What Is Testosterone? Why Is It Important?

Testosterone In Men and Women + What Levels Are Normal

Man and woman in plank position, man with facial hair and bulkier muscles due to increased testosterone
Testosterone is responsible for facial hair and bulkier muscles in men. Low testosterone can lead to infertility in men, while high testosterone in women can lead to ovulation problems. Westend61 / Getty Images

Quick Testosterone Definition

Testosterone is an androgen hormone produced by the adrenal cortex, the testes (in men), and the ovaries (in women). It is often considered the primary male sex hormone. Testosterone stimulates the development of male secondary sex characteristics (like body hair and muscle growth) and is essential in the production of sperm. In women, testosterone plays a role in egg development and ovulation.

While testosterone is essential to both male and female health, it plays a much bigger role in men. Healthy males who have gone through puberty have 20 times the levels of testosterone compared to a healthy female.

Low levels of testosterone in men can cause male infertility. High testosterone levels in women are associated with female infertility.

Testosterone levels can be evaluated with blood work. This is usually part of an infertility workup in men and women.

Testosterone’s Chemical Definition

Testosterone is a potent steroid hormone whose chemical formulation is C 19 28 2.  Testosterone is also known as 17-beta-hydroxy-4-androstene-3-one.

Testosterone Hormonal Supplementation Definition

Prescription testosterone is used to treat hypogonadism related conditions in men, breast cancer in women, and delayed puberty in boys.

Off-label, it may be used to treat symptoms of perimenopause, fatigue, and low sex drive. 

It is available as a gel, topical solution, a patch, an injection, a pellet (to be implanted), or oral capsule.

Brand names include Androderm, AndroGel, AndroGel Pump, Aveed, Axiron, Depo-Testosterone, First-Testosterone, First-Testosterone MC, Fortesta, Natesto, Striant, Testim, Testopel, Vogelxo, and Vogelxo Pump.

Warning: do not attempt to supplement testosterone on your own! It is not harmless and can worsen or cause infertility in both men and women.

Too much testosterone can cause male infertility. More on this below.

Testosterone Levels in Men and Women

If you are having your testosterone levels checked, the blood test will likely be scheduled for the morning. 

There are two kinds of testosterone found in your blood: 

  • Free testosterone (also known as free T) is testosterone that is not chemically bound to anything else. 
  • Bound testosterone makes up the majority of your total testosterone levels. Around 98% of the testosterone in your blood is bound to one of two proteins: albumin or sex hormone binding globulin (SHBG.) 

When you have your testosterone levels tested, your doctor will look at both your free testosterone levels and your total testosterone levels.

Your total testosterone levels include both free and bound testosterone. 

Total testosterone levels vary throughout the day.

They are highest in the morning and lower towards evening. 

Normal ranges for testosterone partially depend on the lab and your doctor's interpretation. With that said, here are some general values.

For a male age 19 or older:

  • Testosterone total should be between 240-950 ng/dL
  • Free testosterone should be between 9-30 ng/dL  

For a female age 19 or older:

  • Testosterone total should be between 8-60 ng/dL
  • Free testosterone should be between 0.3-1.9 ng/dL

What Testosterone Does in Men and Women

Development of sperm: Testosterone is essential to the production of sperm in the testes. It is a delicate balance, however. Both too little and too much testosterone can lead to low sperm counts.

Associated with what are known as “secondary male characteristics”: This includes facial and body hair growth, deepening of the voice, building of muscle mass, increased size of bones, and distribution of fat in the body.

Infant boys and girls have similar levels of testosterone. It is only during puberty that boys begin to produce significantly higher levels of testosterone, leading to the development of the male secondary sex characteristics.

Women with abnormally high levels of testosterone may have problems with facial hair and male pattern balding.

High testosterone levels are also associated with acne in men and women.

Possibly essential to the development of eggs in the ovaries: How testosterone affects egg development in women is not entirely clear. One current theory is that testosterone prevents premature “death” of the egg at an early stage of follicular development.

Testosterone in the ovaries also may make the follicles more sensitive to the hormone FSH. FSH – which stands for follicle-stimulating hormone – helps mature the young oocyte into an ovulated egg.

Sex drive a.k.a. libido: The connection between testosterone and sex drive is well-known. Higher levels of testosterone cause higher levels of sexual appetite.

Building muscle mass: Men are more likely to develop muscle mass than women because of their higher testosterone levels.

Pain tolerance: Testosterone may help men and women regulate pain levels. 

Learning and memory, especially spatial intelligence: Testosterone aids in learning and memory. It is especially associated with spatial intelligence.

Men with abnormally low testosterone levels are at risk for learning disabilities.

Cognitive Empathy: Cognitive empathy involves being able to read another person’s emotions via their body language and facial expressions, especially changes with the eyes.  

Higher testosterone levels are associated with a lessened ability to read people. Women typically outperform men in tests of cognitive empathy. The theory is that lower testosterone levels are the reason women are better at this skill.

More on the reproductive system and essential hormones:

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