Testosterone, the primary sex hormone in men, is produced mainly in the testicles, and causes men to develop a deep voice, large muscles, and body hair.
Testosterone is also essential for the production of sperm.
When levels of testosterone fall known as low testosterone or low T it can have a profound effect on men, both physically and psychologically.
Normal levels of testosterone in healthy men range from about 300 nanograms per deciliter (ng/dL) up to 1,000 ng/dL, according to Medline Plus.
The normal range is broad and varies by age. In older men, the lower end of normal may be lower than in younger men.
Levels of testosterone in the male body rise during and after puberty, then gradually decrease after age 30.
This gradual lowering of testosterone is sometimes referred to as andropause or male menopause.
Testosterone levels may also be lower in men with hypogonadism, a condition in which the body is unable to produce normal amounts of testosterone.
Hypogonadism can occur because of a problem with the testicles or with the pituitary gland, which controls the testicles.
Being obese, having medical conditions such as autoimmune diseases or type 2 diabetes or using drugs including alcohol or opioids, can also contribute to low levels of testosterone.
Women also produce testosterone, although much less than men: A normal testosterone level in women is 15 to 70 ng/dL according to Medline Plus.
In women, testosterone is produced in the ovaries and the adrenal glands.
Similar to men, low levels in women can be caused by a variety of medical conditions as well as by advancing age.
In the years leading up to menopause, women normally experience a drop in testosterone levels.
Low levels in women may be associated with decreased libido, low energy, and depressed mood.
There is some evidence that testosterone replacement therapy can increase sex drive and help with other sexual problems in some women.
But the long-term safety of this treatment is unknown, and some experts say it may raise the risk of breast cancer, though there has yet to be conclusive evidence.
Currently, testosterone preparations are not approved for use in women by the Food and Drug Administration (FDA).
Male hypogonadism may be present from birth or may develop later in life as a consequence of injury or infection.
In infants, symptoms of hypogonadism include:
In boys around the age of puberty, symptoms include:
In men, symptoms of hypogonadism include:
As the level of testosterone in the body decreases with age, men may also experience:
However, these symptoms are nonspecific, meaning they can be caused by many factors, not just low testosterone.
Male hypogonadism is diagnosed based on symptoms, blood tests of testosterone level, and other lab tests, as needed.
Its typically treated with testosterone replacement therapy and other therapies, depending on whats causing it.
In men who are obese, or who have an underlying medical condition such as diabetes that can contribute to low T, losing weight or managing the underlying condition often causes testosterone levels to normalize.
In recent years, many advertising dollars have been spent on promoting testosterone replacement therapy to middle-aged men experiencing such symptoms as fatigue and low libido.
Whether treating such men with testosterone is safe or appropriate, however, is a matter of some controversy. Some doctors are strongly in favor of it, while others are much more cautious.
In 2015, the Food and Drug Administration (FDA) issued a safety alert regarding testosterone therapy for men whose low-T levels are the result of age (as opposed to hypogonadism or other disorders).
"FDA has concluded that there is a possible increased cardiovascular risk associated with testosterone use," the FDA alert stated, noting that some studies pointed to a higher risk of heart attack, stroke, or death among men using testosterone products.
Overall, studies of testosterone replacement therapy in healthy older men have had mixed results: Some men feel better, while others feel unchanged.
Studies have also revealed that taking testosterone comes with other risks for this population, including acne, enlarged breasts, low sperm production, deep vein thrombosis (DVT), and accelerated growth of existing prostate cancer.
The symptoms of low testosterone (or low T) in males can vary depending on the cause of the low level and the age at which it occurs.
In male hypogonadism, a condition in which the body is unable to produce normal amounts of testosterone, symptoms may include underdeveloped genitalia, delayed puberty, and a lack of secondary sexual characteristics such as a deeper voice and facial hair.
In middle-aged or older men experiencing age-related decreases in testosterone, symptoms may include low energy, depressed mood, low sex drive, and erectile dysfunction (ED or impotence).
No matter what the cause, symptoms alone are not enough for a diagnosis of low testosterone: A blood test is needed to confirm that a mans testosterone level is, indeed, low.
Male hypogonadism can occur at any age, as a result of problems affecting the testicles or the pituitary gland.
Signs of hypogonadism in infants include:
In boys, hypogonadism is associated with delayed puberty and can cause symptoms such as:
In adult men, symptoms of hypogonadism include:
Blood testosterone levels in men with hypogonadism are very low and do not fluctuate from day to day, the way they do in healthy men.
Low testosterone in men can also cause nonspecific symptoms such as:
Not all men with age-related low testosterone have or are bothered by symptoms. In addition, the level at which symptoms occur varies from man to man.
Nonspecific signs and symptoms such as fatigue, sleep problems, and low mood can also be caused by other factors such as medication side effects, depression, and excessive alcohol use.
Women also produce testosterone in the ovaries and adrenal glands and they also experience a normal drop in testosterone levels in the time leading up to menopause.
This drop may be associated with a decrease in libido (sex drive), low energy, persistent fatigue, and depressed mood.
Hypogonadism and age-related low testosterone are diagnosed with blood tests that measure the level of testosterone in the body.
The Endocrine Society and the American Association of Clinical Endocrinologists recommend testing for suspected low T with a total testosterone test performed in the morning (when testosterone levels tend to be highest in young men, although this isnt necessarily the case in older men).
The test is often repeated on another day if the results show a low T level.
Sometimes a test for free or bioavailable testosterone is also performed.
The majority of testosterone in the blood is bound to one of two types of protein either albumin or sex hormone binding globulin (SHBG) while a small percentage is unbound, or free.
The portion of testosterone that is bound to SHBG is biologically inactive but, because the bonds between testosterone and albumin are weaker than those between testosterone and SHBG, the portion bound to albumin is biologically active.
Bioavailable testosterone therefore includes free testosterone and albumin-bound testosterone.
A number of medical conditions and medications can raise or lower the amount of SHBG in the blood, consequently altering the amount of bioavailable testosterone.
In those situations, measuring the amount of bioavailable testosterone gives a more accurate indication of the amount of biologically active testosterone in a persons system.
After age 30, most men experience a gradual drop in testosterone.
However, this decrease rarely results in a low T level thats substantially lower than whats considered normal for a middle-aged or older man.
A variety of medical conditions, however, can cause significantly lower-than-normal testosterone levels in boys and men.
Some drugs are also associated with low testosterone levels, including:
Sometimes the cause of low testosterone in a man is unknown, and many men have several different causes.
Obesity, which is epidemic in the United States as well as a growing problem in other parts of the world, has been observed to have a lowering effect on testosterone levels in men.
Obesity is defined as a Body Mass Index (BMI) higher than 30 kg/m2.
A study published in the Journal of Clinical Endocrinology & Metabolism in 2007 found that a 4- to 5-kg/m2 increase in BMI is associated with a drop in testosterone comparable to that associated with 10 years of aging.
A 2008 study in the same journal found that a large waist or an increase in waist circumference over time is an important contributor to low T even more important than age.
The good news is that weight loss is associated with an increase in testosterone levels.
A 2004 study in Diabetes, Obesity and Metabolism found that men who lost weight quickly over 9 weeks on a very-low-calorie diet and maintained their weight loss for 12 months had sustained increases in testosterone.
Studies of men who have had bariatric (weight-loss) surgery have also shown that significant weight loss leads to significant increases in testosterone levels.
But even moderate weight loss can help: In research presented at the 2012 annual meeting of the Endocrine Society, middle-aged obese men who lost modest weight while following a year-long diet and exercise program had modest increases in testosterone level.
While abnormally low testosterone levels in women get less attention than low-T levels in men, they can occur and are caused by a variety of factors, including:
Women also experience a gradual lowering of testosterone production in the years leading up to menopause.
Low testosterone in men is sometimes treated medically, with testosterone supplementation.
In other cases, it can be treated with lifestyle changes such as efforts to lose excess weight and exercise more.
While testosterone therapy has yet to be approved for women in the United States, some doctors do prescribe it off-label to treat sexual problems and other symptoms associated with low testosterone levels in women.
When an underlying medical condition is causing or contributing to "low T," treating that condition can return the testosterone level to normal.
Testosterone can be delivered to the body in a variety of ways:
Men and women who are being treated with testosterone need to have follow-up blood tests to assess how theyre responding to treatment.
When used to treat male hypogonadism, a condition in which the body is unable to produce normal amounts of testosterone because of a problem with the testicles or the pituitary gland, testosterone replacement therapy is rarely associated with serious side effects.
However, in boys with delayed puberty, the treatment can cause the bones to stop growing and fuse prematurely, resulting in short stature as an adult.
Bone development must therefore be checked every six months with x-rays in adolescents receiving testosterone.
When testosterone is used to raise testosterone levels in men with age-related low testosterone, it comes with a number of risks, including:
In March 2015, the Food and Drug Administration (FDA) issued a safety alert regarding the use of testosterone in men whose low-T levels are due to the normal aging process, not hypogonadism or other health conditions.
The alert noted that there is some evidence of an increased risk of heart attack, stroke, and death among men using testosterone products.
"FDA has become aware that testosterone is being used extensively in attempts to relieve symptoms in men who have low testosterone for no apparent reason other than aging," the FDA safety alert stated.
"Healthcare professionals should prescribe testosterone therapy only for men with low testosterone levels caused by certain medical conditions and confirmed by laboratory tests," the FDA statement added.
"Patients using testosterone should seek medical attention immediately if symptoms of a heart attack or stroke are present, such as chest pain, shortness of breath or trouble breathing, weakness in one part or one side of the body, or slurred speech."
The risks of testosterone therapy for women with low testosterone are largely unknown.
Among some doctors, there is concern (but no conclusive evidence) that it may raise the risk of breast cancer.
Its not safe to use testosterone during pregnancy or when breastfeeding.
Testosterone therapy in men also has risks for women and children if they come into contact with testosterone gel (or other topical products) on a mans skin or on clothes, towels, or surfaces he has touched.
Children, in particular, are at risk of developing male secondary sexual characteristics such as enlarged genitals, growth of pubic hair, increased sex drive, and aggressive behavior if exposed to testosterone drugs.
Children who are exposed to testosterone may also experience advanced bone aging, possibly causing them to stop growing prematurely.
Women who come into contact with testosterone may develop irregular menstrual periods, increased acne, body hair growth, male-pattern baldness, or other male characteristics.
Because testosterone can cause birth defects, pregnant women must be very careful to avoid contact with it and to wash their skin thoroughly with soap and water if they think theyve contacted any testosterone preparations.
Contraindications are reasons not to take a certain drug or other therapy. The following are contraindications to testosterone therapy:
Ask your doctor if you have a condition that contraindicates the use of testosterone.
Losing excess weight and being physically active can raise a mans testosterone level.
Any form of exercise done regularly can help, but weight training and other forms of resistance exercise have the biggest effects.
Researchers have found that men who lost weight on a very-low-calorie diet, and maintained their weight loss for 12 months, had sustained increases in testosterone.
Men who had bariatric (weight-loss) surgery have also experienced significant increases in testosterone levels.
Even moderate weight loss helps: Middle-aged obese men who lost modest weight while following a diet and exercise program had modest increases in testosterone level, according to one study.
Hear the words low testosterone (or low T) and you probably think mens health.
But women actually need small amounts of testosterone, too, as part of the mix of hormones that keep mood, energy levels, sex drive, and bodily functions working smoothly.
Product marketing around testosterone creams and services argues that low T could be a reason for low sex drive in women, and that low T in women can be solved with hormone replacement therapy.
How accurate are statements like these?
Chances are, if youve taken a birth control pill, youve already taken a very small dose of a testosterone derivative, points out reproductive endocrinologist David P. Cohen, MD, chief of the section of reproductive endocrinology and infertility at the University of Chicago.
The packet information calls it progesterone, but its really a derivative of testosterone, he explains.
Testosterone, part of a hormone class known as androgens, originates in your ovaries and your adrenal glands.
Even women whose ovaries have been removed probably make enough testosterone with their adrenal glands to meet their needs, Dr. Cohen says.
So while men with low T could be facing low energy, low mood, low sex drive, and changes in sleep patterns, women dont face the same dire effects.
Low testosterone is not much of a concern for women, he points out.
This is important to know because women can be persuaded by advertisements for testosterone creams and other products that slathering on testosterone will cure low sex drive or a blah mood.
Testosterone does play a role in women, though. Healthy testosterone levels help a woman:
Some women may benefit from very small amounts of testosterone, says Cohen, but the side effects of excess testosterone can be worse than the reasons you might want to try hormone replacement therapy in the first place.
These side effects include male pattern hair loss, acne, male pattern hair growth (such as facial hair), changes in menstrual cycles, clitoromegaly (an abnormally enlarged clitoris), and a deepening of one's voice.
Some of these changes, such as clitoromegaly and a deeper voice, could be permanent, Cohen warns.
Women who use testosterone continuously also run the risk of developing lipid profiles similar to those of men, which means an increased risk for heart disease.
In fact, women are more likely at risk for conditions such as polycystic ovarian syndrome (PCOS), in which they might have too much testosterone (or androgens) compared with levels of other hormones, than they are for conditions such as low T.
That said, Cohen says that he does prescribe very small doses of testosterone for some women if its medically appropriate to do so, but the doses are so small that he hasnt seen a clear dose-response relationship.
Dosing is very individualized, he adds, if hormone replacement therapy proves to be needed. You dont titrate to a specific number you titrate to symptoms," he says.
If you feel better, its the right dose. If you start to see some of the side effects, such as male pattern hair loss, your doctor will stop or reduce the amount of testosterone.
So, if youre wondering whether the blue mood, lack of energy, and lack of desire youre struggling with is related to low T, talk with your doctor.
Dont fall for over-the-counter testosterone or bioidentical hormone product marketing and dose yourself. The side effects could be worse than the problems youre trying to solve.