## Mechanism of Calcitonin Action **Key Point:** Calcitonin has the OPPOSITE effect on serum calcium — it DECREASES serum calcium, not increases it. This is a common source of confusion in calcium homeostasis. ### Correct Statements (Options 0, 1, 2): | Statement | Mechanism | Outcome | |-----------|-----------|----------| | PTH → TRPV5 upregulation | Increases apical Ca²⁺ entry in DCT | ↑ Renal Ca²⁺ reabsorption | | Calcitriol → Calbindin-D28K | Enhances intestinal Ca²⁺ binding & transport | ↑ Intestinal Ca²⁺ absorption | | PTH → ↓ Proximal tubular Pi reabsorption | Inhibits Na-Pi cotransport | ↑ Phosphaturia (↓ serum Pi) | **High-Yield:** PTH and calcitriol work synergistically to raise serum calcium. PTH also activates 1α-hydroxylase in the kidney to produce calcitriol. ### Why Option 3 is Wrong (Calcitonin): **Clinical Pearl:** Calcitonin is secreted by parafollicular (C) cells of the thyroid in response to **hypercalcemia**. Its actions are: 1. **Inhibits osteoclasts** → ↓ bone resorption (NOT stimulates) 2. **Increases renal calcium excretion** → ↑ urinary Ca²⁺ (NOT decreases) 3. **Net effect:** ↓ serum calcium (hypocalcemic hormone) **Mnemonic:** **CalciTONin Tones DOWN calcium** — think of it as the "brake" on high calcium, opposing PTH's "accelerator" effect. **Warning:** Students often confuse calcitonin with PTH because both are "bone hormones." Remember: PTH raises calcium; calcitonin lowers it. ### Summary Table: | Hormone | Serum Ca²⁺ Effect | Bone Effect | Renal Ca²⁺ Effect | |---------|-------------------|-------------|-------------------| | **PTH** | ↑ (increases) | ↑ Resorption | ↑ Reabsorption | | **Calcitriol** | ↑ (increases) | ↑ Resorption | ↑ Reabsorption | | **Calcitonin** | ↓ (decreases) | ↓ Resorption | ↑ Excretion | [cite:Guyton & Hall 13e Ch 79]
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