A 52-year-old woman from Delhi presents with muscle cramps, paresthesias around the mouth, and positive Chvostek's sign. Serum calcium is 7.2 mg/dL (normal 8.5–10.5), phosphate is 5.8 mg/dL (normal 2.5–4.5), and PTH is 12 pg/mL (normal 15–65). Albumin is 4.0 g/dL. She has a 10-year history of autoimmune thyroiditis. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Autoimmune Hypoparathyroidism ### Biochemical Pattern Recognition | Parameter | Patient Value | Normal Range | Interpretation | |-----------|---------------|--------------|----------------| | Serum Ca | 7.2 mg/dL | 8.5–10.5 | **Low (hypocalcemia)** | | Serum PO₄ | 5.8 mg/dL | 2.5–4.5 | **High (hyperphosphatemia)** | | PTH | 12 pg/mL | 15–65 | **Inappropriately low** | | Albumin | 4.0 g/dL | 3.5–5.0 | Normal (excludes hypoalbuminemia) | **Key Point:** The combination of **hypocalcemia + hyperphosphatemia + LOW PTH** defines **primary hypoparathyroidism**. The parathyroid glands are failing to respond to hypocalcemia. ### Why Autoimmune Hypoparathyroidism? **Clinical Pearl:** This patient has a strong association with autoimmune thyroiditis (Hashimoto's), which clusters with autoimmune hypoparathyroidism in polyglandular autoimmune syndrome type 1 (PAS-1) or type 2 (PAS-2). **High-Yield:** Autoimmune hypoparathyroidism is the most common acquired form of hypoparathyroidism in adults, often presenting with: - Autoimmune thyroiditis (as in this case) - Addison's disease - Pernicious anemia - Celiac disease ### Pathophysiology ```mermaid flowchart TD A[Autoimmune destruction of parathyroid tissue]:::urgent --> B[Reduced PTH secretion] B --> C[Hypocalcemia]:::outcome B --> D[Hyperphosphatemia]:::outcome C --> E[Neuromuscular hyperexcitability] E --> F[Muscle cramps, paresthesias, Chvostek's sign]:::outcome G[Associated autoimmune thyroiditis]:::outcome -.->|Polyglandular syndrome| A ``` ### Differential Diagnosis Excluded **Why NOT primary hyperparathyroidism?** PTH is LOW, not elevated. In primary hyperparathyroidism, PTH would be high-normal or elevated despite hypercalcemia. **Why NOT pseudohypoparathyroidism (PHP)?** PHP type 1a presents with **end-organ resistance** to PTH, causing elevated PTH (>200 pg/mL) in response to hypocalcemia. This patient's PTH is low, indicating the parathyroid glands themselves are dysfunctional, not the target organs. **Why NOT secondary hyperparathyroidism?** Secondary hyperparathyroidism (e.g., from CKD) would show **elevated PTH** as a compensatory response to hypocalcemia. This patient's PTH is inappropriately low for the degree of hypocalcemia. ### Management Principles **Mnemonic: CHVD** — Calcium, Hydroxy-vitamin D, Vitamin D metabolites 1. **Acute symptomatic hypocalcemia:** IV calcium gluconate (10–20 mL of 10% solution in 50–100 mL 5% dextrose) 2. **Chronic management:** Oral calcium supplements + active vitamin D metabolites (calcitriol 0.25–2 μg BD) 3. **Monitor:** Serum calcium, phosphate, magnesium, and 24-hour urine calcium (target <300 mg/day to prevent nephrolithiasis) 4. **Screen for associated autoimmune conditions:** TSH, cortisol, B₁₂, tissue transglutaminase antibodies **Warning:** Do NOT use PTH replacement — it is not available as a therapeutic agent for hypoparathyroidism. Vitamin D metabolites are the mainstay.