A 68-year-old man from Delhi presents with a 6-month history of fatigue, bone pain, and recurrent urinary tract infections. Laboratory investigations show: serum calcium 11.8 mg/dL (normal 8.5–10.5), phosphate 2.1 mg/dL (normal 2.5–4.5), creatinine 2.4 mg/dL (baseline normal), PTH 112 pg/mL (normal 15–65), and 24-h urinary calcium 385 mg (normal <300). Imaging reveals multiple lytic bone lesions. Which mechanism best explains the elevated PTH in the setting of hypercalcemia?

Explanation

## Differential Diagnosis of Elevated PTH with Hypercalcemia **Key Point:** The presence of hypercalcemia with elevated PTH defines **primary hyperparathyroidism**. In this case, the parathyroid gland(s) have become autonomous and continue to secrete PTH despite serum calcium being above normal—a failure of the normal negative feedback inhibition by hypercalcemia. ### Pathophysiology of Primary Hyperparathyroidism 1. **Autonomous PTH secretion** (usually from an adenoma, less commonly from hyperplasia or carcinoma) 2. **Loss of normal calcium-sensing feedback** → PTH continues even when serum Ca is elevated 3. **PTH drives:** - Increased bone resorption → hypercalcemia and lytic lesions - Increased renal calcium reabsorption → hypercalcemia - Increased 1,25-OH vitamin D production → enhanced intestinal calcium absorption 4. **Result:** Hypercalcemia + elevated PTH + high urinary calcium (due to filtered load exceeding reabsorptive capacity) ### Comparison of PTH-Related Disorders | Condition | Serum Ca | PTH | Mechanism | Phosphate | |-----------|----------|-----|-----------|----------| | **Primary HPT** | ↑ High | ↑ High | Autonomous adenoma/hyperplasia | ↓ Low | | **Secondary HPT** | ↓ Low | ↑ High | Reactive to low Ca or low vitamin D | ↑ High | | **Tertiary HPT** | ↑ High | ↑ High | Autonomous after chronic secondary HPT | Variable | | **Ectopic PTH** | ↑ High | ↑ High | Non-parathyroid malignancy | ↓ Low | **High-Yield:** In **primary hyperparathyroidism**, the set-point for PTH suppression by calcium is shifted rightward—the gland "thinks" calcium is normal when it is actually elevated. ### Clinical Features in This Patient - **Hypercalcemia** → fatigue, bone pain, nephrolithiasis risk - **Lytic bone lesions** → from PTH-driven osteoclast activation (osteitis fibrosa cystica in severe cases) - **Elevated urinary calcium** → hypercalcemia exceeds renal reabsorptive threshold - **Low phosphate** → PTH inhibits proximal tubular phosphate reabsorption - **Mild renal impairment** → from hypercalcemia-induced nephrolithiasis and nephrocalcinosis, not the primary cause **Clinical Pearl:** The combination of hypercalcemia + elevated PTH + low phosphate is virtually diagnostic of primary hyperparathyroidism. Imaging (ultrasound, sestamibi scan, or CT) typically localizes a single adenoma in ~80% of cases. **Mnemonic:** **PRIMARY = PTH elevated + calcium elevated + phosphate low** (the "primary" problem is in the parathyroid gland itself). [cite:Harrison 21e Ch 297; KD Tripathi 8e Ch 12]