Increased Hematocrit Linked to Reduced Death in Men Receiving Testosterone Therapy

January 2022

Real-world evidence studies have shown that men with hypogonadism who receive long-term treatment with testosterone undecanoate injection for up to 12 years have a significant reduction in mortality.1-3 Yet, testosterone therapy increases hematocrit, which theoretically could increase risk of fatal cardiovascular events.4

Here we summarise the results of a study that aimed to specifically examine the relationship between hematocrit and all-cause mortality in men receiving long-term treatment with testosterone undecanoate injection.5,6

Key Points

  • High hematocrit could theoretically increase risk of adverse cardiovascular events.
  • Testosterone-induced hematocrit elevation does not appear to be harmful.
  • Men receiving long-term treatment with testosterone undecanoate injections have reduced mortality despite relatively high hematocrit up to 52%.

What is known about testosterone, hematocrit and mortality

The link between elevated hematocrit and cardiovascular disease (especially venous thromboembolism) is uncertain.4,7 While some population studies suggest that high hematocrit is associated with increased risk of venous thromboembolism,8 cardiovascular disease and all-cause mortality,9 this can be explained by the presence of other medical conditions that cause both a high hematocrit and venous thrombosis or cardiovascular disease.7 For example, in the Scottish Heart Health Extended Cohort Study, hematocrit was significantly associated with cardiovascular disease events and mortality, however after adjusting for lipids, blood pressure, type 2 diabetes, smoking status, family history of cardiovascular disease and fibrinogen, the association disappeared.10

There is a paucity of data on the ratio between hemoglobin and hematocrit. In most individuals, hematocrit (expressed as a percentage) is about 3 times the hemoglobin value (expressed in g/dL).

Changes in hemoglobin, hematocrit and the hemoglobin/hematocrit ratio during testosterone therapy, and how these changes are associated with various outcomes, such as mortality, requires further understanding.

What this study adds

To gain a better understanding of the clinical impact of increased hematocrit during testosterone therapy in men with hypogonadism, the present study investigated the relationship between baseline hematocrit, change in hematocrit over time, and all-cause mortality in 737 men who had been diagnosed with functional hypogonadism (serum total testosterone ≤12.1 nmol/L and symptoms of testosterone deficiency).

Testosterone therapy with testosterone undecanoate injection was started in 353 men, whereas the remaining 384 men who declined testosterone therapy served as a control group. Change in hematocrit, hemoglobin and hematocrit/hemoglobin ratio after 12, 48, 72, and 96 months of treatment with testosterone undecanoate injection was analysed, as well as mortality.

As expected, results showed that hematocrit significantly increased (+4% at month 96) in men receiving testosterone therapy, with most of the increase occurring during the first 48 months, as illustrated in figure 1. At final assessment (month 96) mean hematocrit was 49% (range 47-51%). No subject had hematocrit above 52%. There was no change in hematocrit in men not receiving testosterone therapy.

Figure 1: Change in hematocrit at 12, 48, 72 and 96 months in men receiving testosterone therapy vs. untreated men.5

Figure 1: Change in hematocrit at 12, 48, 72 and 96 months in men receiving testosterone therapy vs. untreated men.5

From Strange RC, König CS, Ahmed A, et al. Testosterone Therapy: Increase in Hematocrit is Associated with Decreased Mortality. Androgens: Clinical Research and Therapeutics. 2021/12/01 2021;2(1):150-159.

HCT, hematocrit; TTh, testosterone therapy

TTh group = 353 men

Untreated group: 384 men

Among men receiving testosterone therapy, a surprising finding was that those who died during the follow-up had significantly (p=0.021) lower hematocrit than men who survived, and baseline hematocrit and change in hematocrit were inversely associated with mortality, even after adjustment for age. Men with final hematocrit above 49% up to 52% suffered lower mortality than men with hematocrit 46-49%. The higher the baseline hematocrit and the greater the increase in hematocrit during testosterone therapy (up to 52%), the lower the risk of mortality. Results were the same when the group of testosterone-treated men was stratified by type 2 diabetes prevalence. This suggests that the mortality risk reduction occurs even at relatively higher hematocrit levels up to 52%.


Interestingly, the hematocrit/hemoglobin ratio increased significantly at all time points (p < 0.0001) compared to baseline), possibly due to increased red blood cell life span, as suggested by the investigators of the T4DM study.


The present study showed that increased hematocrit (up to 52% at final assessment) was independently associated with reduced mortality.5 This confirms the current clinical guideline recommendation of using 54% as a threshold for change in management of men receiving testosterone therapy (e.g. dose reduction or therapeutic phlebotomy).11-15 It should be kept in mind that dehydration can cause a temporary elevation in hematocrit,16 and therefore a high hematocrit reading should be confirmed in a second blood test, ensuring the patient is in a well hydrated state, before action is taken.

The finding that the hematocrit elevation stabilized at month 48 is reassuring.5 This is congruent with results from another long-term real-world evidence study, in which treatment with testosterone undecanoate injection for 10 years increased hematocrit by 3.6%.3

Meta-analyses of randomized controlled trials which showed that despite a higher incidence of elevated hematocrit in men receiving testosterone therapy compared to placebo, no difference in clinical adverse events were reported.17,18 The present study provides reassurance regarding the safety of testosterone therapy, and suggests that long-term treatment with testosterone undecanoate injection can reduce mortality even in the context of relatively high hematocrit levels. Support for this comes from other long-term real-world evidence studies showing that despite increases in hematocrit, there was no increased risk for venous thromboembolism, myocardial infarction, stroke, or mortality.2,19

A possible reason for the lack of adverse effects of testosterone-induced elevation of hematocrit in men receiving testosterone therapy could be that the association between testosterone therapy and increased hemoglobin and hematocrit may be mediated by other mechanisms besides erythropoiesis. For example, it has been shown that testosterone therapy may increase red blood cell lifespan.20 In healthy subjects, there is a loss of 20% of erythrocyte hemoglobin, a phenomenon that increases during the second half of red blood cell life span.21,22 In the context of increased red blood cell life span, the loss of hemoglobin would increase, which in turn would increase the hematocrit/hemoglobin ratio. This could impact not only the accuracy of HbA1c - for more information see our report of the T4DM study23 - but possibly also the link between hematocrit and adverse outcomes. For example, the increased red blood cell lifespan during testosterone therapy may occur as a result of beneficial testosterone-induced modification of the composition and fluidity of red blood cell membranes, contributing to improvement in blood rheology and possibly reduced thrombosis risk.24 If this is the case, an increased hematocrit/hemoglobin ratio could be a marker for beneficial outcomes in the context of hematocrit elevation during testosterone therapy.

In contrast to the present study, where no subject receiving treatment with testosterone undecanoate injection experienced hematocrit above 52%, in the T4DM study23 22% of subjects (106 men) in the testosterone group had at least one reading of hematocrit ≥54%, compared to 1% (6 men) in the placebo group. A possible explanation for this is that the T4DM study was conducted in Australia, where the prevalence of dehydration due to hot weather is relatively common, which may have contributed to rising hematocrit to ≥54%. In contrast, the registry study was conducted in Germany, where dehydration prevalence due to hot weather is likely non-existent. Another possible explanation is that blood testing for measurement of hematocrit was done at different time points between injections.

More studies, both randomized controlled trials and real-world evidence studies, are needed to further investigate the association between hematocrit and mortality, and whether testosterone-induced hematocrit elevation is a cause of concern or just an innocent bystander.7

For related research, see:

Hematocrit elevation following testosterone therapy – does it increase risk of blood clots?


  • Zhang X, Huang K, Saad F, Haider KS, Haider A, Xu X. Testosterone Therapy Reduces Cardiovascular Risk Among Hypogonadal Men: A Prospective Cohort Study in Germany. Androgens: Clinical Research and Therapeutics. 2021/09/01 2021;2(1):64-72. Return to content
  • Saad F, Doros G, Haider KS, Haider A. Differential effects of 11 years of long-term injectable testosterone undecanoate therapy on anthropometric and metabolic parameters in hypogonadal men with normal weight, overweight and obesity in comparison with untreated controls: real-world data from a controlled registry study. Int J Obes (Lond). Jun 2020;44(6):1264-1278. Return to content
  • Saad F, Caliber M, Doros G, Haider KS, Haider A. Long-term treatment with testosterone undecanoate injections in men with hypogonadism alleviates erectile dysfunction and reduces risk of major adverse cardiovascular events, prostate cancer, and mortality. The aging male : the official journal of the International Society for the Study of the Aging Male. Feb 20 2020;23(1):81-92. Return to content
  • Konig CS, Balabani S, Hackett GI, Strange RC, Ramachandran S. Testosterone Therapy: An Assessment of the Clinical Consequences of Changes in Hematocrit and Blood Flow Characteristics. Sex Med Rev. Oct 2019;7(4):650-660. Return to content
  • Strange RC, König CS, Ahmed A, et al. Testosterone Therapy: Increase in Hematocrit is Associated with Decreased Mortality. Androgens: Clinical Research and Therapeutics. 2021/12/01 2021;2(1):150-159. Return to content
  • Lorde N, Maarouf A, Strange RC, et al. Testosterone Therapy in Adult-Onset Testosterone Deficiency: Hematocrit and Hemoglobin Changes. Androgens: Clinical Research and Therapeutics. 2021/09/01 2021;2(1):141-149. Return to content
  • Schreijer AJ, Reitsma PH, Cannegieter SC. High hematocrit as a risk factor for venous thrombosis. Cause or innocent bystander? Haematologica. Feb 2010;95(2):182-4. Return to content
  • Braekkan SK, Mathiesen EB, Njølstad I, Wilsgaard T, Hansen JB. Hematocrit and risk of venous thromboembolism in a general population. The Tromso study. Haematologica. Feb 2010;95(2):270-5. Return to content
  • Gagnon DR, Zhang TJ, Brand FN, Kannel WB. Hematocrit and the risk of cardiovascular disease-- the Framingham study: a 34-year follow-up. Am Heart J. Mar 1994;127(3):674-82. Return to content
  • Peters SA, Woodward M, Rumley A, Tunstall-Pedoe HD, Lowe GD. Plasma and blood viscosity in the prediction of cardiovascular disease and mortality in the Scottish Heart Health Extended Cohort Study. European journal of preventive cardiology. Jan 2017;24(2):161-167. Return to content
  • Salonia A, Bettocchi C, Carvalho J, et al. EAU Guidelines on Sexual and Reproductive Health (available at Health-2021.pdf accessed Deember 2021). Return to content
  • Mulhall JP, Trost LW, Brannigan RE, et al. Evaluation and Management of Testosterone Deficiency: AUA Guideline (unabridged). J Urol. Aug 2018;200(2):423-432. Return to content
  • Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. May 1 2018;103(5):1715-1744. Return to content
  • Hackett G, Kirby M, Edwards D, et al. British Society for Sexual Medicine Guidelines on Adult Testosterone Deficiency, With Statements for UK Practice. The journal of sexual medicine. Dec 2017;14(12):1504-1523. Return to content
  • Lunenfeld B, Mskhalaya G, Zitzmann M, et al. Recommendations on the diagnosis, treatment and monitoring of testosterone deficiency in men. The Aging Male. 2021/01/01 2021;24(1):119-138. Return to content
  • Billett HH. Chapter 151. Hemoglobin and Hematocrit. Available from: 3rd ed. Clinical Methods: The History, Physical, and Laboratory Examinations. Butterworths; 1990. Return to content
  • Fernandez-Balsells MM, Murad MH, Lane M, et al. Clinical review 1: Adverse effects of testosterone therapy in adult men: a systematic review and meta-analysis. J Clin Endocrinol Metab. Jun 2010;95(6):2560-75. Return to content
  • Ponce OJ, Spencer-Bonilla G, Alvarez-Villalobos N, et al. The efficacy and adverse events of testosterone replacement therapy in hypogonadal men: A systematic review and meta-analysis of randomized, placebo-controlled trials. J Clin Endocrinol Metab. Mar 17 2018; Return to content
  • Yassin A, Haider A, Haider KS, et al. Testosterone Therapy in Men With Hypogonadism Prevents Progression From Prediabetes to Type 2 Diabetes: Eight-Year Data From a Registry Study. Diabetes Care. Jun 2019;6(42):1104-1111. Return to content
  • Solomon LR, Hendler ED. Androgen therapy in haemodialysis patients. II. Effects on red cell metabolism. Br J Haematol. Feb 1987;65(2):223-30. Return to content
  • Willekens FL, Bosch FH, Roerdinkholder-Stoelwinder B, Groenen-Döpp YA, Werre JM. Quantification of loss of haemoglobin components from the circulating red blood cell in vivo. Eur J Haematol. Apr 1997;58(4):246-50. Return to content
  • Leal JKF, Adjobo-Hermans MJW, Bosman G. Red Blood Cell Homeostasis: Mechanisms and Effects of Microvesicle Generation in Health and Disease. Frontiers in physiology. 2018;9:703. Return to content
  • Wittert G, Bracken K, Robledo KP, et al. Testosterone treatment to prevent or revert type 2 diabetes in men enrolled in a lifestyle programme (T4DM): a randomised, double-blind, placebo-controlled, 2-year, phase 3b trial. The lancet Diabetes & endocrinology. Jan 2021;9(1):32-45. Return to content
  • Angelova P, Momchilova A, Petkova D, Staneva G, Pankov R, Kamenov Z. Testosterone replacement therapy improves erythrocyte membrane lipid composition in hypogonadal men. The aging male : the official journal of the International Society for the Study of the Aging Male. Sep 2012;15(3):173-9. Return to content