## Distinguishing Feature: Gonadotropin Dominance ### Follicular Phase Characteristics **Key Point:** The follicular phase is defined by FSH-driven recruitment and growth of primary follicles. - FSH rises early in the cycle (day 1–5) due to declining progesterone and estrogen from the previous cycle - FSH stimulates granulosa cell proliferation and aromatase expression - Estrogen rises gradually as follicles mature (days 5–13) - LH remains relatively low and stable - No corpus luteum is present - Endometrium is in proliferative phase (thickens progressively) ### Luteal Phase Characteristics **Key Point:** The luteal phase is defined by LH-driven corpus luteum function and progesterone dominance. - LH becomes the dominant gonadotropin after the LH surge (day 14) - Corpus luteum secretes high progesterone (and some estrogen) - FSH is suppressed by progesterone and inhibin - Endometrium enters secretory phase - Progesterone > estrogen in absolute concentration ### Comparison Table | Feature | Follicular Phase | Luteal Phase | | --- | --- | --- | | **Dominant gonadotropin** | **FSH** | **LH** | | Primary ovarian structure | Growing follicles | Corpus luteum | | Progesterone level | Low (<1 ng/mL) | High (10–20 ng/mL) | | Estrogen pattern | Rising (proliferative) | Biphasic (peak + decline) | | Endometrial phase | Proliferative | Secretory | | Duration | ~14 days | ~14 days | **High-Yield:** FSH dominance in the follicular phase is the **single best discriminator** because it directly causes the morphological and biochemical changes unique to that phase. LH is present but suppressed; it only becomes dominant after the surge triggers ovulation. **Clinical Pearl:** A low FSH:LH ratio in the follicular phase (normally 1:1 to 3:1) suggests inadequate follicular recruitment and may indicate diminished ovarian reserve or PCOS. **Mnemonic:** **F**ollicular phase = **F**SH dominant; **L**uteal phase = **L**H dominant. 
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