A 52-year-old woman from rural Maharashtra presents with progressive muscle weakness, paresthesias in her hands and feet, and two episodes of tetany over the past month. On examination, she has positive Chvostek's and Trousseau's signs. Laboratory investigations reveal: serum calcium 6.8 mg/dL (normal 8.5–10.5), serum phosphate 5.2 mg/dL (normal 2.5–4.5), serum albumin 3.8 g/dL, and serum PTH 12 pg/mL (normal 15–65). Urinary calcium is 45 mg/24 hours (normal 100–300). What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis **Key Point:** The combination of hypocalcemia, hyperphosphatemia, LOW PTH, and clinical signs of hypocalcemia (tetany, Chvostek's, Trousseau's) is pathognomonic for hypoparathyroidism. ### Diagnostic Features | Feature | Patient's Value | Normal Range | Interpretation | |---------|-----------------|--------------|----------------| | Serum Calcium | 6.8 mg/dL | 8.5–10.5 | ↓ Hypocalcemia | | Serum Phosphate | 5.2 mg/dL | 2.5–4.5 | ↑ Hyperphosphatemia | | PTH | 12 pg/mL | 15–65 | ↓ Inappropriately LOW | | Urinary Calcium | 45 mg/24 h | 100–300 | ↓ Low (kidney response to hypocalcemia) | | Albumin | 3.8 g/dL | 3.5–5.0 | Normal (corrected Ca still low) | **High-Yield:** In hypoparathyroidism, the kidneys CANNOT respond to hypocalcemia because there is insufficient PTH. This distinguishes it from vitamin D deficiency, where PTH is elevated (secondary hyperparathyroidism). ### Pathophysiology 1. **PTH deficiency** → inability to increase serum calcium 2. **Loss of PTH-mediated phosphate excretion** → hyperphosphatemia 3. **Loss of PTH-mediated 1,25-dihydroxyvitamin D activation** → reduced intestinal calcium absorption 4. **Result:** Severe hypocalcemia with hyperphosphatemia and low-normal/low urinary calcium **Clinical Pearl:** Hypoparathyroidism commonly follows thyroid or parathyroid surgery (surgical hypoparathyroidism is the most common cause in India). Autoimmune and genetic forms also occur. **Mnemonic:** **"HypoParathyroidism = Low PTH + Low Ca + High PO₄"** — the PTH is the key differentiator. ### Why This Is Hypoparathyroidism, Not Other Conditions - **NOT vitamin D deficiency:** In vitamin D deficiency, PTH is markedly elevated (secondary hyperparathyroidism) as the parathyroids attempt to correct hypocalcemia. This patient's PTH is LOW. - **NOT pseudohypoparathyroidism (PHP):** PHP presents with hypocalcemia and hyperphosphatemia BUT also has characteristic skeletal features (short stature, round face, short metacarpals) and end-organ PTH resistance (high PTH despite low calcium). This patient has no mention of these features. - **NOT primary hyperparathyroidism:** Primary hyperparathyroidism causes hypercalcemia (not hypocalcemia) and low phosphate (not high).