Testosterone
Testosterone is a hormone that your gonads (testicles or ovaries) mainly produce. Testosterone levels are naturally much higher in males. If testosterone levels are too high or too low, it can cause certain symptoms.
Testosterone is a hormone that your gonads (sex organs) mainly produce. More specifically, both testicles and ovaries 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 males.
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.
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.
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 testicles in male infants. The testicles 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 pubescent males, including:
- An increase in height.
- Body and pubic hair growth.
- Enlargement of their penis, testes and prostate gland.
- Increasing libido (sex drive).
Testosterone and adult males
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 adult females
For adult females, testosterone enhances libido. However, the majority of testosterone produced in the ovaries is converted to the primary female sex hormone, estradiol.
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 (testicles 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 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 males
It’s unlikely — and difficult to tell — that a male adult has higher-than-normal levels of testosterone.
Excess testosterone in male children can lead to precocious (early) puberty, which is when puberty begins before the age of nine.
High levels of testosterone in females
High levels of testosterone in female infants may lead to enlargement of their clitoris 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 female adults, high levels of testosterone may be a sign of polycystic ovary syndrome (PCOS). This condition is very common — up to 15% of females 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 males. 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 testicles, 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 testicles, 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 male children, 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 testicles.
- Less voice deepening.
- Lower-than-normal strength and endurance.
Low testosterone in adults
Testosterone levels in males naturally decline with age. However, adult males 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.
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.
References
Chen Z, Legro RS, Ehrmann DA, Wei D. Androgen excess disorders in women. In: Robertson RP, ed. DeGroot’s Endocrinology: Basic Science and Clinical Practice. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 124.
Haddad NG, Eugster EA. Endocrinology of pubertal disorders. In: Robertson RP, ed. DeGroot’s Endocrinology: Basic Science and Clinical Practice. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 103.
Swerdloff RS, Wang C. The testis and male hypogonadism, infertility, and sexual dysfunction. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 216.
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