The hypothalamic-pituitary-ovarian axis and oral contraceptives: Regulation and function

Abstract

This chapter provides an overview of the hypothalamic-pituitary-ovarian (HPO) axis, otherwise known as the female reproductive axis. The HPO axis is the major regulator of the female reproductive hormones: estrogen and progesterone. Gonadotropin-releasing hormone is released from the hypothalamus at the onset of puberty in young women, and induces the release of luteinizing hormone (LH) and follicular stimulating hormone (FSH) from the anterior pituitary. These two hormones bind to ovarian receptors and signal the release of estrogen and progesterone. Estrogen and progesterone are released in fluctuating concentrations throughout the menstrual cycle, resulting in the follicular (low estrogen) phase and the luteal (high estrogen) phase. These phases are separated by ovulation and end with either fertilization or menstruation in a eumenorrheic woman. Oral contraceptives (OCs) prevent unwanted pregnancy by producing high estrogen levels during the follicular phase. This increase in estrogen inhibits the mid-cycle gonadotropin surge necessary for ovulation. OCs generally consist of both estrogen and progesterone. Combination pills, both biphasic and triphasic, help reduce the dose of estrogen and help lower any side effects such as water retention. There is currently not enough evidence to determine if combination pills increase incidence of accidental pregnancy since most OCs available outside of the US are monophasic. Despite possible adverse side effects and incidence of unwanted pregnancy, OCs are used by millions of women, and this number will most likely continue to increase as worldwide health care expands.