FSH Timing for Women vs Men
For women evaluating ovarian function or infertility, FSH must be drawn during the early follicular phase (cycle days 2-5) if oligomenorrheic, or randomly if amenorrheic; for men, FSH can be drawn at any time without cycle-specific timing requirements.
Women: Cycle-Specific Timing is Critical
When Menstruating (Oligomenorrhea)
- Draw FSH on cycle days 2-5 (early follicular phase) 1, 2
- Must be accompanied by estradiol measurement to rule out pre-ovulation peak 3
- During pre-ovulation LH/FSH peak, estradiol is typically 150-200 pg/mL per mature follicle, which would falsely elevate FSH 3
When Amenorrheic
- Draw FSH randomly (no specific timing needed) 2
- If initial FSH is elevated (>25 IU/L) with low estradiol, repeat 1 month later to confirm diagnosis of premature ovarian insufficiency (POI) 3
- The repeat assay is essential because FSH can fluctuate, and diagnosis requires two elevated measurements 3
Critical Pitfalls to Avoid in Women
Hormonal contraception interference: Women using combined estrogen-progestin contraception or macro-progestins will have falsely suppressed FSH levels. Stop antigonadotropic treatment and wait at least 3 months before drawing FSH 3. Ideally wait 2 months after stopping oral contraceptive pills before assessment 2.
Interpretation thresholds for women:
- FSH >25 IU/L (outside pre-ovulation peak) indicates POI in women under 40 years 3
- Elevated FSH with low estradiol (<20 pg/mL) and progesterone (<1 ng/mL) indicates ovarian insufficiency 1
- High FSH with estradiol >150 pg/mL suggests midcycle surge rather than ovarian failure 1
Men: No Specific Timing Required
FSH can be drawn at any time in men without regard to time of day or cyclical variation 4, 5. The male reproductive axis does not have the cyclical fluctuations seen in women.
When to Draw FSH in Men
Draw FSH (along with testosterone and LH) when 4, 5:
- Sperm concentration <5 million/mL with elevated FSH or testicular atrophy
- Azoospermia suspected to be from impaired sperm production
- Clinical findings suggest endocrinopathy or impaired sexual function
Interpretation in Men
- FSH <7.6 IU/L with normal testicular size suggests obstructive azoospermia 4
- FSH >7.6 IU/L with testicular atrophy suggests non-obstructive azoospermia (spermatogenic failure) 4
- FSH >4.5 IU/L may be associated with abnormal semen parameters (sperm concentration and morphology), suggesting the "normal" range may need reconsideration 6
Concurrent Testing
Always measure total testosterone and LH alongside FSH in men 4, 5:
- Low testosterone with low/normal LH and FSH indicates hypogonadotropic hypogonadism
- Low testosterone with elevated LH and FSH indicates primary testicular failure
- If testosterone is low, add prolactin to the panel 4
Summary Algorithm
For women with regular cycles: Draw FSH + estradiol on days 2-5
For women with amenorrhea: Draw FSH + estradiol randomly, repeat in 1 month if elevated
For women on hormonal contraception: Stop for 2-3 months, then draw FSH + estradiol on days 2-5
For men: Draw FSH + testosterone + LH at any time when clinically indicated