to increased renal calcium reabsorption despite systemic hypercalcemia.

Key Concept

Interpretation of calcium, phosphorus, and PTH laboratory profiles to diagnose underlying disorders of calcium metabolism.
Estimated Time:2m 0s
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Topics

Endocrinology and Metabolism

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