Androgens stimulate red blood cell production, so hematocrit climbs over months of use. Above the normal range the blood thickens and clot, stroke and heart attack risk rise. The Endocrine Society threshold for stopping testosterone therapy is 54%. It falls after stopping; hydration, cardio and physician-managed blood donation lower it while using. Test hydrated and not after training.
- Hematocrit is the percentage of blood volume that is red cells; normal is roughly 41–50% in men, 36–44% in women.
- Erythrocytosis is the most frequent adverse effect of testosterone therapy in the guideline literature.
- The Endocrine Society guideline: hematocrit above 54% means stop testosterone until it falls, then reassess.
- Dehydration, altitude, smoking and sleep apnoea all raise hematocrit independently and add to the androgen effect.
- Therapeutic phlebotomy lowers it quickly; it is a medical procedure decided with a physician, not a routine.
What the number is
Hematocrit: the share of your blood that is red cells. Men roughly 41–50%, women 36–44%. Androgens stimulate erythropoietin and iron handling, so red cell mass rises. More oxygen carried, and that is part of the performance effect. Past a point, the blood becomes viscous: harder to pump, more likely to clot.
Why it matters
Thick blood raises blood pressure, strains the heart, and raises the risk of venous clots, stroke and heart attack. Erythrocytosis is the most common adverse effect in the testosterone-therapy literature, and the most common reason physicians pause treatment.
The threshold
The 2018 Endocrine Society guideline: above 54%, stop testosterone until hematocrit returns to a safe level, then reassess. Anything above your lab’s upper limit is already the warning zone. Symptoms of hyperviscosity at any level (headache, flushing, breathlessness, blurred vision, calf pain or swelling) are urgent; calf pain or swelling is a possible clot and is an emergency.
Test it right
| Do | Because |
|---|---|
| Hydrate well the day before and the morning of | Dehydration concentrates blood and reads several points high |
| Morning, before training | Exercise and sweating shift plasma volume |
| No sauna, alcohol or long flight the day before | All dehydrate |
| Same lab each time | Methods differ slightly |
A single high reading after a hard week in the heat is not a diagnosis. A trend across two hydrated samples is.
What raises it besides androgens
Dehydration, smoking, sleep apnoea, altitude, some diuretics. Each adds to the androgen effect. Sleep apnoea in particular: common in heavier lifters, raises hematocrit and blood pressure both, and treatable.
What lowers it
| Measure | Effect |
|---|---|
| Stop or reduce exposure | Falls over weeks to months |
| Hydration | Corrects the measurement, modest true effect |
| Regular cardio | Modest, plus everything else it does |
| Treat sleep apnoea, stop smoking | Meaningful |
| Blood donation or therapeutic phlebotomy | Fast, several points per unit; arranged by a physician |
On donation: it works and physicians use it. Doing it repeatedly on your own schedule depletes iron, causes fatigue and can mask the problem instead of managing it. It is a tool a physician uses with a plan, not a monthly habit.
The pattern to avoid
Hematocrit accumulates with continuous exposure. The HAARLEM cohort, using for a defined period and stopping, saw it rise and then normalise. Continuous use without breaks is how it stays high, and sustained hyperviscosity is one of the mechanisms behind the cardiovascular events in long-term users. Defined periods with confirmed normalisation between them are the structural fix.
Frequently asked questions
What hematocrit is dangerous?
Above the lab's upper limit is a warning; above 54% is the threshold at which guidelines say stop testosterone. Symptoms of thick blood (headache, red face, breathlessness, visual changes, calf pain) at any level are urgent.
How fast does it rise?
Over months. It typically peaks after several months of continuous use and keeps climbing with continued exposure. It is the reason continuous use without breaks accumulates risk.
How do I test it properly?
Well hydrated, in the morning, no hard training for 48 hours, not after a sauna or a flight. A dehydrated sample reads several points high and causes unnecessary alarm.
What lowers hematocrit?
Stopping or reducing exposure, hydration, regular cardio, treating sleep apnoea, not smoking, and blood donation or therapeutic phlebotomy arranged by a physician. Do not self-manage with repeated donations; iron depletion is its own problem.
Does high hematocrit cause symptoms?
Often none until high. Flushed face, headaches, fatigue, breathlessness, blurred vision and calf pain or swelling are signs. The last is a possible clot and is an emergency.
Sources
- Bhasin S et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. JCEM, 2018.
- Pope HG et al. Adverse health consequences of performance-enhancing drugs: an Endocrine Society scientific statement. 2014.
- Smit DL et al. Health effects of androgen abuse: a review of the HAARLEM study. 2022.