A 47-year-old woman comes to the clinic reporting worsening fatigue, increased thirst, and frequent urination over the past three months. She has a history of chronic cough and was told she had "enlarged lymph nodes" in her chest a year ago, but she did not follow up. Her temperature is 37.0°C (98.6°F), blood pressure is 128/82 mmHg, and pulse is 78/min. Physical examination reveals a few erythematous nodules on her anterior lower legs. Laboratory studies show:
| Laboratory Parameter | Result | Reference Range |
|---|---|---|
| Calcium, serum | 12.8 mg/dL | 8.4-10.2 mg/dL |
| Phosphorus, serum | 4.6 mg/dL | 2.5-4.5 mg/dL |
| Parathyroid hormone (PTH) | 9 pg/mL | 10-65 pg/mL |
| Creatinine, serum | 1.1 mg/dL | 0.6-1.1 mg/dL |
Which of the following cellular mechanisms best explains this patient's current laboratory abnormalities?
- ASecretion of a peptide homologous to the amino-terminal portion of parathyroid hormone
- BDecreased sensitivity of calcium-sensing receptors leading to a blunted suppression of parathyroid hormone
- Unregulated conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D by activated macrophagesAnswer
- DEnhanced calcium reabsorption in the distal convoluted tubule due to an unlisted diuretic medication
- EIncreased bone resorption mediated by osteoclast-activating factors and inflammatory cytokines
Answer
Unregulated conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D by activated macrophages
The correct answer is the unregulated conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D by activated macrophages. This patient's presentation of hypercalcemia, suppressed parathyroid hormone (PTH), polyuria, and constitutional symptoms, along with a history of bilateral hilar lymphadenopathy and erythema nodosum, is highly suspicious for sarcoidosis. In sarcoidosis and other granulomatous diseases, disease-associated macrophages ectopically express the enzyme 1-alpha-hydroxylase. This enzyme converts 25-hydroxyvitamin D into the active form, 1,25-dihydroxyvitamin D (calcitriol), independent of normal physiologic feedback mechanisms. The excess calcitriol increases intestinal absorption of both calcium and phosphorus, leading to hypercalcemia and normal-to-elevated serum phosphorus. The resultant hypercalcemia suppresses endogenous PTH secretion from the parathyroid glands via negative feedback.
Step-by-Step Solution
Key Concept
The primary mechanism of hypercalcemia in sarcoidosis and other granulomatous diseases is the unregulated, ectopic production of 1,25-dihydroxyvitamin D (calcitriol) by 1-alpha-hydroxylase expressed in activated macrophages.