A 52-year-old woman from Delhi presents with muscle cramps and paresthesias in her fingers and around her mouth for 3 days. She has a history of thyroid surgery 2 weeks ago for nodular goiter. On examination, Chvostek's sign is positive. Serum calcium is 7.2 mg/dL (normal 8.5–10.5), ionized calcium 3.0 mg/dL (normal 4.5–5.3), phosphate 5.8 mg/dL (normal 2.5–4.5), and magnesium 1.8 mg/dL (normal 1.7–2.2). PTH level is 18 pg/mL (normal 15–65). What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Hypoparathyroidism ### Case Analysis The patient presents with **acute symptomatic hypocalcemia** (serum Ca 7.2, ionized Ca 3.0) with **positive Chvostek's sign**, indicating neuromuscular irritability. The temporal relationship to thyroid surgery 2 weeks ago is the critical clue. ### Laboratory Interpretation | Parameter | Patient Value | Normal Range | Interpretation | | --- | --- | --- | --- | | Serum Calcium | 7.2 mg/dL | 8.5–10.5 | ↓↓ Low | | Ionized Calcium | 3.0 mg/dL | 4.5–5.3 | ↓↓ Low | | Phosphate | 5.8 mg/dL | 2.5–4.5 | ↑ Elevated | | Magnesium | 1.8 mg/dL | 1.7–2.2 | Normal | | PTH | 18 pg/mL | 15–65 | Low-normal (inappropriately low) | **Key Point:** The combination of **low calcium + high phosphate + low-normal/low PTH** is pathognomonic for hypoparathyroidism. In true hypocalcemia, PTH should be elevated (>65 pg/mL); here it is inappropriately suppressed. ### Pathophysiology 1. Thyroid surgery → inadvertent injury/removal of parathyroid glands (most commonly the inferior parathyroids) 2. Loss of PTH secretion → inability to mobilize calcium from bone or increase renal calcium reabsorption 3. Unopposed hyperphosphatemia (no PTH to inhibit phosphate reabsorption) 4. Acute symptomatic hypocalcemia with tetany and paresthesias **Clinical Pearl:** Hypoparathyroidism after thyroid/parathyroid surgery typically manifests within **24 hours to 2 weeks** post-operatively, matching this patient's timeline. ### Why This Is Hypoparathyroidism (Not Other Diagnoses) - **Vitamin D deficiency rickets:** Would show elevated PTH (secondary hyperparathyroidism), not low PTH. Also develops over months, not acutely post-surgery. - **Hungry bone syndrome:** Occurs after parathyroidectomy for hyperparathyroidism; PTH is removed intentionally. This patient had thyroid surgery, not parathyroid surgery. - **Pseudohypoparathyroidism (PHP):** A genetic disorder with PTH resistance; presents in childhood with short stature and subcutaneous ossifications (Albright hereditary osteodystrophy). Acute post-surgical onset rules this out. **High-Yield:** Hypoparathyroidism is the most common cause of **permanent hypocalcemia** after thyroid surgery (incidence ~0.5–2%), and the **most common cause of hypoparathyroidism overall** in developed countries. ### Management Approach 1. **Acute:** IV calcium gluconate (10–20 mL of 10% solution in 50 mL saline over 10–20 min) to raise ionized calcium above 3.5 mg/dL and stop symptoms 2. **Chronic:** Oral calcium supplementation + active vitamin D (calcitriol 0.25–2 µg BD) to maintain serum calcium in low-normal range (7.5–8.5 mg/dL) and keep PTH suppressed 3. **Monitor:** Serum calcium, phosphate, and 24-hour urinary calcium (target <300 mg/day to prevent nephrolithiasis) [cite:Harrison 21e Ch 297]