What does the Follicle Stimulating Hormone test measure?

Why is Follicle Stimulating Hormone measured?

FSH measured on cycle days 2 to 5 provides a conventional assessment of ovarian reserve, with elevation indicating diminished reserve, and is central to distinguishing primary ovarian insufficiency from hypothalamic or pituitary causes of amenorrhoea. It is interpreted only alongside oestradiol, since a raised oestradiol suppresses FSH and can mask diminished reserve. Anti-Mullerian hormone and antral follicle count are less cycle-dependent and have largely superseded FSH for ovarian reserve assessment in fertility practice.

Reference ranges

Standard reference

Cycle-phase dependent in menstruating women — follicular phase approximately 3–10 mIU/L; postmenopausal typically >25 mIU/L

Functional medicine optimal

Not applicable — interpreted against cycle-phase-specific intervals rather than an independent target

Units

mIU/mL, IU/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.