Hormone levels vary during different parts of a normal ovulatory cycle. The hypothalamus regulates the reproductive hormones, signaling increased or decreased production. It accomplishes this by varying the levels of gonadotropin-releasing hormone (GnRH), which stimulate the pituitary to manufacture reproductive hormones.

Three particularly key hormones to monitor are follicle stimulating hormone (FSH), luteinizing hormone (LH), and estrogen. During the first part of the ovulatory cycle, the pituitary increases FSH production, which is necessary for follicular growth. As healthy follicles mature, they produce estrogen. Increases in estrogen cause the pituitary to reduce FSH production. Once the follicles mature, a surge of LH from the pituitary triggers ovulation approximately 36 hours later.

A hormonal evaluation measures the levels of FSH, LH, and estrogen in the bloodstream. Elevated FSH and/or estrogen levels are two indicators of diminished ovarian reserve, which causes eggs to lose their capacity to fertilize and develop.