LH and FSH Peak at Mid-Cycle (Day 14)
Both luteinizing hormone (LH) and follicle-stimulating hormone (FSH) peak at the middle of the menstrual cycle, coinciding with ovulation around day 14 in a typical 28-day cycle 1, 2, 3.
Timing and Characteristics of the Gonadotropin Surge
The mid-cycle gonadotropin surge is the defining hormonal event that triggers ovulation:
- LH surge: The most dramatic hormonal change of the menstrual cycle, occurring approximately 24-36 hours before ovulation
- FSH surge: Occurs simultaneously with the LH surge, though the LH peak is typically more pronounced 1, 2
- Trigger mechanism: A critical concentration of estradiol from the dominant follicle causes positive feedback at the hypothalamus, resulting in increased GnRH secretion and the subsequent LH surge 4
Hormonal Pattern Throughout the Cycle
Early Follicular Phase (Days 1-7)
- FSH levels are higher than LH levels during this phase 1
- This FSH predominance drives follicular recruitment and growth
- Both hormones remain at relatively low baseline levels
Mid-Follicular Phase (Days 8-12)
- Gradual rise in estradiol from developing follicles
- LH and FSH remain relatively suppressed but begin to increase
Ovulatory Phase (Days 13-15)
- Peak LH and FSH occur simultaneously at mid-cycle 1, 2, 3
- The LH surge is the most reliable laboratory indicator of impending ovulation
- High FSH and LH levels with estradiol >150 pg/mL and progesterone <2 ng/mL indicate the mid-cycle gonadotropin surge 5
Luteal Phase (Days 16-28)
- Both LH and FSH decline after ovulation
- LH levels during the luteal phase are higher than during the follicular phase 2
- FSH levels during the luteal phase are lower than during the follicular phase 2
- Progesterone becomes the dominant hormone, rising 10-15 times above follicular phase levels 1
Clinical Interpretation Pitfalls
Common mistake: Interpreting normal or low-normal LH and FSH levels as always indicating hypogonadism. In functional hypothalamic amenorrhea (FHA), LH and FSH are often normal despite low estradiol, which confuses clinicians 5. The pattern of hormones matters more than absolute values.
Key distinguishing features:
- Mid-cycle surge: High FSH and LH with E2 >150 pg/mL indicates ovulation 5
- Ovarian insufficiency: Elevated FSH and LH with E2 <20 pg/mL indicates absent ovarian reserve 5
- FHA: Low or low-normal LH, normal FSH (usually higher than LH), with E2 <50 pg/mL 5
The basal body temperature rise remains the most reliable clinical indicator of ovulation, occurring after the LH surge 3.