Testosterone and the Vascular Endothelium

The endothelium is a single layer of cells lining every blood vessel in the body, and it is not passive plumbing — it decides how wide a vessel opens, how sticky its surface is to white cells, and whether blood flowing over it clots. It is also the first thing to fail in cardiovascular disease, years before a scan shows plaque. Testosterone acts on this layer. That is the honest core of the topic, and it is both more interesting and more limited than either the enthusiasts or the sceptics allow.

The endothelium is an organ, not a lining

Endothelial cells manufacture nitric oxide, prostacyclin and endothelin, and the balance between them sets vessel tone moment to moment. They expose or hide adhesion molecules, deciding whether circulating monocytes stick to the wall and burrow in — the opening move of atherosclerosis. They regulate clotting, and they form the barrier that determines which lipoproteins enter the wall at all.

Because that is chemistry rather than structure, it can be measured before anything anatomical changes. Flow-mediated dilation is the usual method: occlude an artery briefly with a cuff, release it, and measure by ultrasound how much the vessel widens in response to the surge of flow. A healthy endothelium senses the shear stress, releases nitric oxide, and the vessel opens; a dysfunctional one opens less. It is a functional test of a chemical process, which is why it moves years before imaging does.

How the layer fails

Endothelial dysfunction is, mechanistically, a shortage of usable nitric oxide, and there are two routes there that usually run together.

The first is producing less. Endothelial nitric oxide synthase (eNOS) makes NO from arginine, and its output depends on the enzyme being present, correctly assembled and supplied with cofactors. Under metabolic stress it can "uncouple" — staying active but producing superoxide instead, which is worse than doing nothing.

The second is destroying more. Nitric oxide is a short-lived free radical that reacts almost instantly with superoxide, so anything raising oxidative stress in the vessel wall — hyperglycaemia, high ApoB particle counts, smoking, sleep apnoea, visceral adiposity — mops it up before it reaches smooth muscle. Endothelial function therefore tracks the whole metabolic picture, not any single hormone.

What low testosterone looks like at the vessel wall

Men with low testosterone, as a group, show reduced flow-mediated dilation, lower nitric oxide bioavailability, higher markers of vascular oxidative stress and inflammation, and much higher rates of erectile dysfunction, which is itself a vascular event.

The caveat belongs immediately next to that list: low testosterone travels with visceral adiposity, insulin resistance, poor sleep and low activity, each of which independently damages the endothelium. Cross-sectional associations cannot separate the hormone from its company. The pattern is real and worth investigating rather than dismissing — the low testosterone checklist covers an appropriate work-up.

How testosterone acts on the vasculature

Several routes have been described, operating on different timescales.

The slowest is genomic: testosterone binding the androgen receptor alters transcription, including of eNOS itself, so more enzyme is available over days to weeks. Androgens also appear to reduce reactive oxygen species and inflammatory cytokines in the vessel wall, protecting nitric oxide after it is made rather than increasing synthesis.

Faster and stranger is a direct relaxant effect on vascular smooth muscle. Isolated arteries relax in response to testosterone even when the endothelium has been stripped away, which means the hormone is acting on the muscle layer itself rather than through nitric oxide signalling (Yue et al., Circulation 1995). The concentrations used in those preparations are typically higher than circulating physiological levels, so the finding establishes that the mechanism exists rather than that it dominates in a living man.

An indirect route may matter more than any of these: testosterone improves insulin sensitivity and body composition, and a better metabolic environment is itself the largest available intervention on endothelial function. Some of what looks like a hormonal effect is the metabolic change it enables. And part of it is not testosterone at all — a portion aromatises to estradiol, which has its own vascular actions, discussed in estradiol in men.

Erectile dysfunction is the early warning, not the whole problem

An erection is a vascular event with a neurological trigger. Nitric oxide released from nerve endings and endothelium relaxes smooth muscle in the penile arteries and sinusoids; inflow rises, outflow is compressed, and the tissue becomes rigid. Every step depends on endothelial competence.

The penile arteries are also small. A given thickness of plaque or degree of dysfunction narrows a small vessel proportionally more than a large one, which is why erectile symptoms commonly appear before angina — the same disease process reaching a symptomatic threshold in the smaller vessel first. Meta-analysis of prospective cohorts found erectile dysfunction predicts later cardiovascular events (Vlachopoulos et al., Circ Cardiovasc Qual Outcomes 2013).

The decision that follows is the useful part. New erectile dysfunction in a man over about 40 is a reason to check blood pressure, ApoB, glucose and HbA1c — not only testosterone. Treating the symptom without asking what it was signalling is the common and expensive mistake. PDE5 inhibitors prevent the breakdown of the messenger nitric oxide triggers, which is also why they work poorly when there was little nitric oxide to begin with.

What treatment does, and what it has not shown

In men with genuinely low testosterone, treatment tends to improve the measurable vascular markers — flow-mediated dilation, some inflammatory markers, and often blood pressure by a small margin. Those are real, and they are surrogates.

The hard endpoint evidence says something narrower. The TRAVERSE trial, designed specifically to test cardiovascular safety in middle-aged and older men with hypogonadism and existing cardiovascular risk, found that testosterone therapy was not inferior to placebo for major adverse cardiovascular events (Lincoff et al., N Engl J Med 2023). That resolved a genuine safety question that had hung over the field for a decade. It did not demonstrate cardiovascular benefit, and the trial was not built to. Anyone told that testosterone therapy will reduce their cardiac risk is being told something the evidence does not currently support. Testosterone and cardiovascular outcomes works through what that trial did and did not settle.

There is also a monitoring obligation attached. Testosterone raises red cell production, and a rising haematocrit thickens blood in a way that is not helpful to any vascular bed — which is why it is checked on a schedule rather than when someone thinks to ask. Haematocrit management covers how that is handled.

The levers that move the endothelium more

Ranked honestly by expected effect on endothelial function, hormone optimisation is not at the top of the list. Not smoking is first, by a distance. Lowering ApoB is next, because atherogenic particles entering the wall are the substrate of the disease — see ApoB, the real cardiovascular number. Then blood pressure, then regular aerobic and resistance training, which raise shear stress repeatedly and upregulate eNOS by exactly the mechanism the cuff test measures. Then sleep, particularly treating sleep apnoea. Then glucose.

Testosterone belongs on that list but not at the top of it, and a man who fixes his hormone and none of the rest has addressed one contributor while leaving the larger ones running. The 60-second assessment is the entry point to an evaluation that looks at the whole picture rather than a single number.

Bottom line

Testosterone supports endothelial function through several routes: more eNOS expression, less oxidative destruction of nitric oxide, lower vascular inflammation, a direct relaxant effect on smooth muscle, and an indirect benefit through improved body composition and insulin sensitivity. Low testosterone is associated with measurably worse endothelial function, and treatment usually improves the surrogate markers. What has been demonstrated at the level of hard outcomes is safety, not benefit. The sensible reading is that erectile symptoms deserve a cardiovascular work-up, that vascular markers are worth measuring before and during treatment, and that the largest levers on the endothelium remain smoking, ApoB, blood pressure, training and sleep.

Educational content, not medical advice. Laboratory interpretation and any treatment decision are made by a licensed physician after individual evaluation. Individual results vary.