Match each of the following serum laboratory profiles with the most likely underlying clin… — USMLE Step 2 CK

Match each of the following serum laboratory profiles with the most likely underlying clinical diagnosis. (Reference ranges: Calcium $8.4-10.2\text{ mg/dL}$ , Phosphorus $2.7-4.5\text{ mg/dL}$ , Intact PTH $15-65\text{ pg/mL}$ )

Calcium $11.5\text{ mg/dL}$ , Phosphorus $2.2\text{ mg/dL}$ , Intact PTH $120\text{ pg/mL}$ Parathyroid adenoma
Calcium $7.2\text{ mg/dL}$ , Phosphorus $5.5\text{ mg/dL}$ , Intact PTH $12\text{ pg/mL}$ Post-surgical hypoparathyroidism
Calcium $7.5\text{ mg/dL}$ , Phosphorus $6.0\text{ mg/dL}$ , Intact PTH $350\text{ pg/mL}$ Chronic kidney disease
Calcium $12.8\text{ mg/dL}$ , Phosphorus $2.0\text{ mg/dL}$ , Intact PTH $8\text{ pg/mL}$ Squamous cell carcinoma of the lung
Calcium $7.8\text{ mg/dL}$ , Phosphorus $2.1\text{ mg/dL}$ , Intact PTH $180\text{ pg/mL}$ Nutritional vitamin D deficiency

Answer

The laboratory profiles correctly map to their underlying pathophysiological states by analyzing whether the parathyroid response is the primary driver (adenoma, surgical absence) or a secondary compensatory mechanism (CKD, vitamin D deficiency, malignancy).

Each profile distinctively tests the feedback loops of the parathyroid axis. Primary abnormalities of the parathyroid gland present with PTH levels that drive the calcium levels, whereas secondary conditions present with PTH levels responding appropriately to the primary calcium or phosphate derangement, or being suppressed by a tumor-derived PTH analog.

Step-by-Step Solution

Evaluate the serum calcium level.

Classify the patient's state as hypercalcemic (profiles 1 and 4) or hypocalcemic (profiles 2, 3, and 5).

Establishing the initial calcium derangement directs the diagnostic algorithm toward primary versus secondary parathyroid disorders.

Analyze the intact PTH level in the context of the calcium level.

Identify if the PTH is mediating the calcium abnormality (e.g., high PTH causing high calcium) or appropriately responding to it (e.g., high PTH attempting to correct low calcium).

This differentiates primary hyperparathyroidism (high Ca, high PTH) from PTH-independent hypercalcemia (high Ca, low PTH), and primary hypoparathyroidism (low Ca, low PTH) from secondary hyperparathyroidism (low Ca, high PTH).

Assess the serum phosphorus level.

Distinguish between the different causes of secondary hyperparathyroidism.

In hypocalcemia with high PTH, a low phosphorus indicates impaired gut absorption (vitamin D deficiency), whereas a high phosphorus indicates impaired renal excretion (chronic kidney disease).

Key Concept

Interpretation of the calcium, phosphorus, and PTH axes to diagnose bone and mineral metabolism disorders.

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