Calcium & Phosphate Homeostasis
| Hormone | Action on Ca²⁺ | Action on Phosphate |
|---|---|---|
| PTH | Increases (bone resorption, renal absorption) | Decreases (renal excretion) |
| Vitamin D (1,25-(OH)₂D) | Increases (intestinal absorption) | Increases (intestinal absorption) |
| Calcitonin | Decreases (bone mineralization) | Minimal impact |
High-Yield Core Realities:
- PTH Feedback: PTH is the primary regulator. It increases serum calcium while dumping phosphate into the urine to prevent the formation of calcium-phosphate salts in tissues.
- Hyperparathyroidism: Leads to “stones (renal), bones (pain), groans (abdominal/GI), and psychiatric overtones.” PTH is high, Calcium is high, Phosphate is low.
- Hypoparathyroidism: Often iatrogenic (post-thyroidectomy). PTH is low, Calcium is low, Phosphate is high. Classic sign: Trousseau sign (carpal spasm with cuff inflation) and Chvostek sign (facial twitching).