E2; estradiol; 17-beta (o)estradiol
Oestradiol is a steroid hormone made from cholesterol and is the strongest of the three naturally produced oestrogens. It is the main oestrogen found in women and has many functions, although it mainly acts to mature and maintain the female reproductive system. A natural increase in blood oestradiol concentrations during the menstrual cycle causes an egg to mature and be released; that is, to be ovulated. Another important role of oestradiol is to thicken the lining of the uterus so that the egg can implant if it becomes fertilised. Oestradiol also promotes development of breast tissue and increases both bone and cartilage density.
In premenopausal women, oestradiol is mostly made by the ovaries. Oestradiol levels vary throughout the monthly menstrual cycle, being highest at ovulation and lowest at menstruation. Oestradiol levels in women reduce slowly with age, with a large decrease occurring at the menopause when the ovaries ‘switch off’. In pregnant women, the placenta also produces a lot of oestradiol especially towards the end of the pregnancy.
Men also produce oestradiol; however, the amounts produced are much lower than in women. Within the testes, some testosterone is changed into oestradiol and this oestradiol is essential for the production of sperm. In both sexes, oestradiol is also made in much smaller amounts by fat tissue, the brain and the walls of blood vessels.
The production of oestradiol in women’s ovaries is controlled by hormones released from both the hypothalamus in the brain and the pituitary: this is called the reproductive axis in the female and is also known as the hypothalamic–pituitary–ovarian (or gonadal) axis. The hypothalamus in the base of the brain releases a hormone called gonadotropin-releasing hormone. Gonadotropin-releasing hormone then acts on the pituitary gland to cause the release of two further hormones, luteinising hormone (LH) and follicle stimulating hormone (FSH). LH and FSH enter the blood and stimulate the ovary; in particular, LH and FSH act on the cells that surround each egg (these cells plus the egg form a unit called a follicle) stimulating the follicle to grow and develop. In the last stages of growth and development the cells surrounding the egg will produce oestradiol. After the egg has been ovulated, the ovulated follicle will become a corpus luteum. The corpus luteum produces both progesterone and oestradiol and the primary role of these two hormones is to ensure that the lining of the uterus is fully prepared for implantation, if fertilisation occurs. The amount of oestradiol (and progesterone) in the circulation communicates with the hypothalamus and pituitary to control the development of an egg, ovulation and the menstrual cycle.
In women, too much oestradiol can have a number of effects. In mild cases, excess oestradiol may cause acne, constipation, loss of libido and depression. More severe effects can include, weight gain, female infertility, stroke, heart attack and an increased risk of developing uterine and/or breast cancer.
In men, too much oestradiol can also cause sexual dysfunction, loss of muscle tone, increased body fat and the development of female characteristics, such as breast tissue. Oestradiol becomes more dominant as a man ages and his testosterone production reduces, which scientists think may be a contributing factor in the development of prostate cancer.
The combined oral contraceptive pill (the pill) contains synthetic forms of both progesterone and oestradiol. The pill works by preventing ovulation, making it nearly 100% effective in preventing pregnancy. As well as preventing ovulation, the synthetic hormones make the cervical mucus thicker and therefore more difficult for the sperm to move through, thereby reducing their chances of getting to the uterus and oviducts. The synthetic oestradiol was added to prevent breakthrough bleeding, which sometimes occurs with the progesterone-only pill (the mini pill).
Oestradiol is necessary for bone development, so people with low oestradiol tend to have skeletal problems like inadequate bone growth and osteoporosis. Girls will also encounter problems at puberty such as a delay in, or failure of, breast development, a disrupted or absent menstrual cycle and infertility. Oestradiol also has important roles in the brain, where low levels can cause depression, fatigue and mood swings.
A woman’s oestradiol production falls naturally at the menopause and causes many of its symptoms. Initially these include night sweats, hot flushes, vaginal dryness and mood swings, while in the long term she is more likely to develop osteoporosis. Oestradiol is used in hormone replacement therapy to relieve these symptoms of the menopause in women. There are many recognised pros and cons to hormone replacement therapy. See the articles on menopause and ‘What is HRT?’ for more information.
Last reviewed: Mar 2018