## Biochemical Profile of Osteomalacia ### Key Biochemical Abnormalities in Osteomalacia **High-Yield:** Osteomalacia is characterized by defective mineralization of bone matrix in adults, resulting in a predictable pattern of biochemical derangements. | Parameter | Finding in Osteomalacia | Mechanism | |-----------|------------------------|----------| | 25-hydroxyvitamin D | **Markedly reduced** (<20 ng/mL) | Primary deficiency or malabsorption | | Serum calcium | **Low** (7.5–8.5 mg/dL) | Impaired intestinal absorption + PTH-mediated renal loss | | Serum phosphate | **Low** (2–3 mg/dL) | PTH-induced renal phosphate wasting | | Alkaline phosphatase | **Elevated** (2–4× normal) | Increased osteoblastic activity | | Parathyroid hormone | **Elevated** | Secondary response to hypocalcemia | | 1,25-dihydroxyvitamin D | **Variable** | May be elevated (compensatory) or normal | ### Why Serum Phosphate is NOT Elevated **Key Point:** Serum phosphate is **LOW**, not elevated, in osteomalacia. This is a critical distinction. **Clinical Pearl:** The mechanism of hypophosphatemia in osteomalacia: 1. Low 25-hydroxyvitamin D → impaired 1α-hydroxylase activity 2. Secondary hyperparathyroidism develops (due to hypocalcemia) 3. PTH causes **increased renal phosphate wasting** via inhibition of sodium-phosphate cotransporters in the proximal tubule 4. Result: **Serum phosphate falls**, not rises **Mnemonic: "CHAP" for Osteomalacia** — **C**alcium low, **H**yperparathyroidism, **A**lkaline phosphatase elevated, **P**hosphate low. **Warning:** Do not confuse osteomalacia (low phosphate) with hyperphosphatemia seen in renal failure or hypoparathyroidism. The phosphate-wasting effect of PTH is the dominant mechanism in osteomalacia. [cite:Harrison 21e Ch 379]
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