A 68-year-old man from Delhi presents with fatigue, bone pain, and recurrent kidney stones. Serum investigations show: calcium 11.2 mg/dL (normal 8.5–10.5), phosphate 2.8 mg/dL (normal 2.5–4.5), PTH 78 pg/mL (normal 15–65), alkaline phosphatase 92 U/L (normal 30–120), and serum creatinine 1.4 mg/dL (normal 0.6–1.2). 24-hour urinary calcium is 320 mg (normal 100–300). Imaging shows lytic bone lesions and a 1.2 cm mass in the right upper parathyroid gland on ultrasound. What is the mechanism of hypercalcemia in this patient?

Explanation

## Diagnosis: Primary Hyperparathyroidism **Key Point:** This patient has PRIMARY hyperparathyroidism due to a parathyroid adenoma. The elevated PTH in the presence of hypercalcemia (not suppressed) is the diagnostic hallmark. ### Mechanism of Hypercalcemia in Primary Hyperparathyroidism ```mermaid flowchart TD A[Parathyroid Adenoma]:::outcome --> B[Autonomous PTH Secretion]:::outcome B --> C{PTH Actions}:::decision C -->|Kidney| D[Increased Ca²⁺ Reabsorption<br/>in Distal Tubule]:::action C -->|Kidney| E[Increased 1,25-DHVD Synthesis<br/>→ Intestinal Ca²⁺ Absorption]:::action C -->|Bone| F[Activation of Osteoclasts<br/>Bone Resorption]:::action D --> G[Hypercalcemia]:::urgent E --> G F --> G ``` ### Three-Pronged Mechanism | Mechanism | PTH Action | Result | |-----------|-----------|--------| | **Renal tubular reabsorption** | PTH increases calcium reabsorption in distal convoluted tubule via cAMP | ↑ Serum Ca²⁺ | | **Intestinal absorption** | PTH stimulates 1α-hydroxylase → ↑ 1,25-dihydroxyvitamin D → enhanced intestinal Ca²⁺ absorption | ↑ Serum Ca²⁺ | | **Bone resorption** | PTH activates osteoclasts via RANKL on osteoblasts → increased bone resorption | ↑ Serum Ca²⁺ | **High-Yield:** In primary hyperparathyroidism, PTH is ELEVATED (not suppressed) despite hypercalcemia. This is the key distinguishing feature — the parathyroids have lost normal negative feedback regulation. ### Diagnostic Confirmation **Clinical Pearl:** The combination of: - Hypercalcemia (11.2 mg/dL) - Elevated PTH (78 pg/mL) — inappropriately high for the calcium level - Hypophosphatemia (2.8 mg/dL) — due to PTH-induced phosphate wasting - High urinary calcium (320 mg/24 h) — despite hypercalcemia, PTH increases renal calcium reabsorption but cannot fully suppress the filtered load - Imaging showing parathyroid adenoma ...confirms PRIMARY hyperparathyroidism. **Mnemonic:** **"PHPt = Primary Hyperparathyroidism: Parathyroid mass + PTH elevated + Phosphate low"** ### Why Not Other Causes? - **NOT PTHrP secretion:** PTHrP-secreting malignancies (lung, kidney, breast) suppress endogenous PTH. Here, PTH is elevated, indicating autonomous parathyroid production. - **NOT vitamin D intoxication:** Vitamin D excess suppresses PTH (negative feedback). This patient's PTH is elevated. - **NOT osteolytic metastases:** Metastases cause hypercalcemia via local osteolytic factors (IL-6, TNF) or PTHrP, but PTH would be suppressed, not elevated.