What is the most likely diagnosis for a patient with normal parathyroid hormone, hypercalcemia, and elevated serum angiotensin‑converting enzyme levels?

Sarcoidosis

The most likely diagnosis for a patient presenting with normal PTH, hypercalcemia, and elevated serum ACE levels is sarcoidosis, a granulomatous disease that causes PTH-independent hypercalcemia through excessive, unregulated production of calcitriol by granulomatous tissue.

Diagnostic Reasoning

The clinical triad you describe—normal PTH, elevated calcium, and raised ACE—is pathognomonic for sarcoidosis. Here's the algorithmic approach:

Step 1: Classify the Hypercalcemia Pattern

When PTH is normal or low in the setting of hypercalcemia, you've immediately excluded primary hyperparathyroidism and narrowed to six main categories: malignant, granulomatous, genetic, immobilization-related, endocrine, or drug-induced 1.

Step 2: Interpret the Elevated ACE

Elevated serum ACE is highly specific for sarcoidosis in the context of hypercalcemia. Studies demonstrate that ACE levels are markedly elevated (mean 55.2 ± 19.8 U/ml) in hypercalcemic sarcoidosis patients, while remaining normal in other hypercalcemic conditions including primary hyperparathyroidism (18.6 ± 4.7 U/ml) and malignancy-related hypercalcemia (21.8 ± 5.9 U/ml) 2. The combination of hypercalcemia with elevated ACE serves as a rapid diagnostic guide specifically pointing toward sarcoidosis 2.

Step 3: Understand the Mechanism

Sarcoidosis causes hypercalcemia through excessive, unregulated production of calcitriol (1,25-dihydroxyvitamin D) by granulomatous tissue or macrophages via extra-renal 1-alpha-hydroxylase 1. This mechanism:

• Suppresses PTH (explaining the normal/low PTH)
• Increases intestinal calcium absorption
• May elevate phosphate levels due to increased intestinal phosphate absorption 1

Step 4: Confirm with Additional Testing

To solidify the diagnosis:

• Measure 1,25-dihydroxyvitamin D levels: Expect elevation 3
• Check PTH-related peptide (PTHrP): Should be normal, excluding humoral hypercalcemia of malignancy 3
• Obtain chest imaging: Look for hilar/mediastinal lymphadenopathy, lung parenchymal involvement 3, 4
• Consider tissue biopsy: Non-caseating granulomas confirm the diagnosis 3, 5

Clinical Pearls and Pitfalls

Serial ACE monitoring is valuable for treatment response. Studies show that ACE levels fall in parallel with calcium normalization during corticosteroid therapy 6. Seven of eight patients demonstrated concordant decreases in both ACE and calcium levels with treatment 6.

Common pitfall: The presence of lymphadenopathy, constitutional symptoms (fatigue, weight loss, nausea), and renal dysfunction can mimic malignancy, creating diagnostic uncertainty 3. However, the elevated ACE distinguishes sarcoidosis from malignancy-related hypercalcemia.

Renal complications are frequent: Hypercalcemia in sarcoidosis commonly presents with acute kidney injury, nephrocalcinosis, and urolithiasis 3, 7, 5. A significant correlation exists between serum calcium and creatinine levels (r = 0.76, p < 0.02) 7.

Treatment Approach

Once sarcoidosis is confirmed:

1. Immediate management of severe hypercalcemia: IV hydration, bisphosphonates (zoledronic acid), and calcitonin if symptomatic 3

2. Definitive treatment: Corticosteroids (prednisone) are the mainstay 3, 6. Alternate-day regimens are effective for long-term management 6

3. Monitor response: Serial calcium and ACE measurements guide therapy duration 6

The prognosis is excellent with appropriate treatment—calcium levels and renal function typically normalize with corticosteroid therapy 3, 7, 5.