Walk into any men's health clinic or scroll through any optimization forum and you'll hear the same narrative: low testosterone causes ED, and testosterone replacement therapy (TRT) fixes it. The reality is considerably more nuanced, and understanding the actual relationship can save you from an expensive, unnecessary treatment — or point you toward one you actually need.
What Testosterone Actually Does for Erections
Testosterone's primary role in sexual function is desire, not mechanical erectile function. The hormone acts on the brain to maintain libido — the subjective experience of wanting sex. When testosterone drops below a threshold (generally around 300 ng/dL total, though individual sensitivity varies), libido declines.
Testosterone also has secondary roles in erectile physiology: it helps maintain nitric oxide synthase expression in penile tissue (the enzyme that produces the trigger molecule for erections) and supports the health of smooth muscle and connective tissue in the corpus cavernosum. But these effects are permissive, not driving — like oil in an engine, not the fuel.
The practical implication: a man with very low testosterone (below 200–250 ng/dL) may have both reduced desire and impaired erectile mechanics. A man with mildly low testosterone (250–350 ng/dL) is more likely to notice decreased desire while mechanical erectile function remains intact.
When TRT Helps ED
The TRAVERSE trial (2023) — the largest randomized, placebo-controlled trial of TRT to date (over 5,000 men) — showed modest improvements in sexual function scores with testosterone gel in men with hypogonadism. But the improvements were primarily in desire and arousal domains, not in erectile hardness per se.
TRT is most likely to help when:
- Testosterone is genuinely low (below 300 ng/dL on two morning measurements)
- Low libido is a primary complaint, not just ED
- PDE5 inhibitors have been tried but haven't fully resolved the issue
- No other obvious cause of low T exists (obesity, opioids, pituitary disease)
When TRT Doesn't Help ED
TRT is unlikely to meaningfully improve erectile function when:
- Testosterone is normal (>400 ng/dL). Raising a normal T level to supraphysiological doesn't improve erections — it just adds risk.
- ED is primarily vascular. A man with diabetes, hypertension, and atherosclerosis has ED from blood vessel damage. Testosterone doesn't repair arteries.
- ED is primarily psychogenic. Performance anxiety, depression-related ED, and relationship-driven ED aren't testosterone problems.
- ED is medication-induced. SSRI-related ED won't respond to TRT — the serotonin mechanism is the issue.
The Combination Approach
Where testosterone replacement shows the most compelling ED data is in combination with PDE5 inhibitors. Several studies have shown that men with low T who failed PDE5 monotherapy improved when TRT was added. The mechanism makes sense: PDE5 inhibitors amplify the nitric oxide signal, but adequate testosterone helps maintain the nitric oxide production system in the first place.
The combination of TRT + PDE5 inhibitor, in hypogonadal men, addresses both the desire component (testosterone) and the mechanical component (PDE5 inhibitor). Neither alone may be sufficient when both systems are compromised.
Getting the Right Test
If you suspect low testosterone is contributing to ED, the workup should include:
- Total testosterone — drawn in the morning (before 10am), fasting. Testosterone follows a circadian rhythm and peaks early.
- Free testosterone — the fraction not bound to proteins, which is the biologically active portion. Especially important in older men and men with obesity (both increase SHBG, which binds testosterone and reduces the free fraction).
- SHBG — sex hormone-binding globulin. High SHBG can make total T look normal while free T is genuinely low.
- LH and FSH — to distinguish primary hypogonadism (testicular problem) from secondary (pituitary/hypothalamic problem).
- Prolactin — elevated prolactin suppresses testosterone and causes ED independently.
Two low morning readings on separate days, combined with symptoms, is the standard diagnostic threshold for hypogonadism. Don't rely on a single test or an afternoon draw.