## Pathophysiology of Postmenopausal Osteoporosis **Key Point:** Postmenopausal osteoporosis results from estrogen deficiency, which leads to increased osteoclast activity and bone loss. Understanding the mechanism guides appropriate therapeutic selection. ### Correct Mechanisms | Mechanism | Explanation | |-----------|-------------| | **Estrogen deficiency → ↑ RANKL** | Loss of estrogen removes inhibition of RANKL (receptor activator of NF-κB ligand) on osteoblasts and bone marrow stromal cells, promoting osteoclastogenesis | | **Bisphosphonates** | Inhibit farnesyl pyrophosphate synthase in osteoclasts, leading to apoptosis and reduced bone resorption | | **Vitamin D** | Increases intestinal calcium absorption via VDR-mediated mechanisms AND directly promotes osteoblast differentiation through VDR signaling | ### Why Teriparatide Is NOT First-Line **High-Yield:** Teriparatide (recombinant PTH 1–34) is an **anabolic agent** that stimulates osteoblast activity and increases bone formation. However, it is **NOT first-line** for postmenopausal osteoporosis because: 1. **Cost:** Significantly more expensive than bisphosphonates or denosumab 2. **Indication:** Reserved for severe osteoporosis, multiple fractures, or failure/intolerance to other agents 3. **Duration:** Limited to 24 months of therapy due to risk of osteosarcoma in animal models 4. **First-line agents:** Bisphosphonates (alendronate, risedronate) or denosumab are preferred initial therapy **Clinical Pearl:** Teriparatide may be considered in patients with T-score < −3.5, prior fragility fractures, or inadequate response to antiresorptive therapy. **Warning:** Confusing anabolic agents (teriparatide, abaloparatide) with antiresorptive agents (bisphosphonates, denosumab) is a common exam trap. Anabolics build bone; antiresorptives prevent loss.
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