Testosterone: What It Is, Function & Levels (2024)

What is testosterone?

Testosterone is a hormone that your gonads (sex organs) mainly produce. More specifically, the testicl*s in people assigned male at birth (AMAB) and the ovaries in people assigned female at birth (AFAB) produce testosterone.

Your adrenal glands also produce the hormone dehydroepiandrosterone (DHEA), which your body transforms into testosterone and estrogen.

Testosterone is the main androgen, meaning it stimulates the development of male characteristics. Levels of testosterone are naturally much higher in people AMAB than in people AFAB.

Is testosterone a steroid?

Natural testosterone is a steroid — an anabolic-androgenic steroid. "Anabolic" refers to muscle building, and "androgenic" refers to increased male sex characteristics.

However, when you hear people use the term “anabolic steroids” they are generally referring to synthetic (made in a lab) variations of testosterone that are injected into your body.

Healthcare providers use synthetic testosterone to treat and manage various medical conditions.

Synthetic testosterone is the main drug of masculinizing hormone therapy, which is a gender-affirming treatment that someone AFAB uses to produce the secondary sex characteristics associated with being assigned male at birth (AMAB).

Some athletes and bodybuilders misuse synthetic testosterone (anabolic steroids) by taking very high doses of them in an attempt to boost performance or change their physical appearance. Misuse of these drugs can cause several unpleasant symptoms and lead to long-term dangerous health problems, including blood clots, stroke, and potentially increase the risk of prostate cancer.

What does testosterone do?

Testosterone has different roles in different life stages, including:

• Fetal development.
• Puberty for male children.
• Adulthood.

Testosterone and fetal development

At around week seven in utero, the sex-related gene on the Y chromosome initiates the development of the testicl*s in male infants. The testicl*s produce testosterone.

Testosterone triggers the development of the male internal and external reproductive organs during fetal development.

Testosterone and puberty

Testosterone is responsible for many of the changes seen in children assigned male at birth during puberty, including:

• An increase in height.
• Body and pubic hair growth.
• Enlargement of their penis, testes and prostate gland.
• Increasing libido (sex drive).

Testosterone and adults assigned male at birth

Testosterone is essential for the production of sperm. It also:

• Signals your body to make new red blood cells.
• Ensures that your bones and muscles stay strong.
• Enhances libido (sex drive) and a sense of well-being.

Testosterone and adults assigned female at birth

For adults assigned female at birth, testosterone enhances libido. However, the majority of testosterone produced in the ovaries is converted to the primary female sex hormone, estradiol.

How are testosterone levels controlled?

Your body controls the levels of testosterone in your blood. Levels are usually highest in the morning and decline during the day.

Your hypothalamus and pituitary gland control the amount of testosterone your gonads (testicl*s or ovaries) produce and release.

Your hypothalamus releases gonadotropin-releasing hormone (GnRH), which triggers your pituitary gland to release luteinizing hormone (LH). LH then travels to your gonads and stimulates the production and release of testosterone. (LH more often stimulates the production of estrogen and progesterone in ovaries.)

As testosterone in your blood increases, it suppresses the production of gonadotropin-releasing hormone, which helps maintain normal levels of testosterone.

If any of these organs — your hypothalamus, pituitary gland or gonads — aren’t working normally, that can cause abnormal testosterone levels.

What tests measure testosterone levels?

If your healthcare provider suspects you may have irregular testosterone levels, they may order one or more tests, including:

• Total testosterone blood test (this is usually performed in the morning since testosterone levels are highest then).
• Follicle-stimulating hormone (FSH) blood test.
• Luteinizing hormone (LH) blood test.

What are normal testosterone levels by age?

The two charts below list the general normal ranges of testosterone based on age and sex. The level is measured in nanograms per deciliter (ng/dL).

It’s important to note that the normal ranges for testosterone levels can vary based on the type of blood test done and the laboratory where it is done. Your provider will always reference your laboratory’s normal ranges when interpreting your results. Talk to them if you have questions.

Normal testosterone levels for people assigned male at birth:

Normal testosterone levels for people assigned female at birth:

What causes high testosterone levels?

Several conditions can cause your body to produce too much testosterone, including:

• Polycystic ovarian syndrome (PCOS): This is a hormonal imbalance that affects people with ovaries. It happens when the ovaries create excess androgens (testosterone). This imbalance in reproductive hormones causes physical symptoms like excess body hair and weight gain.
• Congenital adrenal hyperplasia: In CAH, genetic mutations (changes) cause shortages of enzymes in the adrenal gland. These enzymes normally help the body to produce cortisol, a hormone the body needs to respond to stress. When the enzymes don’t work normally, the adrenal glands produce an excess of testosterone and other hormones instead of cortisol.
• Ovarian or testicular tumors: Androgen-producing ovarian and testicular tumors can release excess testosterone.
• Adrenal tumors: Sex-hormone-producing adrenal tumors are rare tumors that make too much androgen (testosterone), estrogen or both.

Excess testosterone affects your body differently depending on your sex and age.

High levels of testosterone in people assigned male at birth

It’s unlikely — and difficult to tell — that an adult assigned male at birth has higher-than-normal levels of testosterone.

Excess testosterone in children assigned male at birth can lead to precocious (early) puberty, which is when puberty begins before the age of nine.

High levels of testosterone in people assigned female at birth

High levels of testosterone in infants assigned female at birth (AFAB) may lead to enlargement of their cl*tor*s that can look almost like a penis. It can also lead to precocious (early) puberty, which is when puberty begins before the age of eight.

In adults assigned female at birth, high levels of testosterone may be a sign of polycystic ovary syndrome (PCOS). This condition is very common — up to 15% of people AFAB of reproductive age have it.

PCOS causes certain symptoms, including:

• Acne.
• Excess body and facial hair (hirsutism).
• Irregular menstruation (periods).
• Balding at the front of your hairline.
• Deeper voice.

What happens when testosterone levels are too low?

Lower-than-normal testosterone levels typically only cause symptoms in people assigned male at birth. This condition is called male hypogonadism.

There are two main types of male hypogonadism: classical (congenital or acquired) and late-onset.

Classical male hypogonadism is when low testosterone levels are due to an underlying medical condition or damage to your testicl*s, pituitary gland or hypothalamus. For example, a person can be born with a condition (congenital) that leads to low testosterone, such as Klinefelter syndrome or Kallmann syndrome, or they can develop it later (acquired), such as from an something that changes the normal functioning of their testicl*s, hypothalamus or pituitary gland.

Late-onset male hypogonadism happens when the decline in testosterone levels is linked to general aging and/or age-related conditions, particularly obesity and Type 2 diabetes. Late-onset hypogonadism affects about 2% of men over the age of 40.

The symptoms of low testosterone vary based on your age.

Low testosterone in fetal development

Testosterone deficiency during fetal development doesn’t allow male characteristics to develop normally. This is called androgen insensitivity syndrome (AIS) and occurs when someone is genetically male but is insensitive to androgens (male sex hormones).

AIS is a disorder of sex differentiation. It affects male fetuses as they develop in the uterus, as well as teenage sexual development during puberty. AIS prevents male genitals from developing as they should and almost always results in infertility (it’s difficult or impossible to father children) during adulthood.

Low testosterone in puberty

For children assigned male at birth testosterone deficiency during puberty can result in the following:

• Slowed growth in height, but their arms and legs may continue to grow out of proportion with the rest of their body.
• Reduced development of pubic hair.
• Reduced growth of their penis and testicl*s.
• Less voice deepening.
• Lower-than-normal strength and endurance.

Low testosterone in adults

Testosterone levels in adults assigned male at birth (AMAB) naturally decline as they age. However, adults AMAB can experience even lower testosterone levels, which can lead to the following:

• Unexplained reduction in muscle mass and an increase in body fat.
• Loss of body hair.
• Depressed mood.
• Erectile dysfunction.
• Low sex drive.
• Osteoporosis (weak bones).
• Difficulty with concentration and memory.

When should I see my doctor about my testosterone levels?

If you think you may have symptoms of low testosterone or excess testosterone, talk to your healthcare provider. They can order some tests or send you to a specialist.

A note from Cleveland Clinic

Testosterone is an essential part of reproductive health and overall health for people assigned male at birth. It’s natural for testosterone levels to vary depending on your age and overall health. If they’re consistently high or low, you may experience unpleasant symptoms worth discussing with your provider. Treatments are available that may help.