A 38-year-old man with symptomatic hypoparathyroidism (post-thyroidectomy) and a 41-year-old woman with pseudohypoparathyroidism (PHP) both present with hypocalcemia and elevated PTH. Which feature best distinguishes true hypoparathyroidism from pseudohypoparathyroidism?

## Distinguishing Hypoparathyroidism from Pseudohypoparathyroidism ### Pathophysiology **Key Point:** Both conditions present with hypocalcemia and hyperphosphatemia, but the mechanism and clinical features differ fundamentally. | Feature | True Hypoparathyroidism | Pseudohypoparathyroidism (PHP) | |---------|------------------------|---------------------------------| | **PTH level** | Low or absent | High (elevated) | | **Serum calcium** | ↓ | ↓ | | **Serum phosphate** | ↑ | ↑ | | **Renal response to PTH** | Normal | Blunted/absent | | **Mechanism** | Parathyroid gland failure | PTH resistance (end-organ) | | **Physical features** | None (unless genetic syndrome) | Albright hereditary osteodystrophy (AHO) | | **AHO stigmata** | Absent | Short stature, round facies, subcutaneous ossifications, short 4th/5th metacarpals | ### The Discriminating Feature **High-Yield:** The presence of **Albright hereditary osteodystrophy (AHO) features** — short stature, round facies, short 4th and 5th metacarpals, subcutaneous ossifications — is pathognomonic for pseudohypoparathyroidism and is the single best clinical discriminator. In true hypoparathyroidism (post-thyroidectomy), there are no characteristic skeletal or somatic features. The patient simply has low PTH due to parathyroid gland destruction or agenesis. **Clinical Pearl:** PHP is caused by end-organ (kidney and bone) resistance to PTH, typically due to mutations in the GNAS gene affecting the PTH1 receptor signaling pathway. The AHO features reflect the systemic nature of this genetic defect in G-protein signaling. ### Laboratory Distinction Both conditions present with: - Hypocalcemia - Hyperphosphatemia - Elevated PTH (PHP) vs. low PTH (true HPT) The PTH level itself is the *biochemical* discriminator: elevated in PHP (because kidneys don't respond), low in true hypoparathyroidism. However, among the given options, the *clinical* discriminator (AHO features) is more specific and easier to recognize at the bedside. **Mnemonic:** **PHP-AHO** = **P**seudohypoparathyroidism with **A**lbright **H**ereditary **O**steodystrophy. [cite:Harrison 21e Ch 297]