What hematocrit means, and why it matters on testosterone

The number in plain terms

A hematocrit of 45 means that 45% of your blood volume is red blood cells and the rest is mostly plasma. MedlinePlus notes that normal ranges vary with sex, age, smoking and altitude, so your result is read against the lab's range and your own history.

Red cells carry oxygen, so some increase is not harmful in itself. The concern is that a very high proportion makes blood thicker, which may raise the risk of clots. How much risk a modest rise adds is still debated, which is why the trend over time matters as much as any single result.

Why testosterone pushes it up

Testosterone stimulates the bone marrow to make more red blood cells. It appears to do this partly by raising erythropoietin, the kidney hormone that drives red cell production, and partly by making more iron available to the marrow. The effect is expected and tends to be dose-related.

Forms that create high peaks, such as larger injections spaced two or more weeks apart, are generally associated with bigger rises than steadier forms. The Endocrine Society notes, for example, that transdermal gels cause less erythrocytosis than injectable testosterone.

What level prompts action

Both the American Urological Association and the Endocrine Society call for checking hemoglobin and hematocrit before starting therapy and monitoring during treatment, and the Endocrine Society lists an elevated hematocrit as a reason not to start. Many clinicians use a hematocrit of about 54% as the point at which therapy is paused or changed, a threshold that also appears as the definition of erythrocytosis in the trial data the Endocrine Society reviewed.

That figure is not a hard line drawn from randomized trials. A result that climbs quickly toward it, or sits just above it on a single test, is handled with judgment. Oral testosterone labeling, for instance, advises stopping when hematocrit becomes elevated and checking roughly every three months.

Other things that raise hematocrit

Because dehydration alone can push a borderline result over the line, a high reading is often repeated after you have been drinking normally. If hematocrit keeps rising, screening for sleep apnea is sensible, especially if you snore or wake unrefreshed.

• Dehydration, which MedlinePlus calls the most common cause, because it shrinks plasma volume
• Obstructive sleep apnea, where repeated drops in oxygen overnight stimulate red cell production
• Smoking, which reduces oxygen delivery
• Chronic lung disease or some heart conditions
• Living at high altitude
• Polycythemia vera, a rare bone marrow disorder

What happens if it climbs

The usual first step is to repeat the test and check hydration. Then your clinician looks at the dose and schedule. Lowering the dose, splitting injections into smaller, more frequent doses or switching to a different form can bring hematocrit down while keeping you on treatment.

If the level stays high, therapy may be paused until it falls. In some cases, a therapeutic blood draw (phlebotomy) is used, although repeated draws can deplete iron over time. If hematocrit rises again after restarting, stopping testosterone altogether may be the safest choice.

Things you can do between tests

Stay well hydrated, particularly on the day before and the morning of a blood test, so the result reflects your true level. If you smoke, stopping helps on many fronts, including red cell counts. If you snore or wake unrefreshed, ask about a sleep study rather than assuming tiredness is from testosterone.

Do not try to manage a high hematocrit by donating blood on your own or by changing your dose without guidance. Blood donation centers may not accept donations made for this purpose, and unplanned dose changes make it harder for your clinician to interpret your labs.

How often it is checked

Monitoring is front-loaded, because most of the rise happens in the first months. Expect a baseline test, a check within the first few months and periodic tests after that. Your clinician may check more often if your number is trending upward or after a change in dose or route.

Between tests, report symptoms that could suggest thick blood, such as headaches, dizziness, unusual redness of the face, or signs of a clot like leg swelling or chest pain. Chest pain or sudden shortness of breath needs emergency care.

Do you need it checked if you are not on testosterone?

A hematocrit is part of a routine complete blood count, so many people have it measured during standard checkups. Outside of testosterone therapy, a high result is investigated for causes such as dehydration, lung disease or a bone marrow problem, and a low result can signal anemia.

HealifyMD's testosterone replacement therapy is $249 for the first month, then $149/month, with no membership fee. The first month includes the lab panel, and your price is confirmed at the eligibility review.