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Last updated October 28, 2019. 7 minute read

Testosterone: what is it and how does it affect your health?

Often thought of as a “male hormone,” testosterone is an important hormone in both men and women. In males, Leydig cells, special cells in the testicles, are responsible for producing testosterone.

Dr Tzvi Doron Do Written by Tzvi Doron, DO
Reviewed by Mike Bohl, MD, MPH

Testosterone (T) is one of the biggest concerns men have about their health. Are my levels high enough? Would my sex life be better if I had higher T levels? Would I finally lose those extra pounds if I had higher T levels? Would I be more assertive at work and finally get that promotion I’ve been gunning for? These are just some of the questions men may ask themselves. 

On the flip side, some are concerned that testosterone may hurt them because of what they’ve heard or read. Does testosterone cause heart disease? What about high T levels in women? Does high T cause or exacerbate acne? Stay tuned as we demystify testosterone and answer these and other questions for you.

What is testosterone?


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Testosterone is usually thought of as a “male hormone,” but testosterone is an important hormone in both men and women. Hormones are chemicals that travel through the blood and act as messengers in the body. Testosterone is classified as a sex steroid hormone. Sex hormones are hormones that affect sexual development, and reproduction and steroid hormones are a group of hormones that are made out of cholesterol. 

Testosterone is a key male sex hormone that has both androgenic and anabolic functions. Androgenic refers to its effects on male sexual characteristics, and anabolic refers to its tissue-building functions. During puberty, T levels increase in males and are responsible for:

  • An increase in the size of the penis and testicles (androgenic)
  • Increase in muscle mass (anabolic)
  • A deepening of the voice (androgenic)
  • Strengthening of the bones (anabolic)
  • Increase in height (anabolic)
  • Increase in libido and aggression (androgenic

Testosterone levels usually begin to decline slowly at around age 30 and continue to decline by about 1% per year. Aside from aging, there are many other causes of decreases in testosterone levels (discussed below). 

If you have low testosterone, it’s important to be evaluated by a healthcare provider for the cause instead of just jumping to treat the low testosterone. If and when a cause is found, treating it directly can sometimes trigger the testosterone levels to normalize.

What does testosterone do?

While testosterone is important during puberty for developing sexual maturity in males, it continues to be important in men and women throughout life. In men, testosterone plays a major role in:

  • Libido
  • Erectile function
  • Sperm production
  • Maintaining bone density and muscle mass
  • Red blood cell production
  • Growth of facial and body hair
  • Mood regulation

This is why abnormal testosterone levels can cause so many different symptoms. Testosterone is also important in women, playing roles in:

  • Libido
  • Maintaining bone density and muscle mass
  • Mood regulation
  • Maintaining the health of certain vaginal tissues

How is testosterone produced?

In males, Leydig cells, special cells in the testicles, are responsible for producing testosterone. After puberty, the hypothalamus (a part of the brain) releases a hormone called gonadotropin-releasing hormone (GnRH) in pulses. GnRH stimulates the pituitary gland (also in the brain) to release luteinizing hormone (LH). LH reaches the testicles through the blood, where it stimulates the Leydig cells to produce testosterone from cholesterol through a process involving several enzymes. High levels of testosterone signal the hypothalamus and pituitary to stop releasing GnRH and LH, respectively, keeping tight control of the system.

In females, testosterone is made by the theca cells of the ovary, but most of it is converted to estrogen by an enzyme called aromatase. Females typically only have 5–10% of the amount of testosterone as males, but it is still crucial for normal sexual function. 

In both sexes, a small amount of testosterone is also made in the adrenal glands, which are glands that are part of the endocrine system and sit on top of the kidneys.

Testosterone imbalances

In both males and females, it’s important to have testosterone levels that are within a normal range. T levels that are too high or too low can cause problems, although the issues that they cause are different in males and females. 

What are the symptoms of low T?

As discussed above, testosterone levels typically begin a slow decline around the fourth decade of life. Aside from aging, there are many other causes of decreased testosterone levels, including but not limited to:

  • Obesity
  • Sleep problems, like sleep apnea
  • Genetic diseases (e.g., Klinefelter syndrome)
  • Autoimmune diseases
  • Certain types of infections (e.g., mumps, HIV)
  • Drugs (e.g., glucocorticoids, opioids, some antifungals)
  • Chronic diseases (e.g., chronic kidney disease, chronic liver disease, diabetes)
  • Chemotherapy 
  • Radiation
  • Tumors (e.g., prolactinomas)

A healthcare provider will take a detailed history, perform a physical exam, and order specific blood tests and sometimes imaging to determine if there is a secondary cause for low T.

Since testosterone affects so many bodily functions, low levels (also known as hypogonadism) can have various seemingly unrelated signs and symptoms. Low T can cause:

  • Decreased libido
  • Erectile dysfunction (with loss of morning erections) 
  • Fatigue
  • Loss of muscle mass
  • Increased fat gain
  • Anemia
  • Osteoporosis

Testosterone levels normally vary quite a bit throughout the day, with levels being highest in the morning. Levels can also vary day to day depending on many factors, which is why doctors usually require two early-morning testosterone levels (usually 8–10 am) on different days to be low before diagnosing hypogonadism in men. This suggests that low T levels are chronic and ongoing. A low level is usually defined as being <300 ng/dL. However, some sources and laboratories use a cutoff of <270 ng/dL instead.

Can you have too much testosterone?

Having too much testosterone is also not a good thing, and it can happen to both men and women. In men, the most common cause is using anabolic steroids or taking too high a dose of testosterone prescribed by a healthcare provider. Anabolic steroids are drugs that are similar to testosterone but are designed to have greater anabolic effects and less androgenic effects. Technically, these drugs cause testosterone levels to decrease because they inhibit the body’s own natural testosterone production. Symptoms of having too much testosterone (or too much androgenic hormones in the case of anabolic steroid use) in men include:

  • Acne, including body acne (aka “backne” when it appears on the back)
  • Enlarged prostate
  • Breast enlargement (gynecomastia) due to some of the testosterone being converted to estrogen by a process called “aromatization.”
  • Worsening of sleep apnea 
  • Fluid retention
  • Decreased testicle size 
  • Decrease in sperm count
  • Increase in red blood cells (erythrocytosis)

Women can suffer from hyperandrogenism, a condition where testosterone or other androgens (such as DHEA) are too high. There are several causes of hyperandrogenism in females.

  • Polycystic ovary syndrome (PCOS)
  • Non-classical congenital adrenal hyperplasia (an enzyme disorder leading to high T levels among other things)
  • Idiopathic hyperandrogenism
  • Ovarian or adrenal tumors
  • Cushing syndrome
  • Drugs

The most common cause of high T levels in women is PCOS, which occurs in up to 12% of women and is the most common female endocrine disorder. Signs of hyperandrogenism in females include:

  • Menstrual irregularities
  • Infertility
  • Hirsutism (excess facial and body hair in a male pattern)
  • Acne
  • In some cases, deepening of the voice, growth of the larynx, or “Adam’s apple” and clitoral enlargement (clitoromegaly). These signs are concerning for tumors and should be investigated urgently.
  • Women with these symptoms should have a detailed history and physical exam by a healthcare provider, and lab tests (and sometimes imaging) are necessary to determine the cause of hyperandrogenism as the treatment varies depending on the cause.

Testing and treatment of low T

Diagnosis of low T requires two morning T levels between 8 am, and 10 am to be low (below 300 ng/dL). This is important because the signs and symptoms of low T can be caused by other things in people with normal T levels. Some healthcare providers will consider treating slightly higher T levels in people who are symptomatic, and some will also use free T levels when making treatment decisions. Free T refers to the T that is not bound to proteins in the blood. 

Once low T is diagnosed, causes must be sought out before deciding on a treatment option. When a cause is found, treatment should begin by treating the cause of the low T. Examples would be weight loss in men with obesity and treating sleep apnea in those who have it. 

If no cause is found, there are a few drug options that can be used in different scenarios. Clomiphene citrate is a drug known as a selective estrogen receptor modulator (SERM). Clomiphene works by tricking the pituitary into making more LH and FSH. The increased LH levels lead to increased T levels in men provided that their testicles are able to manufacture testosterone, and by raising FSH levels, clomiphene increases sperm count in some men. It is commonly used in men who have low T and want to preserve their fertility because taking testosterone has a detrimental effect on sperm production.

Another drug that can be used in some men to increase T levels is human chorionic gonadotropin (hCG). HCG is a hormone found in high levels in women during pregnancy and is actually the hormone that is being tested in both at-home and laboratory pregnancy tests. HCG is very similar to LH and can increase testosterone in men with low T and functioning testicles.

Testosterone replacement therapy (TRT) is used in men who cannot manufacture their own T and in men who no longer desire fertility. It comes in various forms, including:

  • Injections
  • Topical gels
  • Patches
  • Mouth patch or troche (a small lozenge that dissolves in the cheek or under the tongue)
  • Pellets that are implanted under the skin
  • A new oral formulation (FDA, 2019)

All testosterone products have a boxed warning that they may increase the risk of cardiovascular disease, including heart attacks and strokes. This was based on some studies showing that TRT may increase these risks. Other studies have shown that TRT did not increase the risks of cardiovascular events and may even reduce these risks (Cheetham, 2007). There is a need for ongoing research to determine the true cardiovascular risks of TRT.


  • Often thought of as a “male hormone,” testosterone is an important hormone in both men and women.
  • In males, Leydig cells, special cells in the testicles, are responsible for producing testosterone. 
  • Testosterone levels usually begin to decline slowly at around age 30 and continue to decline by about 1% per year.
  • Diagnosis of low T requires two morning T levels between 8 am, and 10 am to be low (below 300 ng/dL).