Follicle-stimulating hormone (FSH) is made by the pituitary gland in the brain and signals the ovaries to develop egg follicles each cycle.
As the ovaries become less responsive in perimenopause, the brain releases more FSH to try to stimulate them, so FSH levels rise. A high FSH can therefore point toward the menopause transition.
However, because FSH and estrogen swing widely from day to day during perimenopause, a single blood test is usually unreliable for diagnosis — menopause is normally identified from symptoms and menstrual pattern instead. See menopause hormone testing.