## Ovulation Induction Agents — Mechanism and Pharmacology ### Correct Statements **Key Point:** Clomiphene citrate is a selective estrogen receptor modulator (SERM) that blocks estrogen feedback at the hypothalamus and pituitary, leading to increased GnRH and FSH secretion. This is the mechanism by which it induces ovulation in anovulatory women. **High-Yield:** Letrozole is an aromatase inhibitor that reduces local estrogen production in the ovary and peripherally. Unlike clomiphene, it does not deplete estrogen receptors and has a shorter half-life, making it preferred in PCOS where clomiphene may cause excessive follicle recruitment or adverse lipid effects. **Clinical Pearl:** GnRH agonists (leuprolide, goserelin) initially cause a surge in FSH and LH (flare effect) followed by downregulation and suppression of endogenous gonadotropins. This is exploited in long protocols for controlled ovarian hyperstimulation to prevent premature LH surge. ### The Incorrect Statement **Warning:** hMG does NOT contain FSH and LH in a fixed 1:1 ratio. The ratio varies between 1:1 and 3:1 depending on the source and batch. More importantly, modern preparations like recombinant FSH (rFSH) and recombinant LH (rLH) allow precise control of the FSH:LH ratio, which is critical for optimizing follicle development in different patient populations (e.g., higher LH in poor responders, lower LH in PCOS). | Agent | Source | FSH:LH Ratio | Key Feature | | --- | --- | --- | --- | | hMG | Postmenopausal urine | Variable (1:1 to 3:1) | Unpredictable ratio | | rFSH | Recombinant DNA | FSH only | Precise, no LH | | rLH | Recombinant DNA | LH only | Allows titration | | hCG | Pregnancy urine | LH-like activity | Used for final oocyte maturation | **Mnemonic:** **SERM-AROMATASE-GnRH-hMG** — Remember that hMG has a **variable**, not fixed, ratio, and modern protocols use recombinant preparations for better control.
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