NEW ORLEANS – Evidence seems to be mounting that the link between testosterone replacement therapy and increased hematocrit doesn’t lead to more cardiac or thrombotic events in men.
The association between testosterone and secondary erythrocytosis has been known for some time, Dr. Wayne J. G. Hellstrom said at the annual meeting of the American Urological Association. An increase in hematocrit almost invariably follows testosterone supplementation. “The question is, is there a causal relation between testosterone replacement therapy–induced erythrocytosis and venous thromboembolism or major cardiac events?” said Dr. Hellstrom of Tulane Medical Center, New Orleans. “The available evidence doesn’t support this claim.”
Erythrocytosis is defined as a packed red blood cell volume exceeding 125% of the age-predicted mass. This may be primary – an intrinsic alteration of the hematopoietic stem cells – or secondary. “And it may actually be a physiologically appropriate response to something, as in anemia,” Dr. Hellstrom said. “In fact, some anemias are primarily treated with testosterone.”
In the presence of exogenous testosterone, the condition may be due to a couple of things, he noted, such as:
• An overall increase in the erythropoietin set point.
• Increased availability of iron in the liver.
• The conversion of testosterone to estradiol, which tends to stimulate the bone marrow.
Erythrocytosis, obviously then, increases blood viscosity – and this is the primary concern for cardiovascular events.
Intramuscular testosterone is the only form that significantly increases hematocrit above normal levels. However, it does so strongly, with up to a 6% change from baseline. The runner-up is testosterone gel, with an average increase of 2.5% over baseline levels.
But despite concerns – which in March prompted the FDA to require on labeling a warning about the risk of cardiovascular events – the relationship has never been thoroughly investigated, Dr. Hellstrom said.
“We only have retrospective data, primarily extrapolating from the nephrology literature. When we look at the renal literature, we see that 10%-20% of kidney transplant patients develop polycythemia – an increase of both red and white cells, with hematocrit values of more than 51% or 52%.”
This has led to a recommendation by the American Society of Nephrology for frequent complete blood cell counts in the year after transplant and annual measurements thereafter.
The highest-quality mortality data for kidney transplant patients come from a 2013 study of 365 patients; the investigators found that those with polycythemia were 2.7 times more likely to die over 4 years. “But this is a true primary polycythemia,” which is often accompanied by procoagulative changes. It is not the secondary condition induced by testosterone, Dr. Hellstrom said.
Older studies suggested a significant link between increased hematocrit and cardiovascular or thrombotic events, especially after surgery. But prospective data from the Atherosclerosis Risk in Communities and Cardiovascular Health Studies have found no increased risk of cardiovascular death by increasing tertiles of either hematocrit or hemoglobin, with respective cut points of 43% and 14.5 g/dL.
In fact, a recent transgenic mouse model with hematopoietic overexpression, reaching an 85% hematocrit, found no evidence of either lung or cardiovascular thromboses. “This seems to be related to a reduction in clot strength and increased osmotic fragility in the presence of increasing hematocrit. It seems to mechanically deter the interaction of platelets and fibrin in the extravascular space and endothelium.”
He referred to an in-press mouse study showing that a short course of high-dose testosterone did raise whole blood viscosity and hematocrit. “But over time, this returned to normal, even with supraphysiolgic testosterone levels, so it seems likely that there is an adaptive mechanism that occurs in these animals.”
Additionally, he said, men who live at high altitudes develop naturally high hematocrits as a response to decreased oxygen in the atmosphere. “We routinely see men from these locations with hematocrits of 57% and 59% who have no problems at all.”
Extrapolating all these data to the testosterone/thrombosis link is confusing. The most recent study, however, provided some measure of reassurance. The large meta-analysis comprised 75 randomized, placebo-controlled trials involving about 5,500 men; they all examined cardiovascular risk and testosterone therapy.
“Our analyses, performed on the largest number of studies collected so far, indicate that testosterone supplementation is not related to any increase in cardiovascular risk, even when composite or single adverse events were considered,” wrote Dr. Giovanni Corona of the Maggiore-Bellaria Hospital, Bologna, Italy. “In randomized trials performed in subjects with metabolic derangements, a protective effect … was observed. … Our results are in agreement with a large body of literature from the last 20 years supporting testosterone supplementation of hypogonadal men as a valuable strategy in improving a patient’s metabolic profile, reducing body fat, and increasing lean muscle mass, which would ultimately reduce the risk of heart disease