TRT for Erectile Dysfunction: Does Testosterone Replacement Therapy Actually Work?

What Is the Relationship Between Testosterone and Erections?

Testosterone does not directly cause an erection the way a PDE5 inhibitor does, but it sets the neurochemical conditions that make erections possible. Penile nitric oxide synthase (NOS) expression depends on adequate androgen signaling. Without enough testosterone, nitric oxide production in the cavernosal smooth muscle drops, the tissue becomes hypoxic over time, and the erectile response becomes unreliable. A 2007 study published in the Journal of Sexual Medicine demonstrated that castrate-level testosterone in animal models reduced NOS expression by more than 50%, an effect reversed by androgen repletion [1].

In clinical terms, testosterone works on two parallel tracks. The first is central: testosterone acts on hypothalamic androgen receptors to generate sexual motivation and mental arousal. The second is peripheral: androgens maintain the structural integrity of penile vascular tissue. When testosterone falls below the clinical threshold of 300 ng/dL set by the Endocrine Society's 2018 Clinical Practice Guideline, both tracks are compromised [2].

Low libido and ED are related but genuinely different problems. Low libido is the absence of sexual desire. ED is the mechanical failure to achieve or maintain an erection sufficient for intercourse. A man can have normal testosterone and severe ED from arterial disease. Conversely, he can have profoundly low testosterone, no vascular disease, and find that TRT alone fully restores function. Separating these two conditions before prescribing is the entire point of a proper work-up.

How Common Is Hypogonadism Among Men With ED?

Hypogonadism explains a meaningful but minority fraction of ED cases. Population studies are instructive here. The European Male Ageing Study (EMAS), which followed 3,369 men aged 40 to 79, found that a syndromic definition of late-onset hypogonadism (three sexual symptoms plus total testosterone below 11 nmol/L and free testosterone below 220 pmol/L) applied to only 2.1% of the overall cohort [3]. Among men already reporting ED, however, the prevalence of biochemical hypogonadism is higher, with some clinic-based series reporting rates of 10 to 40%, depending on whether total or free testosterone is used.

The overall burden of ED is substantial. The Massachusetts Male Aging Study found that some degree of ED affected 52% of men between 40 and 70 years old [4]. Because both ED and declining testosterone become more common with age, they co-occur frequently without one necessarily causing the other. Age-related arterial insufficiency, metabolic syndrome, and psychological stress each account for more ED cases than testosterone deficiency does.

This co-occurrence matters clinically. A man who presents with ED and is found to have low testosterone should have the testosterone treated, but he should not assume TRT will fully resolve his ED if there is concurrent vascular disease or diabetes.

What Does the Trial Evidence Actually Show?

The strongest TRT-specific ED data come from the Testosterone Trials (TTrials), a coordinated set of seven placebo-controlled trials funded by the National Institute on Aging. The sexual function sub-trial enrolled 790 men aged 65 and older with total testosterone below 275 ng/dL. At 12 months, men randomized to testosterone gel 1% (titrated to maintain levels of 500 to 800 ng/dL) showed an improvement in the Sexual Desire subdomain of the PDSS and a statistically significant improvement in the IIEF Erectile Function domain score of 4.0 points versus 1.6 points in the placebo group (P<0.001) [5]. The Endocrine Society's 2018 guideline cites this trial directly in its recommendation to offer TRT to hypogonadal men with sexual dysfunction.

A 2016 meta-analysis in the Journal of Clinical Endocrinology and Metabolism pooled data from 14 randomized controlled trials (N=2,298) and found that testosterone therapy significantly improved IIEF scores compared with placebo, with the largest effect sizes in men with the lowest baseline testosterone [6].

One key observation from the TTrials data: men with baseline testosterone above 200 ng/dL showed smaller improvements than men below 200 ng/dL. This dose-response relationship supports the idea that TRT for ED works best in men who are genuinely testosterone-deficient, not in men at the low end of the normal range.

Primary Versus Secondary Hypogonadism: Why the Cause Matters for Treatment

The distinction between primary and secondary hypogonadism changes both the treatment approach and the prognosis for fertility.

Primary hypogonadism means the testes themselves are failing. The pituitary gland releases LH and FSH in high amounts trying to stimulate testosterone production, but the testes cannot respond. Causes include Klinefelter syndrome (47,XXY), orchitis after mumps infection, chemotherapy, radiation, and testicular torsion with ischemic injury. In primary hypogonadism, the hypothalamic-pituitary axis is intact and trying to compensate. TRT replaces the missing testosterone, but because exogenous testosterone suppresses LH via negative feedback, it will further reduce sperm production. Men with primary hypogonadism who want to preserve fertility face a different conversation.

Secondary hypogonadism (also called hypogonadotropic hypogonadism) involves a failure at the pituitary or hypothalamus. LH and FSH are low or inappropriately normal despite low testosterone. Causes include hyperprolactinemia, hemochromatosis, Kallmann syndrome, pituitary adenoma, opioid use, anabolic steroid abuse, obesity-related suppression, and age-related decline in hypothalamic GnRH pulsatility. In secondary hypogonadism, the testes are potentially functional. For men who want to restore fertility, clomiphene citrate or hCG can stimulate endogenous testosterone production rather than replacing it exogenously.

The Endocrine Society 2018 guideline specifies that clinicians should measure LH, FSH, and prolactin in every man with confirmed low testosterone to distinguish the two types before starting treatment [2]. Skipping this step means potentially missing a pituitary tumor or hemochromatosis, both of which require direct treatment beyond TRT.

Andropause and Late-Onset Hypogonadism: The Gradual Decline

"Andropause" is the colloquial term for the gradual age-related testosterone decline that begins around the fourth decade of life. The preferred clinical term is late-onset hypogonadism (LOH). Unlike female menopause, LOH is neither universal nor abrupt. Harman et al., analyzing data from the Baltimore Longitudinal Study of Aging, showed that total testosterone declines at approximately 1.6% per year and free testosterone at approximately 2-3% per year after age 30, with substantial inter-individual variation [7].

Men with LOH often present with a cluster of symptoms rather than a single complaint: reduced libido, mild ED, fatigue that does not improve with sleep, decreased morning erections, difficulty concentrating, and reduced muscle mass. The Aging Males' Symptoms (AMS) scale is a validated 17-item questionnaire used to screen for this syndrome, though it has lower specificity than a direct serum testosterone measurement.

The challenge with LOH is that symptoms are non-specific. Fatigue and reduced libido are also features of depression, sleep apnea, hypothyroidism, and anemia. The Endocrine Society guideline explicitly recommends against making a diagnosis of LOH on symptoms alone and requires two separate morning testosterone measurements below the threshold [2]. This is not excessive caution; it reflects the fact that testosterone has diurnal variation of up to 35%, so a single low value may not be reproducible.

TRT for Low Libido: Separating Desire From Erection

Low libido and ED are co-located in the testosterone story but respond differently to treatment.

Testosterone's effect on libido is more direct and typically faster than its effect on erections. Hypothalamic androgen receptors modulate dopaminergic pathways that drive sexual motivation. In the TTrials sexual function sub-trial, the PDSS Sexual Desire subdomain showed improvement within 3 months, somewhat ahead of the erectile function improvements [5]. A 2000 randomized crossover trial by Seftel et al. showed that hypogonadal men on TRT reported significant increases in sexual desire at 6 weeks [8].

When a man has poor libido with entirely normal erectile function on PDE5 inhibitors, low testosterone becomes the more probable explanation. When a man has low libido alongside ED that does not respond to sildenafil 100 mg, checking testosterone is a standard next step.

One complication: low libido can also persist after testosterone is normalized if the underlying driver is depression or relationship distress rather than hormonal. TRT is not a substitute for addressing psychological contributors to sexual dysfunction.

TRT for Fatigue: A Real but Narrowly Applicable Benefit

Fatigue is among the most common presenting complaints in men who seek TRT, and the evidence for benefit is specific rather than broad.

The TTrials vitality sub-trial enrolled 790 men aged 65 and older with confirmed low testosterone and self-reported fatigue or low vitality. At 12 months, testosterone gel improved FACIT-Fatigue scores by 1.2 points more than placebo on a 52-point scale. The difference was statistically significant (P=0.03) but the magnitude was modest [9]. The authors noted that men with the most severe fatigue at baseline showed larger improvements.

The practical takeaway: TRT reliably reduces fatigue caused by hypogonadism, but its effect size is not dramatic enough to justify empiric treatment in a tired man with normal testosterone. Sleep apnea, iron deficiency, hypothyroidism, and mood disorders each produce fatigue that TRT will not fix. Screening for these conditions before or alongside testosterone testing is standard practice.

Combining TRT With PDE5 Inhibitors

For men who have both confirmed hypogonadism and vascular ED, combining TRT with a PDE5 inhibitor produces better outcomes than either therapy alone.

A 2014 double-blind randomized trial by Spitzer et al. enrolled 140 hypogonadal men with ED who had previously failed PDE5 inhibitor monotherapy. After 14 weeks of testosterone gel plus vardenafil, the combination group achieved IIEF-EF domain scores 6.4 points higher than the placebo plus vardenafil group (P<0.001) [10]. The proposed mechanism is that testosterone upregulates phosphodiesterase-5 expression and restores cavernosal smooth muscle sensitivity to nitric oxide, which then allows PDE5 inhibitors to work as intended.

This has a direct clinical implication. A man who reports that sildenafil "stopped working" or "never really worked" should have his testosterone checked before escalating to intracavernosal injections or penile prosthesis evaluation. Approximately 25 to 35% of PDE5 inhibitor non-responders have biochemical hypogonadism, and testosterone repletion converts a proportion of them to PDE5 inhibitor responders.

The HealthRX clinical decision framework for TRT and ED works as follows. First: confirm hypogonadism with two morning testosterone measurements and LH/FSH. Second: if testosterone is below 300 ng/dL and symptoms are present, start TRT and reassess erectile function at 3 months. Third: if erections remain inadequate after testosterone is in the 500 to 800 ng/dL range, add a PDE5 inhibitor. Fourth: if testosterone is normal and PDE5 inhibitors fail, refer for penile vascular imaging and urological evaluation. This four-step sequence prevents both under-treatment (dismissing a fixable hormonal cause) and over-treatment (assuming every ED case is hormonal).

TRT Formulations Relevant to ED Management

The formulation of TRT influences how consistently testosterone is maintained, which affects sexual outcomes.

Testosterone cypionate or enanthate injected intramuscularly every 7 to 14 days produces supraphysiologic peaks on days 1 to 3 followed by troughs. Libido and erectile quality may mirror this rollercoaster pattern. Switching to every-5-day injections or subcutaneous administration of smaller doses flattens the peaks and troughs and typically stabilizes sexual function more reliably.

Testosterone undecanoate (Aveed, injectable) dosed every 10 weeks produces stable levels after the loading phase and is a reasonable option for men who dislike frequent injections, though the FDA requires administration in a clinical setting due to pulmonary oil microembolism risk [11].

Topical gels (AndroGel 1%, 1.62%, Testim) and solutions are applied daily and maintain stable levels. The primary concern is transference to partners or children through skin contact. Men with ED who share a bed with a female partner should apply the gel to the back or abdomen and cover with clothing before contact.

Testosterone pellets (Testopel) are implanted subcutaneously every 3 to 6 months and provide the most stable steady-state levels of any formulation. Several observational studies report high patient satisfaction for both libido and energy endpoints, though large randomized controlled trial data specific to ED outcomes remain limited.

Monitoring and Safety During TRT

The Endocrine Society 2018 guideline specifies monitoring hematocrit at 3 to 6 months, then annually; PSA at 3 to 12 months in men over 40; and testosterone levels at 3 months to confirm therapeutic range [2].

The most common adverse effect is erythrocytosis, a rise in hematocrit above 54%, which increases thrombotic risk. It occurs in roughly 3 to 18% of men on TRT depending on the formulation, with injectable testosterone carrying the highest rate due to peak-level stimulation of erythropoiesis. A hematocrit above 54% warrants dose reduction, formulation change, or therapeutic phlebotomy.

TRT suppresses the hypothalamic-pituitary-gonadal (HPG) axis, which reduces intratesticular testosterone and stops spermatogenesis. Men who want to father children should discuss alternatives (clomiphene, hCG monotherapy, or hCG co-administration with TRT) before starting exogenous testosterone.

A 2023 FDA drug safety communication re-confirmed that TRT carries a cardiovascular risk signal requiring individualized assessment, particularly in men with pre-existing coronary artery disease [11]. The TRAVERSE trial (N=5,204), published in the New England Journal of Medicine in 2023, found that testosterone gel was non-inferior to placebo for major adverse cardiovascular events (MACE) in men with hypogonadism and high cardiovascular risk, though it did increase rates of atrial fibrillation and acute kidney injury [12].

Who Is Not a Candidate for TRT?

The Endocrine Society guideline lists absolute contraindications: metastatic prostate cancer, breast cancer in men, an unexplained PSA above 4 ng/mL without urological evaluation, hematocrit above 54%, untreated obstructive sleep apnea, and active desire for fertility without concurrent hCG co-administration [2].

Men with borderline testosterone (300 to 400 ng/dL) and no clear symptoms are not standard candidates. The Endocrine Society states: "We suggest against making a diagnosis of androgen deficiency in men with symptoms that are non-specific for hypogonadism who have total testosterone concentrations within the laboratory reference range" [2]. Treating borderline testosterone without confirmed symptoms exposes men to the HPG suppression and hematocrit risks of TRT without a reliable likelihood of benefit.