- Estrogen is a group of steroid hormones that play a pivotal role in the development, regulation, and maintenance of the female reproductive system and secondary sexual characteristics. Although present in both sexes, estrogen is primarily considered a female sex hormone, with significantly higher levels in females, particularly during the reproductive years.
- The three main naturally occurring forms of estrogen in humans are estradiol (E2), estrone (E1), and estriol (E3), each differing slightly in structure and predominant during different life stages. Estradiol is the most potent and predominant form during reproductive age, estrone is more prominent after menopause, and estriol is dominant during pregnancy.
- Estrogen is synthesized mainly in the ovaries, specifically by the granulosa cells of developing follicles, under the influence of follicle-stimulating hormone (FSH). During the menstrual cycle, estrogen levels rise in the follicular phase, stimulating the proliferation of the endometrium (the uterine lining), preparing it for potential embryo implantation. Estrogen also triggers the luteinizing hormone (LH) surge, which leads to ovulation—the release of an egg from the ovary. After ovulation, estrogen continues to work in concert with progesterone to maintain the endometrium in the luteal phase.
- In addition to its reproductive functions, estrogen plays a crucial role in the development of female secondary sexual characteristics, such as breast development, widening of the hips, and distribution of body fat. It also contributes to skeletal health by promoting bone density, making it a protective factor against osteoporosis. Estrogen helps maintain skin elasticity, supports cardiovascular function, and influences mood and cognition through its action on the central nervous system.
- Estrogen acts by binding to estrogen receptors (ERα and ERβ), which are widely distributed throughout the body in tissues including the uterus, brain, liver, bones, and cardiovascular system. Upon binding, the hormone-receptor complex influences the transcription of target genes, affecting cellular function and development. This widespread receptor distribution explains why estrogen has systemic effects far beyond the reproductive tract.
- Throughout life, estrogen levels fluctuate significantly. They peak during the reproductive years, vary during the menstrual cycle, surge during pregnancy, and decline during menopause. The sharp drop in estrogen levels during menopause is responsible for many of the symptoms experienced by menopausal women, such as hot flashes, vaginal dryness, mood swings, and bone loss.
- Clinically, estrogen has a wide range of applications. It is a key component in hormonal contraceptives, where it helps suppress ovulation and regulate the menstrual cycle. Estrogen is also used in hormone replacement therapy (HRT) to alleviate menopausal symptoms and prevent postmenopausal osteoporosis. In addition, selective estrogen receptor modulators (SERMs) and aromatase inhibitors, which modify or block estrogen’s effects, are used in the treatment of estrogen-sensitive cancers, particularly breast cancer.
