High haematocrit: when thick blood is a problem for athletes

A raised haematocrit means your blood contains relatively many red blood cells, and the cause determines whether it is a problem. In athletes it often comes from dehydration or training, but in users of testosterone therapy it can climb structurally. That last case is the one you want to watch together with your doctor.

I often see haematocrit read wrongly. A one-off high value after a hard, dry training day is something very different from a persistently high value on TRT.

What is haematocrit, really?

Haematocrit is the percentage of your blood made up of red blood cells. A higher value means thicker, more viscous blood, which can make the heart work harder to pump it around. Haematocrit and haemoglobin usually move up together.

A snapshot says little: hydration alone can make your value swing noticeably.

Why does your haematocrit rise?

The causes range from harmless and temporary to something that needs attention. This table helps you separate them.

Testosterone stimulates the production of red blood cells, among other things by raising erythropoietin and suppressing the hormone hepcidin (Bachman, 2010).

When is a high haematocrit a problem?

A persistently high haematocrit can raise the viscosity of your blood, which is a reason to take it seriously. On testosterone therapy, a raised haematocrit is a known side effect whose clinical meaning depends on the level and your wider risk profile (Ohlander, 2018). Risk factors include injectable administration, a higher dose and older age (Agrawal, 2023).

A one-off slightly raised value without context is rarely cause for panic, a persistently high one is worth discussing.

What do you do with a high haematocrit?

Start with the simple things and leave the rest to your doctor. Good hydration prevents apparently high values, so drink enough around your measurements. If you use TRT and your haematocrit is structurally high, discuss with your doctor what is sensible, because adjusting your dose or having blood drawn yourself does not belong without supervision.

To track your haematocrit, compose a panel through our custom blood test. If you use TRT, look at our TRT monitoring blood test.

Also read the overview blood values for strength athletes and estradiol on TRT.

My advice: always read your haematocrit in the context of your hydration and your situation. If the value stays high, certainly on TRT, raise it with your GP instead of solving it yourself.

References

1. Ohlander SJ, Varghese B, Pastuszak AW. Erythrocytosis following testosterone therapy. Sexual Medicine Reviews. 2018. PMID: 28526632.
2. Bachman E, Feng R, Travison T, et al. Testosterone suppresses hepcidin in men: a potential mechanism for testosterone-induced erythrocytosis. The Journal of Clinical Endocrinology and Metabolism. 2010. PMID: 20660052.
3. Agrawal P, Singh SM, Hsueh J, et al. Management of erythrocytosis in men receiving testosterone therapy: clinical consultation guide. European Urology Focus. 2023. PMID: 36335038.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to walk into the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.