Is a male hormone that plays an important role in the development of the male reproductive system and sperm.

Testosterone is perhaps the most famous male sex hormone. It plays a part in a number of different and important functions within the body, including:

  • The development of the penis and testicles
  • The deepening of the voice during puberty
  • The appearance of pubic and facial hair, starting at puberty
  • Muscle size and strength
  • Bone growth and strength
  • Libido (sex drive)
  • Sperm production

It has also been suggested that testosterone may have a role in baldness in later life.

The production of testosterone is controlled by the pituitary gland. This is a small gland underneath the brain which influences nearly every part of the human body. 

When controlling production, the pituitary gland  produces LH hormone which makes the testes produce testosterone, and a “feedback loop” that regulates the secretion of hormones will ensure that only a specific amount of the hormone is present in the blood. When levels of testosterone are too high in the body,  The high levels of testosterone supress LH production via negative feedback.

Even though testosterone is known as a male hormone, it is also produced in (and is important for) biologically female bodies. In a biologically female person, testosterone is produced in the ovaries and the adrenal glands. It is one of several male sex hormones, also called “androgens” that contribute to the health of a female body. 

For women, non-binary, and transgender people who were born female, testosterone is thought to have an important effect on:

  • Ovarian function (how the ovaries work)
  • Bone strength
  • Sexual behaviour, including normal libido

It must be noted that the last of these points does not have conclusive evidence, despite the fact that it has been theorised by experts. 

A delicate balance between testosterone (as well as other androgens) and oestrogen is needed in order for the ovaries to function as they should. While the research is still unclear, it has also been suggested that androgens such as testosterone also have a role to play in normal brain function. Within the brain, it may have a part in controlling factors such as mood, sex drive, and cognitive function.

Among biologically female patients, the most common condition associated with having too much testosterone is polycystic ovary syndrome (PCOS). It is not yet known if one causes the other.

Having too much naturally-occurring testosterone is not a common problem among people who are biologically male. However, this may be due to the difficulty in defining normal testosterone levels in patients. 

Blood levels of testosterone will vary greatly over time, and can even fluctuate over the course of a day, so measuring testosterone levels to find an average often produces extremely different results. In addition, things which may seem to be symptoms of particularly high testosterone may be a symptom of something else.

Symptoms associated with abnormally high levels of testosterone include:

  • Heart muscle damage
  • High blood pressure and cholesterol
  • An increased risk of blood clots
  • Prostate enlargement, with difficulty urinating
  • Liver disease
  • Acne
  • Headaches
  • Insomnia
  • Fluid retention (with particular swelling in the legs and feet)
  • Weight gain (which may be related to an increase in appetite)
  • Stunted growth in adolescents
  • Mood swings

As people who are biologically male age, their testosterone levels will gradually decrease. This will happen at a rate of about 1-2% per year. When this starts to happen, the pituitary gland will send out fewer amounts of LH to the testes to produce testosterone. A protein known as sex hormone binding globulin (SHBG) will also start to increase. Altogether, these reduce the active form of testosterone in the body.

Symptoms associated with a decreased amount of testosterone include:

  • Reduced face and body hair
  • Low libido, impotence, small testicles, and generally reduced fertility
  • Hot flashes
  • Mood swings
  • Brittle bones
  • Increased breast size

For people who are biologically female, the symptoms of testosterone deficiency are not as evident as for people who are biologically male. However, they may experience a low libido, mood swings, or reduced bone strength.