Testosterone has become the male hormone in the same way estrogen became shorthand for female biology, which is to say the picture is more crowded than the branding suggests. It matters, but it is one hormone among many, and the story of its decline with age is often oversold by clinics that make money treating it.
What follows is what testosterone actually does, what a real deficiency looks like, and when a lab draw earns its place.
What testosterone does
In adult men, testosterone contributes to libido, erectile function, sperm production, muscle mass, bone density, red blood cell production, mood, and energy. It does not do these things alone, and it does not do them predictably at any given level. Two men with identical total testosterone can feel and function differently.
Production is regulated by a feedback loop between the hypothalamus, pituitary, and testes. When that loop is disrupted, testosterone falls. The clinically important distinction is between primary hypogonadism, where the testes themselves fail, and secondary hypogonadism, where signaling from the brain drops. The workup differs, and so does the treatment.
What actually causes low testosterone
Age lowers testosterone gradually, roughly one to two percent per year after the thirties, but age alone rarely drops a healthy man into a clearly deficient range. Most low readings in men under sixty trace to something else:
- Obesity, particularly visceral fat, which converts testosterone to estradiol and lowers signaling from the brain
- Poor sleep, especially untreated obstructive sleep apnea
- Chronic opioid or corticosteroid use
- Diabetes and metabolic syndrome
- Prior or ongoing anabolic steroid use, which suppresses the natural axis, sometimes long-term
- Testicular injury, chemotherapy, or genetic conditions like Klinefelter syndrome
The AUA guideline is direct on this: identify and treat reversible causes before starting hormone therapy. Losing meaningful weight, treating sleep apnea, or tapering an offending medication often restores levels without a prescription.
When to test
Symptoms drive testing, not curiosity. The AUA recommends measuring testosterone in men with symptoms consistent with deficiency, which include reduced libido, erectile dysfunction, decreased spontaneous erections, loss of body hair, hot flashes, low energy, depressed mood, and reduced muscle mass or strength.
How you test matters:
- Draw in the morning, ideally before 10 a.m., when levels peak
- Confirm a low reading with a second morning draw; a single low value is not a diagnosis
- Measure free testosterone if total is borderline or if conditions like obesity or thyroid disease may alter binding proteins
- Include LH and FSH to distinguish primary from secondary causes
Direct-to-consumer testosterone panels sold without symptoms tend to generate anxiety without changing outcomes. If you have no symptoms, a number in isolation is not a reason to treat.
The case for and against treatment
For men with confirmed hypogonadism and symptoms, testosterone therapy improves libido, erectile function, lean mass, bone density, and mood in most studies. For men with borderline levels and vague symptoms, the benefits are smaller and harder to distinguish from placebo.
Risks are real and worth naming. Testosterone therapy:
- Suppresses sperm production and can cause infertility, sometimes persistently
- Raises hematocrit and can require dose adjustment or blood donation
- May worsen untreated sleep apnea
- Requires ongoing monitoring of PSA and hematocrit
Fertility is the risk most often glossed over. If you may want biological children, discuss sperm banking, hCG, or alternative strategies before starting therapy.
The cardiovascular question, whether testosterone therapy raises or lowers cardiac risk, has been contested for years. The TRAVERSE trial, published in 2023, found no increased risk of major cardiac events in hypogonadal men with elevated cardiovascular risk, though certain arrhythmias and pulmonary embolism appeared more frequently. Treatment is not the acute danger some earlier work suggested, but it is also not risk-free.
The bottom line
Testosterone deficiency is a real diagnosis with a defined workup and clear indications for treatment. It is also a diagnosis that has been stretched to cover normal aging and lifestyle-driven declines. Test when symptoms warrant it, confirm before you treat, and address the reversible causes first.