Testosterone is the main sex hormone in men that regulates and interacts with most of their body. It gives men their characteristics, such as facial hair and a deeper voice. It’s also important for sex drive (libido), erections, and sperm production.
Testosterone is categorized as “bound” and “free.” Most of a man’s testosterone travels through the bloodstream “bound” to proteins. These proteins are most often albumin and sex hormone binding globulin (SHBG). Binding to these proteins makes it easier for testosterone to travel to where it is needed in the body. However, “bound” testosterone is considered inactive most of the time.
The remaining 2-3% is considered “free” because it is not bound to any other substance. “Total” testosterone measures bound and free testosterone together. When doctors check testosterone levels, they look at total (serum) testosterone readings first.
Measuring Testosterone Deficiency vs. Low Testosterone
When a man shows signs of testosterone deficiency (hypogonadism), a doctor may suggest testing. Symptoms of testosterone deficiency include:
- Fatigue and lethargy
- Mood changes and irritability
- Depression
- Decreased libido
- Erectile dysfunction (ED)
- Fewer spontaneous erections (including overnight and morning wood)
- Muscle weakness
- Shrinking testicles
- Less body and pubic hair
- Hot flashes
- Poor concentration
- Increased body fat
- Decreased bone density
- Low appetite and weight loss
The test itself is a simple blood test. It is usually performed in the morning, between 8 am and 10 am, when testosterone levels are highest. Any serum testosterone level under 300 ng/dL (nanograms per deciliter) is considered low for healthy men.
If testosterone is low for two tests in a row and the patient has any associated symptoms, this is considered testosterone deficiency. Otherwise, a low testosterone result by itself does not establish testosterone deficiency.
Potential Causes
As men age, their testosterone levels start to decline in a natural process called androgen deficiency of the aging male (ADAM). Some men compare ADAM to women’s menopause, but men’s testosterone declines much less dramatically than women’s estrogen. Around age 40, men’s testosterone levels start to decrease by about 1% (100 ng/dL) each year.
Some genetic disorders may also contribute to low testosterone or testosterone deficiency:
- Klinefelter syndrome: men have an extra X chromosome.
- Undescended testicles: the testicles don’t “drop” before birth.
- Hemochromatosis: the body absorbs too much iron.
- Kallman syndrome: the body doesn’t produce normal amounts of sex hormones.
Testosterone levels are also influenced by lifestyle choices. Tobacco, alcohol, obesity, and lack of sleep are some of the lifestyle factors that may decrease testosterone.
Treatment
The standard treatment depends on whether the patient has testosterone deficiency or low testosterone. Testosterone deficiency (low testosterone with symptoms) is recommended for testosterone therapy.
Testosterone therapy works to return testosterone levels to a normal range (between 300-900 ng/dL). This could come in the form of gels, liquids, patches, pellets, and injections. All forms of testosterone therapy come with their own risks and benefits. It’s beneficial to speak with a healthcare provider to determine what the best option is.
For low testosterone (no symptoms present), it is recommended that lifestyle changes be made. This may include reducing tobacco and alcohol use, dieting, exercising, and getting more sleep.
For men wanting to preserve fertility, testosterone therapy is not the best choice. There are other options, like selective estrogen modulator receptors (SERMs), that are recommended.
Men at risk of prostate cancer should have their prostate-specific antigen (PSA) levels checked often. While testosterone may increase PSA levels by a little, there is still concern about its effects on the prostate.
Key Takeaways
- Testosterone is the primary sex hormone in men.
- It interacts with and is responsible for many body functions, such as libido, erectile function, fertility, and mood.
- Testosterone is considered low after two early-morning tests result in less than 300 ng/dL.
- Low testosterone with symptoms suggests testosterone deficiency.
- Aging, genetic conditions, and lifestyle factors may cause low testosterone or testosterone deficiency.
- Testosterone deficiency may be treated with testosterone therapy.
- Low testosterone may be treated with lifestyle changes.
- Men who want to remain fertile should discuss other options with their healthcare provider.
- If any symptoms of low testosterone arise, it may be beneficial to speak with a healthcare provider.
Resources
Kazmi SRH, Can AS. Luteinizing Hormone Deficiency. [Updated 2023 Sep 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK562219/
Krakowsky, Y., & Grober, E. D. (2015). Testosterone Deficiency – Establishing A Biochemical Diagnosis. EJIFCC, 26(2), 105–113.
Mulhall JP, Trost LW, Brannigan RE et al: Evaluation and management of testosterone deficiency: AUA guideline. J Urol 2018; 200: 423.
Sizar O, Leslie SW, Schwartz J. Male Hypogonadism. [Updated 2024 Feb 25]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK532933/