Testosterone: What it is, functions, levels, and how it affects health

Both males and females secrete testosterone, but it is a primary reproductive hormone in males. Females only need this hormone in a minimal amount for functions like improved bone health and increased sexual desire. In males, testosterone drives different male functions and characteristics. Learn more about testosterone in this article.

Image of a doctor, wearing a nose mask holding a test tube with a sample for research

Key takeaways:

  • Testosterone is a very important hormone in the human body, especially in males. It is a primary male hormone responsible for the development of male sexual characteristics. 
  • Testosterone levels can rise and fall in response to different conditions in the body at a time, with corresponding consequences.
  • When testosterone levels are too high or too low, it can cause health issues in both males and females. However, there are available treatments, such as testosterone replacement therapy, that can help balance the hormone or improve symptoms. 

There are many chemicals that are secreted by the glands and certain parts of the brain that control various functions in the human body. They are called hormones. In men, male sex hormones, called androgens, modulate the development and maintenance of male characteristics. The most important of these androgens is testosterone.

What is testosterone?

Testosterone is a hormone that is mostly produced in the testicles of males and the ovaries of females. In females, it is produced in relatively smaller amounts. The adrenal glands also produce a hormone called dehydroepiandrosterone, which is later transformed into testosterone.

Testosterone is the main androgen, as it stimulates the development of male characteristics. Natural testosterone produced in the body is also anabolic, meaning it promotes muscle building.

The functions of testosterone

From when life begins through adulthood, testosterone subserves various functions. Below is a summary of the functions of testosterone during the development of a baby in the womb, the puberty stage, and adulthood. 

1. Testosterone functions during fetal development

As early as seven weeks of pregnancy, the testicles begin to develop rapidly in the fetus. Once formed, they start to produce testosterone. The testosterone produced then activates the development of the rest of the male reproductive system. Internally, the male reproductive system includes the prostate gland, vas deferens, and urethra, and externally, it includes the penis and scrotum.

2. Testosterone function during puberty

As males approach puberty, there is a surge in testosterone production. This causes facial and pubic hair to grow, as well as increased muscle and bone mass. It also causes the voice to deepen and the reproductive organs to begin to mature.

3. Testosterone function in adult males

Testosterone is responsible for sperm production and increased sexual drive, called libido. It also helps the body synthesize new red blood cells for optimal tissue functioning.

4. Testosterone function in adult females

Testosterone also increases sexual drive in female adults. It is also involved in other functions, such as increasing muscle mass and bone density. However, most of the testosterone produced in them is converted into the female sex hormone, estradiol.

Normal testosterone levels by age

Although information exists about normal testosterone levels across various age groups, most are not foolproof. A 2022 study on 1,486 men revealed normal ranges of testosterone measured in nanograms per deciliter (ng/dL) as follows:

  • Males aged 20–24 years old: 409–558 ng/dL
  • Males aged 25–29 years old: 413-575 ng/dL
  • Males aged 30-34 years old: 359–498 ng/dL
  • Males aged 35–39 years old: 352-478 ng/dL
  • Males aged 40–44 years old: 350–473 ng/dL

Therefore, cutoffs for low testosterone levels were set at 409, 413, 359, 352, and 350 ng/dL, respectively.

How to measure testosterone levels

In order to assess testosterone levels, doctors request tests like:

  • Total testosterone blood test: This is a test to measure the level of testosterone hormone circulating in the blood. An early morning blood sample is used for this test. This is because testosterone levels fluctuate throughout the day but are highest in the morning.
  • Luteinizing hormone (LH) assay: This test measures the amount of LH in the blood. LH is a very important hormone in the female reproductive system. Its release triggers ovulation in females. However, it is also produced in males. In males, LH stimulates the testes to produce testosterone. So, doctors sometimes use this test as a supportive test when measuring testosterone levels.
  • Follicle-stimulating hormone (FSH) assay: In females, FSH stimulates the growth of ovarian follicles, from which a mature egg gets released every month during ovulation. But in males, FSH stimulates certain cells in the testes called sertoli cells to stimulate sperm production. It provides a suitable environment needed for sperm production. An FSH assay is sometimes used as a supportive test for assessing testosterone levels, too.

What happens when testosterone level is too high?

It is rare to find a male adult with too high testosterone levels. However, high testosterone levels in children can cause them to attain puberty too early. Puberty earlier than 8 years in girls and 9 years in boys is described as precocious.

In adult females, high testosterone levels is one of the diagnostic criteria for polycystic ovarian syndrome (PCOS). PCOS is a female medical condition characterized by ovarian cysts, irregular menstruation, as well as excess body and facial hair (hirsutism).

What happens when testosterone level is too low?

The most important effect of low testosterone in women is a loss of sexual desire. In males, on the other hand, low testosterone is diagnosed as a medical condition called hypogonadism. Hypogonadism is a medical condition that occurs when the sex glands (also called gonads), which are the testes in males and the ovaries in females, produce little to no sex hormones.

Hypogonadism is of two types: primary and secondary hypogonadism.

  • Primary hypogonadism: The low level of testosterone in primary hypogonadism is usually due to a defect in testicular function. It arises due to an issue in the testicles. This is the case in conditions like Klinefelter’s syndrome, undescended testes, and mumps orchitis. 
  • Secondary hypogonadism: Secondary hypogonadism arises from defects in the hypothalamus or pituitary gland. Some other well-known causes of this type of hypogonadism are testicular trauma, obesity, HIV, surgery, and stress.

The problems that low testosterone causes vary with age and are explained below.

Effects of low testosterone during in-utero development

Low testosterone during the development of a fetus in the womb hinders the normal development of male characteristics. This results in a baby that is genetically male but does not have male genitals. Instead, they have the female external genitalia.

Effects of low testosterone in puberty

Slow growth of the pubic hair, penis, and testicles are the hallmarks of low testosterone during puberty. Low testosterone during puberty can also cause a slow increase in height, less voice deepening, and reduced bone and muscle mass.

Effects of low testosterone in male adults

The level of testosterone in male adults declines with increasing age. Nevertheless, if this happens at a faster-than-normal rate, it can cause significant problems. The most worrisome of them is erectile dysfunction, popularly known as impotence. People with erectile dysfunction are unable to get and sustain an erection long enough for penetrative sexual intercourse.

Low testosterone levels in adult males can also cause:

  • Significant reduction in sexual desire
  • Low sperm count
  • Reduction in muscle mass
  • loss of body hair, and
  • low mood.

Testosterone replacement therapy

Low testosterone can be treated using testosterone replacement therapy (TRT). A doctor might recommend it when the testosterone level is abnormally low and the patient has significant symptoms.

However, older people with age-related low testosterone levels are often not eligible for TRT.

Testosterone replacement therapy is effective in increasing testosterone levels and comes in various forms, including:

  • Transdermal patch: This skin patch is applied on the arm or anywhere in the upper trunk region. This patch contains testosterone hormone and continuously delivers it into the body to help restore a steady testosterone level in the blood.
  • Mouth patch: This works similarly to the transdermal patch, but in this case, it sticks to the upper gum, releasing testosterone slowly through the oral mucosal lining.
  • Gels: TRT gels are applied to the skin of the abdomen, upper arm, or shoulders. Nasal testosterone gels are also available for administration through the nose.
  • Injections and implants: Testosterone can be administered as an intramuscular injection. It can also be given as an implant beneath the skin.

These testosterone replacement therapy methods are typically preferred because they deliver testosterone directly into the blood without going through the liver. However,  it is worthy of note that testosterone pills are also available. But, there is evidence to show that some of them damage the liver.

The only currently approved oral pill for testosterone replacement is undecanoate (Jatenzo)⸺with a recommended starting dose of 237 mg twice per day)⸺which the FDA approved in 2019 for treating low testosterone levels due to specific medical conditions like damage to the pituitary gland.  

Wrap up

Testosterone is a very important hormone in the human body, particularly in males. Therefore, the necessity of knowledge of its functions among the male population cannot be overemphasized. This article is intended to add to that body of knowledge and help you understand the role of this hormone and what can happen when there is an imbalance of this hormone.

If you have been diagnosed with high or low testosterone levels, your healthcare team will suggest a treatment method that is specific to your condition. 

Frequently Asked Questions (FAQs)

Does high testosterone cause an enlarged prostate?

Yes, high testosterone levels can cause enlarged prostate. This is because, naturally, testosterone stimulates the growth of the prostate gland. So, at higher-than-normal levels, testosterone will cause an enlarged prostate. This is known as Benign Prostatic Hypertrophy (BPH).

Does testosterone increase body temperature?

An investigation carried out on mice strongly suggests that testosterone may increase body temperature. Their gonads (testes and ovaries), which are the organs that secrete testosterone, were removed, and their core body temperature dropped significantly. While this is an animal study, it suggests that testosterone may be linked to increased body temperature.

Does testosterone make you look older?

A normal level of testosterone is positively correlated with increased oil production, which keeps the skin well moisturized. It also aids in excellent collagen production, which keeps the skin healthy and smooth. So, testosterone may make one look younger, not older.