For decades the great majority of physicians were very cautious about adminstering testosterone over a variety of concerns. My guess is that many physicians considered HRT in the same category as steroids, even though the grossly elevated testosterone levels of steroid users are in no way similar to those of a man on normal testosterone therapy. Still other physicians saw HRT as a useless attempt for a man to maintain his sex life or a hopeless fountain of youth. All of this drama and emotion took the eyes of the medical community at large off of an incredible fact: testosterone improves most of the key cardiovascular risk factors that scientists have discovered to date.
Of course, the significance of this is that heart disease is the #1 killer of men in modern, Western societies, and nothing else really comes close. It is no exaggeration to call it an epidemic and statins are a weak and I believe sometimes dangerous attempt to correct the problem, something I discuss here: The Dangers of Statins. I believe that right under our noses is an additional tool, testosterone replacement therapy, to fight cardiovascular disease in the solid majority of men with low or lowish testosterone levels.
Why do I say the "solid majority?" Why not all men? Well, there are likely exceptions, such as men with clotting disorders, certain men with hypertension or arrhythmias and men with high red blood cell counts, hematocrit or hemoglobin. Men should be screened before going on TRT and, of course, any issues discussed with a knowledgeable physician. But on this page I will present the evidence, and there is a lot of it, that, generally speaking for most guys, boosting low testosterone will be a big help in the fight against heart disease.
NOTE: The #1 way is to Regressing Atherosclerosis. If your diet and lifestyle is putting plaque in your arteries, then testosterone is not going to save you.
Here are just some of the benefits to the cardiovascular system associated with improved testosterone levels:
1. Hardening of the Arteries. Many studies have shown that not only does testosterone protect a man's arteries from hardening, but it also can partially reverse existing issues in many cases.  For more information, see no. 1 is this link on Arterial Hardening Solutions. (Testosterone is not a miracle cure, of course, for poor diet, being overweight and a sedentary lifestyle.)
2. High Blood Pressure. Testosterone therapy can also lower blood pressure in men. The change is usually not huge, but it is significant nonetheless. The reason is that testosterone actually affects nitric oxide - yes the stuff that makes your erections possible. I document all this in my link on Testosterone and Blood Pressure . By the way, one of the Peak Testosterone Forum members saw a huge drop in blood pressure from going on HRT. He went from about 120/80 to 87/67! 
3. Weight Maintenance. Study after study has shown that HRT will help hypogonadal men gain muscle and maintain or even lose weight. For example, one study even showed an increase in fat free mass, essentially muscle, and a decrease in fat mass in men with low normal testosterone.  Of course, testosterone will not keep you from overeating, but it is definitely another tool in your arsenal against the slow and steady weight gain that plagues men in modern societies. And, yes, those extra pounds are a risk factor for cardiovascular disease.
4. Intercourse. In general, the higher the testosterone, the higher the libido and erectile strength (due to increased nitric oxide and improved venous leakage). Testosterone also improves psychological parameters and all of this will increase a man's chances in the bedroom. Sexual intercourse is actually linked to improved health outcomes as I document in my link on Sex is Good For You, and that will almost always be good for the heart.
5. Decreased Anxiety and Improved Depression. Anxiety and depression are both stressors and can increase cortisol and cause a host of direct and indirect cardiovascular-related issues. Testosterone can often help greatly with both of these and is a proven mood booster, something I talk about in my link on Testosterone and Depression . Psychologists are starting to admit the interrelationship of hormones and depression and perhaps one day getting your testosterone and estradiol checked will be a part of any psychological evaluation. On the Peak Testosterone Forum I have had a couple of low T men completely turned around from depression by going on HRT. Of course, it doesn't always work that way as depression is a complex, multi-faceted condition. But having low testosterone is likely only going to make the condition worse and be very hard on one's cardiovascular system.
6. Diabetes. It is no secret that type II, adult onset diabetes is an epidemic in the U.S. And it is no secret that, once a man gets diabetes, his chances of cardiovascular disease sky rocket. One study showed that men with diabetes tripled their chance of another heart attack if they had already had one. And, if they had not yet had a heart attack, they increased their risk six fold!  Testosterone therapy can be a HUGE boost in men with low or lowish testosterone in this area. As I mention in my link on Testosterone and Diabetes, I have spoken with one large HRT clinic where ALL of their men got off of insulin by going on testosterone cypionate injections to youthful levels. This is an incredible stat and, hopefully, more credence will be paid to it over the next decade by the medical community.
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7. Insulin and Inflammation. Probably the two biggest root cause killers of men are what I call The Two I's: insulin and inflammation. As insulin levels and resistance increase, tissues begin aging and a host of nasty metabolic conditions set in, especially in the arteries. Testosterone therapy dramatically decreases both insulin and inflammation levels, something I document in my links on Testosterone and Inflammation and Testosterone and Insulin Resistance.
8. Visceral Fat. Visceral fat is the stomach fat that plagues so many men and is linked to Metabolic Syndrome and heart disease. In one study of hypogonadal diabetic men, testosterone therapy decreased visceral fat.  This will very likely be the case for many of you because diabetes and prediabetes account for a huge block of modern societies.
9. Atherosclerosis. There is now in vitro evidence that testosterone (and estradiol, DHT and DHEA) can inactivate white blood cell activity in the arterial walls, which is so critical in the plaque formation process, and thus help a man prevent atherosclerosis. I cover this in my page on Testosterone, Arterial Plaque and Atherosclerosis.
All of the above are big risks for heart disease and so it would be tragedy to ignore all this research when heart disease is the #1 killer of us guys. However, a big question remained in the eyes of researchers: could testosterone therapy (for men with hypogonadism) improve heart disease outcomes. We all know of research that should have been positive but turned out to be the opposite. Would HRT fall into this category?
The good news is that testosterone therapy has done very well in the studies so far and has actually improved outcomes in a variety of studies. Here are a few examples:
1. Moderate Heart Failure. One study looked at men with moderate heart failure. It found significantly improvement from Androderm therapy even though not all the men were hypogonadal to begin with. 
2. Angina. Heart patients with angina often suffer tremendously with chest pain. One study gave men with stable, chronic angina low dose testosterone in the form of a patch.  The study, which was double blind and placebo-controlled, improved pain scores. And, as expected, in men with lower bioavailable testosterone, the pain scores were improved even more dramatically.
3. Myocardial Ischemia. One (admittedly small) double blind, crossover study showed that testosterone helped heart patients' myocardial ischemia, or insufficient blood flow to an organ or tissue. In this case, the researchers were concerned with proper blood flow to the heart muscle, which can limit tissue damage before and after a heart attack. 
What is remarkable about the above studies, if you stop and think about it, is that no lifestyle modifications were usually required of the participants. In general, cardiovascular disease is a disease of a Western lifestyle, and so it is rather surprising that testosterone can help so much just on its own. Imagine if testosterone therapy is combined with proper diet, exercise and general good, clean livin'? So, if your testosterone is low, talk to your doctor about the pros and cons of HRT. I have many links on the subject including ones on HRT, Pellets, Compounding and so on.
I also highly recommend that one read these two pages for other very important research on the subject: Testosterone Therapy and the Recent VERY Postive Research and Profound Lowering of Cholesterol in Men through HRT.
NOTE: Erectile dysfunction often goes hand-in-hand with cardiovascular disease as well. Your erections, just like your heart and arteries, are dependent on soft, expandable vascular tissues and ample nitric oxide that is not overly aged, injured and covered with plaque. It will be no surprise, then, to learn that testosterone very often improves erectile strength as well, nor that hypogonadism is often a root cause of erectile dysfunction. For more information, see my link on Testosterone and Erectile Dysfunction.
1) European Heart Journal, 27(1):57-64, "Testosterone therapy in men with moderate severity heart failure: a double-blind randomized placebo controlled trial"
2) Circulation, 2000, 102: 1906-1911, "Low-Dose Transdermal Testosterone Therapy Improves Angina Threshold in Men With Chronic Stable Angina"
3) Eur J Endocrinol June 1, 2006 154 899-906, "Testosterone replacement therapy improves insulin resistance, glycaemic control, visceral adiposity and hypercholesterolaemia in hypogonadal men with type 2 diabetes"
4) J Endocrinol, Sep 1, 2003, 178:373-380, "Testosterone as a protective factor against atherosclerosis--immunomodulation and influence upon plaque development and stability"
6) The Journal of Clinical Endocrinology & Metabolism, Jan 1 2008, 93(1):139-146, "Testosterone Therapy Prevents Gain in Visceral Adipose Tissue and Loss of Skeletal Muscle in Nonobese Aging Men"
7) The American Journal of Cardiology, 1999, 83(3):437-9, A9, "Effect of acute testosterone on myocardial ischemia in men with coronary artery disease"
8) N Engl J Med, 1998; 339:229-234, "Mortality from Coronary Heart Disease in Subjects with Type 2 Diabetes and in Nondiabetic Subjects with and without Prior Myocardial Infarction"