Nelson Vergel
Founder, ExcelMale.com
By Nelson Vergel | B.S. Chemical Engineering, MBA | Founder, ExcelMale.com | 34+ years on TRT | NIH and FDA advisory panel service | Author: Testosterone: A Man's Guide, Beyond Testosterone, The Peptide Consensus | Updated July 2026
Most conversations about testosterone and the heart stop at one number: hematocrit. That number matters, but it is downstream of something more interesting. Your arteries have a repair system, and testosterone helps run it. When your level drops, that system slows down. When you replace testosterone, the way you replace it decides whether your blood vessels get steady support or a series of hormonal surges they have to absorb. This article looks at what happens inside the artery wall, why injections and gels send different signals, and what to actually watch on your labs.
The endothelium is the single-cell lining inside every blood vessel. It controls how vessels dilate, how sticky the walls are to white blood cells, and whether the blood tips toward clotting. Testosterone is one of the hormones that keeps that lining working. When it falls, the lining loses some of its protective capacity before any symptom shows up. That silent phase is why lab markers matter.
Endothelial progenitor cells (EPCs) are bone-marrow cells that circulate and home in on damaged spots in the vessel wall, then rebuild the lining. Think of them as the repair crew. Hypogonadal men have measurably fewer of them in circulation. Testosterone helps mobilize this crew through the CXCL12/CXCR4 signaling pathway and by supporting nitric oxide production.
Endothelial microparticles (EMPs) are the opposite signal. These are tiny membrane fragments shed by stressed or dying endothelial cells. High EMP counts mean the lining is actively being damaged, and the fragments themselves carry pro-clotting cargo. Low testosterone raises them.
The worst-case profile is low EPCs alongside high EMPs: no repair capacity and active damage at the same time. This pattern shows up often in men who have both metabolic dysfunction and low testosterone. Restoring testosterone shifts the balance back toward repair in most men, though the size of that shift varies.
One honest caveat: EPC measurement is not standardized across labs. Different centers use different surface markers to identify these cells, so a single absolute count means less than a trend over time. Treat these as research-grade markers that explain mechanism, not as something to order at your next visit.
The fast pathway is where most of the vascular benefit lives. Testosterone activates Src kinase and the PI3K/Akt cascade, which phosphorylates the enzyme eNOS at a site called Ser1177. Activated eNOS produces nitric oxide. Nitric oxide relaxes the vessel to lower blood pressure and keeps platelets from clumping. It also reduces the wall stickiness that lets plaque get a foothold. This activation happens independent of testosterone converting to estradiol, which means testosterone itself drives the effect.
Testosterone also lowers asymmetric dimethylarginine (ADMA), a molecule that blocks nitric oxide production. Hypogonadal men run high on ADMA. Clearing it is part of how testosterone restores healthy dilation.
Short-acting intramuscular esters like cypionate and enanthate create a sharp peak within a day or two of the shot, often pushing well above the normal range, followed by a drop toward a trough before the next dose. Transdermal gels deliver a steady daily level that tracks closer to the body's natural rhythm.
That difference is not cosmetic. Supraphysiological peaks flip testosterone from a vascular protector into a vascular stressor. At those high concentrations, testosterone stimulates NADPH oxidase and mitochondrial reactive oxygen species, and it activates the NLRP3 inflammasome, a molecular alarm that triggers inflammatory cytokine release inside the vessel wall. Steady levels keep eNOS phosphorylated and EPC mobilization stable without setting off that alarm.
Long-acting testosterone undecanoate sits between these two. It avoids the sharp weekly swings of short esters but does not match the daily stability of a gel. Some men also split short-ester doses into smaller, more frequent injections to flatten the peaks, which is a common ExcelMale strategy for keeping hematocrit down without switching formulations.
The peaks from injections push both levers harder than the steady level from a gel. Higher hematocrit means thicker, more viscous blood, which the heart has to work harder to move through small vessels. Above 54%, most guidelines call for action. This rise in viscosity is one reason improved lab markers on TRT do not automatically translate into fewer heart attacks: the thickening can offset some of the benefit from better nitric oxide signaling.
The TRAVERSE trial, published in The New England Journal of Medicine in 2023, followed 5,246 men at high cardiovascular risk. Transdermal testosterone was non-inferior to placebo for the combined endpoint of cardiovascular death, nonfatal heart attack, and nonfatal stroke. That is genuinely reassuring for the major events. TRAVERSE also flagged higher rates of atrial fibrillation, pulmonary embolism, and acute kidney injury in the testosterone group, so "non-inferior for MACE" does not mean "no cardiovascular signals at all."
There is a second wrinkle. A substudy of the Testosterone Trials, published in JAMA in 2017, found that testosterone treatment was associated with greater progression of non-calcified coronary plaque volume in men 65 and older, even while their metabolic markers improved. Non-calcified plaque is the softer, more rupture-prone kind. Better surrogate numbers and more soft plaque showed up in the same population.
The takeaway is not that TRT is dangerous. It is that biomarker improvement and hard-outcome safety are two separate questions, and only the second one is settled by randomized trials with pre-specified endpoints. TRT earns "mechanistic reassurance," and for major events TRAVERSE adds real evidence, but neither one is a promise of cardiovascular immunity.
Check hematocrit at baseline, again at 3 to 6 months, then annually. If it climbs above 54%, the dose needs to come down, the formulation may need to change, or therapeutic phlebotomy comes into play. If your baseline hematocrit is already above 50%, a gel is the more forgiving starting point.
Track blood pressure, a lipid panel, and fasting glucose or HbA1c at least yearly. Report shortness of breath, chest pain, or new leg swelling promptly, since those can signal a clot or fluid problem. And treat TRT as a complement to the basics that move cardiovascular risk on their own: losing visceral fat, training regularly, and sleeping enough all raise testosterone and protect the heart independent of any prescription.
For men with a recent heart attack or stroke, most guidance advises waiting 3 to 6 months before starting TRT until things are stable. Severe, poorly controlled heart failure is generally a reason to hold off entirely.
If you want to go deeper on the hematocrit side of this, start with our guide to lowering high hematocrit on TRT and the member success stories thread.
Most conversations about testosterone and the heart stop at one number: hematocrit. That number matters, but it is downstream of something more interesting. Your arteries have a repair system, and testosterone helps run it. When your level drops, that system slows down. When you replace testosterone, the way you replace it decides whether your blood vessels get steady support or a series of hormonal surges they have to absorb. This article looks at what happens inside the artery wall, why injections and gels send different signals, and what to actually watch on your labs.
Is Low Testosterone Its Own Cardiovascular Risk Factor?
Yes. Hypogonadism is now treated as an independent cardiovascular risk factor in its own right, well beyond its effect on energy and libido. Low testosterone tracks with endothelial dysfunction and arterial stiffness. It also correlates with chronic low-grade inflammation and a blood environment that clots more easily. A 2024 individual-participant meta-analysis in Annals of Internal Medicine found that men with the lowest testosterone concentrations had higher all-cause mortality, and very low levels were linked to cardiovascular death.The endothelium is the single-cell lining inside every blood vessel. It controls how vessels dilate, how sticky the walls are to white blood cells, and whether the blood tips toward clotting. Testosterone is one of the hormones that keeps that lining working. When it falls, the lining loses some of its protective capacity before any symptom shows up. That silent phase is why lab markers matter.
What Blood Vessel Markers Tell You That Cholesterol Doesn't?
A standard lipid panel tells you about circulating fats. It says almost nothing about whether your artery walls are repairing themselves or breaking down. For that, researchers look at two opposing signals.Endothelial progenitor cells (EPCs) are bone-marrow cells that circulate and home in on damaged spots in the vessel wall, then rebuild the lining. Think of them as the repair crew. Hypogonadal men have measurably fewer of them in circulation. Testosterone helps mobilize this crew through the CXCL12/CXCR4 signaling pathway and by supporting nitric oxide production.
Endothelial microparticles (EMPs) are the opposite signal. These are tiny membrane fragments shed by stressed or dying endothelial cells. High EMP counts mean the lining is actively being damaged, and the fragments themselves carry pro-clotting cargo. Low testosterone raises them.
The worst-case profile is low EPCs alongside high EMPs: no repair capacity and active damage at the same time. This pattern shows up often in men who have both metabolic dysfunction and low testosterone. Restoring testosterone shifts the balance back toward repair in most men, though the size of that shift varies.
One honest caveat: EPC measurement is not standardized across labs. Different centers use different surface markers to identify these cells, so a single absolute count means less than a trend over time. Treat these as research-grade markers that explain mechanism, not as something to order at your next visit.
How Does Testosterone Actually Reach the Artery Wall?
Testosterone works on blood vessels through a fast pathway and a slow one. The slow, genomic pathway involves testosterone binding androgen receptors inside the cell and changing which genes get expressed over hours to days. The fast, non-genomic pathway happens in seconds to minutes at the cell membrane.The fast pathway is where most of the vascular benefit lives. Testosterone activates Src kinase and the PI3K/Akt cascade, which phosphorylates the enzyme eNOS at a site called Ser1177. Activated eNOS produces nitric oxide. Nitric oxide relaxes the vessel to lower blood pressure and keeps platelets from clumping. It also reduces the wall stickiness that lets plaque get a foothold. This activation happens independent of testosterone converting to estradiol, which means testosterone itself drives the effect.
Testosterone also lowers asymmetric dimethylarginine (ADMA), a molecule that blocks nitric oxide production. Hypogonadal men run high on ADMA. Clearing it is part of how testosterone restores healthy dilation.
Why Does Injection vs. Gel Change the Cardiovascular Picture?
Because the two methods produce completely different hormone curves, and your blood vessels respond to the shape of that curve as much as to the average level.Short-acting intramuscular esters like cypionate and enanthate create a sharp peak within a day or two of the shot, often pushing well above the normal range, followed by a drop toward a trough before the next dose. Transdermal gels deliver a steady daily level that tracks closer to the body's natural rhythm.
That difference is not cosmetic. Supraphysiological peaks flip testosterone from a vascular protector into a vascular stressor. At those high concentrations, testosterone stimulates NADPH oxidase and mitochondrial reactive oxygen species, and it activates the NLRP3 inflammasome, a molecular alarm that triggers inflammatory cytokine release inside the vessel wall. Steady levels keep eNOS phosphorylated and EPC mobilization stable without setting off that alarm.
| Feature | Transdermal Gel | Short-Acting IM Injection |
|---|---|---|
| Delivery pattern | Steady, near-circadian | Peak then trough |
| Typical testosterone range | Stable, physiological | Supraphysiological peak, then sub-normal |
| Effect on eNOS / nitric oxide | Sustained support | Oscillating |
| Erythrocytosis risk | Lower (15.4% in HEAT) | Higher (43.8% in HEAT) |
| Abnormal hematocrit driver | Minimal | Peaks stimulate EPO, suppress hepcidin |
Long-acting testosterone undecanoate sits between these two. It avoids the sharp weekly swings of short esters but does not match the daily stability of a gel. Some men also split short-ester doses into smaller, more frequent injections to flatten the peaks, which is a common ExcelMale strategy for keeping hematocrit down without switching formulations.
Why Do Injections Raise Hematocrit More Than Gels?
Testosterone drives red blood cell production through two levers. It increases erythropoietin (EPO) output from the kidneys, and it suppresses hepcidin, the liver hormone that limits how much iron reaches the bone marrow. With hepcidin low, iron flows freely to make more red cells. A 2010 study in The Journal of Clinical Endocrinology & Metabolism identified this hepcidin suppression as a core mechanism.The peaks from injections push both levers harder than the steady level from a gel. Higher hematocrit means thicker, more viscous blood, which the heart has to work harder to move through small vessels. Above 54%, most guidelines call for action. This rise in viscosity is one reason improved lab markers on TRT do not automatically translate into fewer heart attacks: the thickening can offset some of the benefit from better nitric oxide signaling.
Do Better Blood Markers Mean a Lower Risk of Heart Attack?
Not necessarily, and this is the part that gets oversold. TRT reliably improves surrogate markers. It raises EPCs and lowers ADMA. It improves insulin sensitivity and often nudges cholesterol in a favorable direction. Cleaner labs feel reassuring. They are not the same as proof of fewer cardiac events.The TRAVERSE trial, published in The New England Journal of Medicine in 2023, followed 5,246 men at high cardiovascular risk. Transdermal testosterone was non-inferior to placebo for the combined endpoint of cardiovascular death, nonfatal heart attack, and nonfatal stroke. That is genuinely reassuring for the major events. TRAVERSE also flagged higher rates of atrial fibrillation, pulmonary embolism, and acute kidney injury in the testosterone group, so "non-inferior for MACE" does not mean "no cardiovascular signals at all."
There is a second wrinkle. A substudy of the Testosterone Trials, published in JAMA in 2017, found that testosterone treatment was associated with greater progression of non-calcified coronary plaque volume in men 65 and older, even while their metabolic markers improved. Non-calcified plaque is the softer, more rupture-prone kind. Better surrogate numbers and more soft plaque showed up in the same population.
The takeaway is not that TRT is dangerous. It is that biomarker improvement and hard-outcome safety are two separate questions, and only the second one is settled by randomized trials with pre-specified endpoints. TRT earns "mechanistic reassurance," and for major events TRAVERSE adds real evidence, but neither one is a promise of cardiovascular immunity.
How Should a Man on TRT Monitor His Heart Safety?
Target a total testosterone in the mid-normal range, roughly 400 to 700 ng/dL, rather than chasing the top of the range. Higher is not better here, and the peaks that come with pushing levels up are exactly what stress the vasculature.Check hematocrit at baseline, again at 3 to 6 months, then annually. If it climbs above 54%, the dose needs to come down, the formulation may need to change, or therapeutic phlebotomy comes into play. If your baseline hematocrit is already above 50%, a gel is the more forgiving starting point.
Track blood pressure, a lipid panel, and fasting glucose or HbA1c at least yearly. Report shortness of breath, chest pain, or new leg swelling promptly, since those can signal a clot or fluid problem. And treat TRT as a complement to the basics that move cardiovascular risk on their own: losing visceral fat, training regularly, and sleeping enough all raise testosterone and protect the heart independent of any prescription.
For men with a recent heart attack or stroke, most guidance advises waiting 3 to 6 months before starting TRT until things are stable. Severe, poorly controlled heart failure is generally a reason to hold off entirely.
Frequently Asked Questions
Is gel or injection safer for the heart?
For cardiovascular safety, gels have the edge on paper because they avoid supraphysiological peaks and cause abnormal hematocrit far less often (15.4% vs 43.8% in the HEAT-Registry). That said, no head-to-head trial has compared them on hard endpoints like heart attack, so this is a pharmacokinetic and hematologic advantage, not proven mortality benefit.What hematocrit level is dangerous on TRT?
Most guidelines flag action at 54%. Above that, blood viscosity rises enough to raise clot risk, and the standard response is dose reduction, a formulation change, or therapeutic phlebotomy. Isolated readings can be thrown off by dehydration, so decisions should rest on repeated measurements.Does testosterone cause heart attacks?
The TRAVERSE trial found transdermal testosterone non-inferior to placebo for major cardiac events in high-risk men. Most of the heart-attack risk historically tied to TRT comes from unmonitored high hematocrit, which is why regular blood testing matters more than the therapy itself.Can TRT improve my blood vessel function?
In most men, yes, at the level of markers. Testosterone increases nitric oxide, lowers ADMA, and mobilizes endothelial repair cells. Whether that translates to fewer real-world cardiac events is a separate question that surrogate markers cannot answer.Why does my hematocrit go up more on injections than gel?
Injection peaks push red blood cell production harder by raising erythropoietin and suppressing hepcidin, which frees up iron for red cell synthesis. The steadier level from a gel applies less of that stimulus, so hematocrit tends to stay lower.The One Thing Most Men Miss About TRT and Heart Health
Here is something that rarely makes it into a doctor's visit: the peak matters more than the average. Two men can have the same monthly average testosterone, one on a daily gel and one on a weekly injection, and their blood vessels see two very different environments. The injection guy's arteries ride a wave up into stress territory and back down every week. If your hematocrit keeps creeping up despite a "normal" testosterone level, the shape of your dosing curve is the first place to look, not the dose alone. Splitting your injections into smaller, more frequent shots, or moving to a cream, is often all it takes.If you want to go deeper on the hematocrit side of this, start with our guide to lowering high hematocrit on TRT and the member success stories thread.
Related ExcelMale Forum Discussions
- TRT and Cardiovascular Disease - Member discussion and study abstracts on the TRAVERSE trial and CV safety data.
- How to Lower High Hematocrit Caused by TRT - Nelson's practical guide to managing elevated hematocrit on testosterone.
- Managing TRT-Induced High Hematocrit: Success Stories - Real member experiences with dose and donation strategies.
- Could Rusfertide Help Men on TRT with High Hematocrit? - The hepcidin connection and an emerging drug approach.
- TRT Side Effects: Complete Guide to Managing Them - Full rundown of side effects including hematocrit and blood pressure.
- How to Lower Red Blood Cell Count and Hematocrit While on Testosterone - Mechanisms and management options for erythrocytosis.
- Transdermal vs. Injections of Testosterone - Long-running thread on the pros and cons of each delivery method.
- New TRT User Frustrated on 1.62% Gel - Discussion touching on transdermal cardiovascular safety and DHT.
- Testosterone Injection Frequency and Its Effect on Hematocrit - Members compare dosing schedules and their effect on blood thickness.
Key References
- Lincoff AM, et al. Cardiovascular Safety of Testosterone-Replacement Therapy. New England Journal of Medicine. 2023. https://doi.org/10.1056/NEJMoa2215025
- Yeap BB, et al. Associations of Testosterone and Related Hormones With All-Cause and Cardiovascular Mortality and Incident Cardiovascular Disease in Men. Annals of Internal Medicine. 2024. https://doi.org/10.7326/M23-2781
- Budoff MJ, et al. Testosterone Treatment and Coronary Artery Plaque Volume in Older Men With Low Testosterone. JAMA. 2017. https://doi.org/10.1001/jama.2016.21043
- Corona G, et al. The HEAT-Registry (HEmatopoietic Affection by Testosterone): comparison of a transdermal gel vs long-acting intramuscular testosterone undecanoate in hypogonadal men. The Aging Male. 2022. https://doi.org/10.1080/13685538.2022.2063830
- Bachman E, et al. Testosterone Suppresses Hepcidin in Men: A Potential Mechanism for Testosterone-Induced Erythrocytosis. The Journal of Clinical Endocrinology & Metabolism. 2010. https://doi.org/10.1210/jc.2010-0864
- Corona G, et al. Cardiovascular disease and testosterone therapy in male hypogonadism. Annals of the New York Academy of Sciences. 2024. https://doi.org/10.1111/nyas.15211
- Francomano D, et al. Acute endothelial response to testosterone gel administration in men with severe hypogonadism and its relationship to androgen receptor polymorphism: a pilot study. Journal of Endocrinological Investigation. 2015. https://doi.org/10.1007/s40618-015-0325-4
- Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. The Journal of Clinical Endocrinology & Metabolism. 2018. https://doi.org/10.1210/jc.2018-00229
