Why we measure it

FSH and LH are the diagnostic bridge between the pituitary and the gonads. In females, rising FSH is the earliest measurable signal of declining ovarian reserve — it rises as the diminishing follicle pool produces less inhibin B and oestradiol, causing the pituitary to increase FSH output in an attempt to recruit remaining follicles. Early-day FSH above 10 IU/L signals significantly reduced ovarian reserve and reduced fertility, even before AMH falls below the typical clinical threshold. In males, the FSH and LH pattern distinguishes primary hypogonadism (testicular failure — high LH and FSH with low testosterone, as the pituitary compensates) from secondary hypogonadism (pituitary or hypothalamic dysfunction — low LH and FSH with low testosterone, confirming the fault is upstream). This distinction fundamentally changes clinical management: primary hypogonadism requires testosterone replacement; secondary may respond to gonadotropin stimulation or treatment of the underlying pituitary pathology. In females with polycystic ovary syndrome (PCOS), an elevated LH:FSH ratio (typically > 2:1) in the early follicular phase is a classical finding reflecting disordered GnRH pulsatility.

Why every 6 months

In premenopausal females, FSH and LH vary significantly across the menstrual cycle — FSH peaks in the early follicular phase and at ovulation; LH surges dramatically at ovulation (triggering egg release). For consistent interpretation, FSH should ideally be measured on days 2–5 of the menstrual cycle (early follicular phase). LH measured at any other time may reflect the mid-cycle surge rather than the basal pituitary output. In males, gonadotropins show less intra-individual variability and can be measured at the same morning draw as testosterone. The six-month interval tracks meaningful changes — the progressive FSH rise of perimenopause, the gonadotropin response to treatment, or the gradual LH suppression that accompanies rising BMI and leptin.

What movement means

Reference ranges vary by sex, age, and menstrual cycle phase. Ranges below are approximate; laboratory-specific values should be consulted. FSH and LH are reported in IU/L (international units per litre).

In the protocol