What is testosterone and what does it do?

Testosterone is one of the most important hormones in the human body. It is often associated exclusively with masculinity, muscle growth, or libido. In reality, however, testosterone performs numerous essential functions in both men and women and influences physical as well as mental processes.

In this article, you will learn what testosterone is, which roles it plays in the body, how testosterone levels change over the course of life, which symptoms may occur in cases of deficiency or excess, and how testosterone levels are medically measured.

Table of contents

• What Is testosterone?
• What functions does testosterone have in the body?
• What are normal testosterone levels?
  
• What happens with testosterone deficiency?
• What symptoms occur with excess testosterone?
• How can testosterone levels be measured?
• When should medical evaluation be considered?

What Is testosterone?

Testosterone is a sex hormone belonging to the group of androgens. In men, it is produced primarily in the testes; in women, smaller amounts are produced in the ovaries. In addition, the adrenal glands of both sexes contribute to testosterone production.

Although testosterone is often referred to as a “male hormone,” it is also essential for women. It acts on numerous tissues and organ systems and plays a role in muscle and bone metabolism, energy balance, libido, and psychological well-being (Cleveland Clinic, 2024).

What functions does testosterone have in the body?

In men:

• Development of male reproductive organs during puberty

• Development of secondary sex characteristics (facial hair, voice deepening, body hair)

• Promotion of muscle mass and bone density

• Support of sperm production

• Regulation of libido and sexual function

• Influence on mood, motivation, and concentration

In women:

• Support of bone health

• Maintenance of muscle strength and physical performance

• Influence on sexual desire and arousal

• Contribution to hormonal balance (e.g., interaction with estrogen)

A balanced testosterone level is therefore not only relevant for reproduction but also for overall physical and mental health (HarvardHealth, 2025).

What are normal testosterone levels?

Testosterone circulates in the blood in two forms:

• Bounded testosterone

• Free testosterone (biologically active)

Total testosterone includes both fractions and is the value most commonly measured in clinical practice.

Reference ranges for total testosterone (nmol/L)*

*Reference ranges may vary depending on the laboratory, measurement method, age, and time of day. Blood sampling is ideally performed in the morning between 8 and 10 a.m.

Source: MedlinePlus, 2024

How do testosterone levels change with age?

In men, testosterone levels begin to decline gradually from around the age of 30 (approximately 1% per year). In women, androgen levels decrease significantly, particularly after menopause.

What happens with testosterone deficiency?

Testosterone deficiency can cause a variety of symptoms.

Typical symptoms in men:

• Persistent fatigue and lack of motivation

• Loss of muscle mass and strength

• Increase in body fat, especially abdominal fat

• Reduced libido or erectile dysfunction

• Mood changes or depressive symptoms

• Sleep disturbances

Possible symptoms in women:

• Reduced sexual desire

• Muscle weakness

• Chronic fatigue

• Decreased bone density

These symptoms are not specific to testosterone deficiency and should be medically evaluated if they occur or persist (Cleveland Clinic, 2022; Cleveland Clinic, 2023).

What are the causes of testosterone deficiency?

Low testosterone levels may be caused by:

• Aging processes

• Chronic stress

• Sleep deprivation

• Severe overweight or obesity

• Certain medications (e.g., corticosteroids, opioids)

• Disorders of the testes, ovaries, or pituitary gland

Source: Cleveland Clinic, 2022

What symptoms occur with excess testosterone?

Persistently elevated testosterone levels are rare and often associated with hormonal disorders or external hormone use (e.g., anabolic steroids).

Possible symptoms in men:

• Acne and oily skin

• Mood swings or increased irritability

• Hair loss

• Suppression of natural sperm production with external testosterone use

Possible symptoms in women:

• Excessive facial or body hair growth (hirsutism)

• Irregular menstrual cycles

• Acne

• Polycystic ovary syndrome (PCOS)
  

Source: Cleveland Clinic, 2025

How can testosterone levels be measured?

Testosterone levels are measured through a blood test. Typically, total testosterone is assessed first. Depending on the clinical question, additional markers such as SHBG, free testosterone, or estradiol may be useful.

Testing can be performed by a physician or through a certified at-home test, such as the Homed-IQ Testosterone Test. This test allows you to take a blood sample conveniently from home and have it analyzed in a certified medical laboratory. At-home tests may provide an initial indication but do not replace medical diagnosis or treatment decisions (MedlinePlus, 2025).

When should medical evaluation be considered?

Medical evaluation is recommended if:

• several of the listed symptoms are present

• symptoms persist over a longer period of time

• abnormal laboratory results are detected

• there is an unfulfilled desire to conceive

Individual medical consultation is essential to clarify causes and initiate appropriate treatment.

Conclusion

Testosterone is a key hormone for health, performance, and well-being in both men and women. A conscious approach to hormonal health and medically sound diagnostics are crucial for properly interpreting changes and maintaining long-term health.