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Difficulty: HardEndocrinology and Metabolism

A medical student is reviewing laboratory data for five different patients evaluated in the endocrinology clinic for suspected calcium and bone metabolism disorders. Based on the diagnostic algorithm for calcium homeostasis, match each of the following complex laboratory profiles to its most likely underlying pathophysiologic diagnosis.

  • High Serum Calcium, Low Serum Phosphorus, High Intact PTH, High 24-hour Urinary CalciumPrimary hyperparathyroidism
  • Low Serum Calcium, High Serum Phosphorus, High Intact PTH, Low 1,25-dihydroxyvitamin DSecondary hyperparathyroidism (due to chronic kidney disease)
  • High Serum Calcium, Low Serum Phosphorus, Suppressed Intact PTH, High PTH-related peptide (PTHrP)Humoral hypercalcemia of malignancy
  • High Serum Calcium, High Serum Phosphorus, Suppressed Intact PTH, High 1,25-dihydroxyvitamin DGranulomatous disease (e.g., Sarcoidosis)
  • Mildly High Serum Calcium, Normal Serum Phosphorus, Normal to mildly High Intact PTH, Very Low 24-hour Urinary CalciumFamilial hypocalciuric hypercalcemia (FHH)

Answer

Profile 1 matches Primary hyperparathyroidism. Profile 2 matches Secondary hyperparathyroidism. Profile 3 matches Humoral hypercalcemia of malignancy. Profile 4 matches Granulomatous disease. Profile 5 matches Familial hypocalciuric hypercalcemia (FHH).
Each laboratory profile uniquely aligns with the pathophysiology of a distinct disease state. High calcium with high PTH signifies a failure of negative feedback at the parathyroid gland (Primary HPT) or altered sensing setpoints (FHH). High calcium with suppressed PTH implies an external driver, such as PTHrP from a tumor or uncontrolled calcitriol production from granulomas. Low calcium with high phosphorus and PTH points to a failing kidney unable to excrete phosphate or activate vitamin D.

Step-by-Step Solution

1
Separate the profiles based on the serum calcium level.
Profile 2 shows hypocalcemia, while Profiles 1, 3, 4, and 5 show hypercalcemia.
Identifying whether the primary derangement is hypercalcemia or hypocalcemia dictates the subsequent diagnostic algorithm.
2
Diagnose the hypocalcemic profile.
Profile 2 matches Secondary hyperparathyroidism due to chronic kidney disease.
The combination of high phosphorus, low active vitamin D, and low calcium with compensatory high PTH is the classic biochemical signature of renal failure.
3
Divide the hypercalcemic profiles based on the intact PTH level.
Profiles 1 and 5 have elevated/unsuppressed PTH (PTH-dependent). Profiles 3 and 4 have suppressed PTH (PTH-independent).
A suppressed PTH correctly indicates a non-parathyroid driver of hypercalcemia, whereas an unsuppressed PTH suggests parathyroid pathology or altered calcium sensing.
4
Differentiate the PTH-dependent hypercalcemias using 24-hour urinary calcium.
Profile 1 (high urine calcium) is Primary hyperparathyroidism. Profile 5 (very low urine calcium) is Familial hypocalciuric hypercalcemia.
FHH features a mutated CaSR that promotes renal calcium retention, yielding hypocalciuria, which distinguishes it from the hypercalciuria typically seen in primary hyperparathyroidism.
5
Differentiate the PTH-independent hypercalcemias using specific markers (PTHrP vs. 1,25-Vit D).
Profile 3 (high PTHrP) is Humoral hypercalcemia of malignancy. Profile 4 (high 1,25-Vit D) is Granulomatous disease.
Malignancies secrete PTHrP directly mimicking PTH effects without activating vitamin D, whereas sarcoidosis macrophages autonomously activate vitamin D, increasing both calcium and phosphorus.

Key Concept

Diagnosis of calcium metabolism disorders relies on evaluating serum calcium in tandem with intact PTH, followed by specific markers like phosphorus, 1,25-dihydroxyvitamin D, PTHrP, and 24-hour urinary calcium to isolate the precise etiology.
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Topics

Endocrinology and Metabolism

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