Testosterone is one of the most important hormones in the male body, yet it is also one of the most misunderstood. Whether you are experiencing fatigue, mood changes, or changes in body composition, understanding your testosterone levels is a critical first step in taking charge of your health. This guide covers what testosterone does, what normal and optimal levels look like, and when testing makes sense. This article is part of our guide to testosterone.
What Testosterone Does
Testosterone is a steroid hormone produced primarily in the testes, with smaller amounts made by the adrenal glands. It plays a central role in a wide range of bodily functions:
- Muscle mass and strength: Testosterone stimulates protein synthesis in muscle tissue, supporting the growth and maintenance of lean muscle mass
- Bone density: Testosterone helps maintain bone mineral density. Low levels increase the risk of osteoporosis and fractures, particularly as men age
- Fat distribution: Adequate testosterone promotes a leaner body composition. Low testosterone is associated with increased visceral fat, particularly around the abdomen
- Red blood cell production: Testosterone stimulates erythropoiesis — the production of red blood cells — which supports oxygen delivery throughout the body
- Sexual function: Testosterone drives libido, supports erectile function, and is essential for sperm production
- Mood and cognitive function: Testosterone influences energy levels, motivation, concentration, and overall sense of well-being. Low levels are linked to irritability, depression, and brain fog
- Hair growth: Testosterone and its derivative DHT regulate facial and body hair growth
Normal Testosterone Ranges
Understanding what constitutes a "normal" testosterone level requires some nuance. Laboratory reference ranges are based on population averages, and they vary between laboratories.
Total Testosterone
Total testosterone measures both free testosterone and testosterone bound to proteins (SHBG and albumin):
- Normal range: Approximately 10-35 nmol/L (300-1000 ng/dL)
- Optimal range: Many endocrinologists consider 15-25 nmol/L (450-700 ng/dL) to be the functional sweet spot for most men
- Low testosterone (hypogonadism): Generally diagnosed below 8-10 nmol/L (230-300 ng/dL), depending on the laboratory and clinical context
Free Testosterone
Only about 2-3% of testosterone circulates in its free, biologically active form. Free testosterone is often a more accurate reflection of your hormonal status, especially if SHBG levels are unusually high or low:
- Normal range: Approximately 0.2-0.6 nmol/L (5-17 ng/dL)
- Low free testosterone with normal total testosterone can explain symptoms even when the total number looks reassuring
SHBG (Sex Hormone Binding Globulin)
SHBG is a protein that binds testosterone and makes it inactive. High SHBG reduces the amount of free testosterone available to your tissues. SHBG tends to increase with age, contributing to the decline in bioavailable testosterone.
Important: Testosterone levels are highest in the early morning and can drop by 30-40% by the evening. For accurate measurement, blood should be drawn before 10:00 AM after an overnight fast.
Signs of Low Testosterone
Low testosterone, also called hypogonadism, can manifest in a variety of ways. Some symptoms are subtle and develop gradually, making them easy to dismiss as normal ageing or stress:
- Persistent fatigue that does not improve with rest
- Decreased libido and reduced sexual desire
- Erectile dysfunction or difficulty maintaining erections
- Loss of muscle mass and strength, even with regular exercise
- Increased body fat, particularly around the midsection
- Mood changes: irritability, low motivation, depression, or anxiety
- Difficulty concentrating and brain fog
- Decreased bone density, increasing fracture risk
- Sleep disturbances and insomnia
- Hot flushes in severe cases
It is important to note that these symptoms can have many causes. Low testosterone should be confirmed through blood testing rather than assumed based on symptoms alone.
Factors That Affect Testosterone
Testosterone levels are influenced by a combination of factors, some within your control and some not:
Age
Testosterone naturally declines by approximately 1-2% per year after the age of 30. This gradual decline is normal, but in some men the decrease is steeper and leads to symptomatic hypogonadism.
Sleep
The majority of daily testosterone production occurs during sleep, particularly during deep and REM sleep phases. Studies show that restricting sleep to five hours per night for just one week can reduce testosterone by 10-15%. Prioritising 7-9 hours of quality sleep is one of the most effective ways to support healthy testosterone levels.
Body Composition
Excess body fat, especially visceral fat, increases the enzyme aromatase, which converts testosterone into estradiol (estrogen). This creates a negative cycle: low testosterone promotes fat gain, and fat gain further lowers testosterone. Maintaining a healthy weight through exercise and nutrition is crucial.
Nutrition
Several nutritional factors directly impact testosterone:
- Caloric intake: Prolonged caloric restriction suppresses testosterone production
- Dietary fat: Very low-fat diets are associated with lower testosterone. Healthy fats from eggs, olive oil, nuts, and fatty fish support hormone synthesis
- Zinc: Essential for testosterone production. Found in red meat, shellfish, and pumpkin seeds
- Vitamin D: Men with sufficient vitamin D levels have significantly higher testosterone than those who are deficient
- Magnesium: Plays a role in testosterone production and is commonly deficient in modern diets
Exercise
Resistance training — particularly compound movements like squats, deadlifts, and bench presses — acutely elevates testosterone. However, excessive endurance training without adequate recovery can suppress hormone levels through elevated cortisol.
Stress
Chronic psychological stress raises cortisol, which directly suppresses testosterone production. Managing stress through exercise, sleep, and relaxation techniques supports hormonal balance.
Alcohol and Substances
Regular heavy alcohol consumption suppresses testosterone. Even moderate drinking can temporarily reduce levels for up to 24 hours. Recreational drugs including opioids and anabolic steroids (when discontinued) can severely disrupt the hormonal axis.
When to Get Tested
Consider testing your testosterone levels if you are experiencing:
- Persistent unexplained fatigue or low energy
- Decreased sex drive or sexual dysfunction
- Difficulty building or maintaining muscle mass
- Unexplained weight gain, especially around the abdomen
- Mood changes, depression, or difficulty concentrating
- If you are over 40 and want to establish a baseline
- If you are considering or currently on testosterone replacement therapy (TRT)
Testing is simple and requires a morning blood draw. A comprehensive hormone panel should include total testosterone, free testosterone, SHBG, estradiol, LH (luteinising hormone), and FSH (follicle-stimulating hormone). LH and FSH help determine whether low testosterone is due to a problem in the testes (primary hypogonadism) or the pituitary gland (secondary hypogonadism).
Understanding Your Results
Interpreting testosterone results requires looking at the full picture, not just a single number:
- Total testosterone within range but low free testosterone: This often occurs when SHBG is elevated. You may still experience symptoms of low testosterone despite a "normal" total level
- Low total and free testosterone with high LH: Suggests primary hypogonadism — the testes are not producing enough testosterone despite strong signalling from the pituitary
- Low total and free testosterone with low LH: Suggests secondary hypogonadism — the pituitary gland is not sending adequate signals to the testes
- Elevated estradiol relative to testosterone: May indicate excessive aromatisation, often seen with higher body fat
Results should always be interpreted in the context of your symptoms, medical history, and overall health. A single test is a starting point — trends over time are more informative. If your levels are borderline, your doctor may recommend retesting after implementing lifestyle changes such as improving sleep, nutrition, and exercise habits.
Understanding your testosterone is the first step toward optimising your health. Whether you are investigating symptoms or proactively monitoring your hormones, regular testing gives you the data you need to make informed decisions.
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