What it measures.
Total testosterone measures all circulating testosterone, including hormone bound to proteins (SHBG and albumin) and the free fraction. In men, healthy levels support muscle mass, bone density, cognitive function, and overall vitality. In women, testosterone contributes to libido, energy, and lean body mass.
The total concentration of testosterone in blood, including ~98% bound to proteins (SHBG and albumin) and ~2% free/bioavailable. Measured in ng/dL or nmol/L.
Why it matters.
Low testosterone is associated with fatigue, reduced muscle mass, depression, and increased cardiovascular mortality risk. Optimal levels support energy, mood, body composition, and longevity. Testing should be done early morning when levels peak.
Physiology.
In men, testosterone is primarily produced by the testes under LH stimulation. In women, smaller amounts are produced by the ovaries and adrenal glands. Testosterone peaks in early morning (7-9 AM) and declines throughout the day. Levels naturally decline 1-2% per year after age 30.
Testing & preparation.
How to prepare
- Test between 7-9 AM when levels peak
- Fasting is preferred
- Avoid strenuous exercise 24 hours before
- Get adequate sleep the night before
When to test
When experiencing symptoms of low T (fatigue, low libido, depression, reduced muscle mass), or as part of comprehensive health assessment.
How often
Annually for monitoring; every 3-6 months if on testosterone therapy.
Interpretation.
High total testosterone
Common causes:
- Testosterone replacement therapy
- Anabolic steroid use
- Adrenal or testicular tumors (rare)
- Polycystic ovary syndrome (women)
Implications:
- May suppress natural production if exogenous
- Risk of polycythemia (elevated red blood cells)
- Potential cardiovascular and liver effects if supraphysiologic
- In women: acne, hair growth, menstrual irregularities
Low total testosterone
Common causes:
- Primary hypogonadism (testicular dysfunction)
- Secondary hypogonadism (pituitary/hypothalamic)
- Aging (natural decline)
- Obesity (increased aromatization to estrogen)
- Chronic illness, stress, poor sleep
- Medications (opioids, steroids)
Implications:
- Fatigue, low energy
- Reduced muscle mass and strength
- Depression, irritability, brain fog
- Low libido, erectile dysfunction
- Increased body fat, especially visceral
- Reduced bone density
Optimization.
Diet
- Adequate calories (restriction suppresses testosterone)
- Healthy fats (20-35% of calories)
- Sufficient protein (1.6-2.2g/kg bodyweight)
- Zinc-rich foods (oysters, red meat, pumpkin seeds)
- Limit alcohol (directly suppresses testosterone)
Lifestyle
- Resistance training 3-4x weekly with compound movements
- Quality sleep 7-9 hours (1 week of <5 hours reduces T by 10-15%)
- Stress management (cortisol suppresses testosterone)
- Maintain healthy body weight (obesity increases aromatization)
- Minimize endocrine disruptors (plastics, pesticides)
Supplements
- Vitamin D if deficient
- Zinc if deficient
- Magnesium for sleep and testosterone support
- Ashwagandha may modestly increase T
FAQs.
Why should testosterone be tested in the morning?
Testosterone follows a circadian rhythm, peaking between 7-9 AM and declining 20-30% by afternoon. Testing later in the day may show artificially low results. For accurate assessment and comparison over time, always test in the early morning, ideally fasting.
What causes low testosterone in younger men?
Modern lifestyle factors significantly impact testosterone: Over 60% of men aged 18-30 with BMI >32.5 have low T. Poor sleep (even 1 week of <5 hours drops T 10-15%), chronic stress, sedentary lifestyle, and low-fat diets all contribute. Population data shows average T dropped from 605 ng/dL (1999-2000) to 451 ng/dL (2015-2016)—a 25% decline in just 16 years.