A 38-year-old woman presents to the clinic with a 2-month history of fatigue, generalized … — USMLE Step 3

A $38$ -year-old woman presents to the clinic with a $2$ -month history of fatigue, generalized weakness, and a persistent dry cough. She also reports the recent development of tender, red nodules on her anterior shins. Her temperature is $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure is $124/78$ mm Hg, pulse is $76$ /min, and respirations are $16$ /min. Physical examination reveals bilateral tender, erythematous nodules on the extensor surfaces of her lower legs. A chest radiograph demonstrates bilateral hilar lymphadenopathy.

Initial laboratory evaluation reveals:

Laboratory Test	Result	Reference Range
Sodium	$140$ mEq/L	$136$ - $145$ mEq/L
Potassium	$4.1$ mEq/L	$3.5$ - $5.0$ mEq/L
Chloride	$102$ mEq/L	$95$ - $105$ mEq/L
Bicarbonate	$24$ mEq/L	$22$ - $28$ mEq/L
Blood urea nitrogen	$14$ mg/dL	$7$ - $18$ mg/dL
Creatinine	$0.8$ mg/dL	$0.6$ - $1.2$ mg/dL
Calcium, serum	$11.5$ mg/dL	$8.4$ - $10.2$ mg/dL
Albumin, serum	$4.0$ g/dL	$3.5$ - $5.0$ g/dL
Phosphorus, serum	$4.2$ mg/dL	$2.5$ - $4.5$ mg/dL

Which of the following additional laboratory profiles is most likely to be found in this patient?

Decreased intact parathyroid hormone, normal 25-hydroxyvitamin D, and increased 1,25-dihydroxyvitamin DAnswer
B
Increased intact parathyroid hormone, normal 25-hydroxyvitamin D, and increased 1,25-dihydroxyvitamin D
C
Decreased intact parathyroid hormone, normal 25-hydroxyvitamin D, and decreased 1,25-dihydroxyvitamin D
D
Decreased intact parathyroid hormone, increased 25-hydroxyvitamin D, and increased 1,25-dihydroxyvitamin D
E
Increased intact parathyroid hormone, decreased 25-hydroxyvitamin D, and decreased 1,25-dihydroxyvitamin D

Answer

The most likely laboratory profile reveals decreased intact parathyroid hormone, normal 25-hydroxyvitamin D, and increased 1,25-dihydroxyvitamin D.

The correct answer identifies the distinct pathophysiological profile of hypercalcemia in sarcoidosis. The patient's clinical presentation of fatigue, cough, erythema nodosum, and bilateral hilar lymphadenopathy is highly characteristic of sarcoidosis. Sarcoidosis and other granulomatous diseases can cause hypercalcemia because activated macrophages within the granulomas possess ectopic 1-alpha-hydroxylase activity. This enzyme converts 25-hydroxyvitamin D into the active form, 1,25-dihydroxyvitamin D (calcitriol), without being subject to the normal negative feedback regulation that occurs in the kidneys. Consequently, the unregulated high levels of 1,25-dihydroxyvitamin D increase intestinal calcium absorption, leading to hypercalcemia. The elevated serum calcium subsequently suppresses the secretion of intact parathyroid hormone (PTH) from the parathyroid glands. Serum 25-hydroxyvitamin D levels typically remain normal, as they are primarily determined by dietary intake and hepatic conversion of vitamin D, not by the ectopic macrophage activity.

Step-by-Step Solution

Analyze the clinical presentation to establish the primary diagnosis.

The combination of bilateral hilar lymphadenopathy, erythema nodosum, and polyarthralgia/fatigue in a young woman points strongly to sarcoidosis (Löfgren syndrome).

Identifying the underlying systemic disease is required before determining the etiology of the laboratory abnormalities.

Evaluate the provided laboratory data in the context of the diagnosis.

The patient has marked hypercalcemia (

11.5

mg/dL) with normal serum phosphorus and renal function.

Hypercalcemia is a known complication of granulomatous diseases like sarcoidosis.

Determine the pathophysiological mechanism of hypercalcemia in the established diagnosis.

In sarcoidosis, activated macrophages within granulomas express 1-alpha-hydroxylase, which converts 25-hydroxyvitamin D to active 1,25-dihydroxyvitamin D independently of physiologic regulation.

Understanding this enzyme activity explains the subsequent downstream laboratory profile.

Predict the levels of PTH, 25-hydroxyvitamin D, and 1,25-dihydroxyvitamin D.

1,25-dihydroxyvitamin D will be elevated. The resulting hypercalcemia will suppress intact PTH via negative feedback. 25-hydroxyvitamin D will remain normal because its levels depend on dietary intake and hepatic synthesis, not the unregulated macrophage enzyme.

Differentiating these values distinguishes sarcoidosis from primary hyperparathyroidism, malignancy, and vitamin D overdose.

Key Concept

Hypercalcemia in granulomatous diseases (e.g., sarcoidosis) is driven by PTH-independent, macrophage-mediated overproduction of 1,25-dihydroxyvitamin D, resulting in suppressed PTH and normal 25-hydroxyvitamin D levels.

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