It may surprise you to know that men don't have a monopoly on testosterone. Testosterone belongs to a class of male hormones called androgens. But women also have testosterone.

The ovaries produce both testosterone and estrogen. Relatively small quantities of testosterone are released into your bloodstream by the ovaries and adrenal glands. In addition to being produced by the ovaries, estrogen is also produced by the body's fat tissue. These sex hormones are involved in the growth, maintenance, and repair of reproductive tissues. But that's not all. They also influence other body tissues and bone mass.

What are hormones?

A hormone is a chemical substance. It's secreted by one tissue and travels by way of body fluids to affect another tissue in your body. In essence, hormones are "chemical messengers." Many hormones, especially those affecting growth and behavior, are significant to both men and women.

The amount and levels of hormones change daily. The sex hormones, estrogen and testosterone, are secreted in short bursts -- pulses -- which vary from hour to hour and even minute to minute. Hormone release varies between night and day and from one stage of the menstrual cycle to another.

What is estrogen?

Estrogen is an entire class of related hormones that includes estriol, estradiol, and estrone.

Estriol is made from the placenta. It’s produced during pregnancy.

Estradiol is the primary sex hormone of childbearing women. It is formed from developing ovarian follicles. Estradiol is responsible for female characteristics and sexual functioning. Also, estradiol is important to women's bone health. Estradiol contributes to most gynecologic problems, including endometriosis and fibroids and even female cancers.

Estrone is widespread throughout the body. It is the main estrogen present after menopause.

Why do estrogen levels fall?

There are many reasons why estrogen levels fall, including:

• Hypogonadism
• Hypopituitarism
• Pregnancy failure (estriol)
• Perimenopause and menopause (estradiol)
• Polycystic ovarian syndrome (PCOS)
• Anorexia nervosa (eating disorder)
• Extreme exercise or training

Drugs that block estrogen include clomiphene, which  tricks the body into thinking it has decreased levels of estrogen. Also, women experience low levels of estrogen immediately after childbirth and also during breastfeeding.

Why are athletes at risk for low levels of estrogen?

Women with low body fat often do not produce sufficient amounts of sex hormones. This can be a problem for women such as athletes, models, and gymnasts. It can also be a problem for women with eating disorders. These women can experience a cessation of menstruation, known as amenorrhea. They may also develop osteoporosis -- thin bones -- and fractures as well as other conditions more common in older women after menopause.

Do estrogen levels fall at menopause?

Yes. Estrogen levels fall at menopause. This is a natural transition for all women between ages 40 and 55. The decline in estrogen can happen abruptly in younger women whose ovaries are removed, resulting in so-called surgical menopause.

Perimenopause is the period of transition before menopause. The first natural decline in estrogen levels starts during this phase. Other physiological changes also start. Women going through perimenopause may experience weight gain along with other menopause symptoms. For instance, there may be irregular menstrual periods, hot flashes, and vaginal dryness.

On average, menopause occurs at age 51. When it does, a woman's body produces less estrogen and progesterone. The drop of estrogen levels at menopause can cause uncomfortable symptoms, including:

• Hot flashes
• Night sweats
• Vaginal dryness or itching
• Loss of libido or sex drive

Some women experience moodiness. That may or may not be related to the loss of estrogen. Lower levels of estrogen may also increase a woman's risk for heart disease, stroke, osteoporosis and fractures.

Why do estrogen levels rise?

During puberty, it's normal for levels of estrogen to rise. That’s because this hormone fuels changes in a young girl's body. For example, it plays a role in the development of breasts, a more mature curved figure, fuller hips, and pubic and underarm hair.

In addition, high levels of estrogen are seen in women who are extremely overweight. Estrogen levels rise during a healthy pregnancy, and increased estrogen levels may be seen with tumors of the ovaries, testes, or adrenal glands.

Some drugs, such as steroid medications, ampicillin, estrogen-containing drugs, phenothiazines, and tetracyclines can increase estrogen levels.

What happens when testosterone levels rise or fall?

If your body produces too much testosterone, you may have irregular or absent menstrual periods. You may also have more body hair than the average woman. Some women with high testosterone levels develop frontal balding. Other possible effects include acne, an enlarged clitoris, increased muscle mass, and deepening of voice.

High levels of testosterone can also lead to infertility and are commonly seen in polycystic ovarian syndrome (PCOS). PCOS is an endocrine condition that is sometimes seen in women of childbearing age who have difficulty getting pregnant. Women with PCOS have symptoms similar to those produced by high testosterone levels. They include:

• Obesity
• An apple-shaped body
• Excessive or thinning hair
• Acne
• Menstrual irregularity

PCOS is associated with:

• Higher levels of circulating male hormones
• Insulin resistance
• Carbohydrate intolerance -- conditions that make you prone to gaining weight
• Low levels of HDL -- ''good'' -- cholesterol
• Elevated triglycerides
• High LDL -- ''bad'' -- cholesterol
• Obesity
• High blood pressure

As women with PCOS age, the presence of these risk factors increases their risk for heart disease.

By the time of menopause, women have experienced a decline in testosterone since their 20’s that does not decrease further.” That decline may be correlated to a reduced libido. Some findings indicate that testosterone replacement therapy may benefit sexual function in certain perimenopausal and postmenopausal women. Testosterone replacement is unadvised in women with breast or uterine cancer. It also may increase the chances of cardiovascular disease or liver disease. So, experts are cautious about recommendations.

How do I know if my hormone levels are too high or too low?

Your doctor can do a physical examination and assess your health situation and symptoms to determine if further laboratory tests are needed to check hormone levels. Those tests may be important if you have a health condition such as PCOS or have stopped menstruating because of excessive athletic training or anorexia nervosa. If the tests show abnormal levels of hormones, your doctor can prescribe effective treatment.