Your red blood cell count rises whenever your body senses it needs more oxygen-carrying capacity, or when certain hormones, medications, and medical conditions push your bone marrow to produce more cells. Normal ranges fall between 4.7 and 6.1 million cells per microliter for men and 4.2 to 5.4 million cells per microliter for women. Anything consistently above those ranges signals that something is driving extra production, or that your blood has become more concentrated.
Low Oxygen Is the Primary Trigger
The most fundamental driver of red blood cell production is oxygen availability. When your body detects that tissues aren’t getting enough oxygen, it activates a signaling chain that ramps up production of a hormone called erythropoietin (EPO) in the kidneys. EPO travels to the bone marrow, where it stimulates precursor cells to multiply and mature into red blood cells. This is the same system that kicks in when you move to a high-altitude city, develop a lung condition, or have a heart defect that reduces oxygen delivery.
At high altitude, this response follows a predictable timeline. EPO surges within 24 to 48 hours after arrival, then gradually declines and stabilizes near baseline levels within weeks as the body adapts. The result is a lasting increase in red blood cell mass that improves oxygen transport in thinner air. It’s the reason athletes sometimes train at elevation before competing at sea level.
Testosterone and Hormonal Changes
Testosterone is one of the strongest hormonal drivers of red blood cell production. It works by boosting EPO output and suppressing hepcidin, a protein that regulates iron availability. With more EPO signaling and more iron accessible for building hemoglobin, the bone marrow produces red blood cells at a higher rate.
This is why men naturally carry higher red blood cell counts than women, and why testosterone replacement therapy (TRT) frequently pushes counts above normal. In studies of men receiving TRT, rates of elevated red blood cell counts reached as high as 66.7%, with injectable formulations, higher doses, and older age all increasing the risk. Transgender men on testosterone see similar effects. If you’re on TRT, routine blood work typically includes monitoring your red blood cell count and hematocrit for this reason.
Dehydration Can Raise Your Numbers Without Raising Your Cells
Not every high reading on a blood test means your body is making more red blood cells. Dehydration reduces the liquid portion of your blood, which concentrates the cells you already have and makes your count appear elevated. This is called relative erythrocytosis. Your total number of red blood cells hasn’t changed; there’s just less plasma diluting them.
This matters because a single high result after a morning of not drinking water, intense exercise, or an illness involving vomiting or diarrhea may not reflect a true increase in cell production. Rehydrating often brings the numbers back to normal. If your count stays elevated after proper hydration, that points toward your bone marrow actually producing more cells.
Smoking and Chronic Lung Disease
Smoking is one of the most common causes of a genuinely elevated red blood cell count. Carbon monoxide from cigarette smoke binds to hemoglobin far more tightly than oxygen does, effectively reducing the blood’s oxygen-carrying capacity. Your body responds by producing more red blood cells to compensate.
Chronic lung diseases like COPD and pulmonary fibrosis work through a similar mechanism. When your lungs can’t efficiently transfer oxygen into your blood, arterial oxygen levels drop below about 92% saturation, and your kidneys respond with sustained EPO release. Over months and years, this leads to a persistently elevated red blood cell count.
Sleep Apnea and Nighttime Oxygen Drops
Severe obstructive sleep apnea creates repeated episodes of oxygen deprivation throughout the night, and your body notices. In patients with severe sleep apnea, EPO levels rise by about 20% after just 3.5 hours of untreated sleep, with oxygen levels dipping to around 77%. That’s a meaningful signal to the bone marrow.
The encouraging finding is that this effect reverses with treatment. After 4 to 5 hours of CPAP therapy, EPO levels returned to baseline in the same patients. People with only mild sleep apnea showed EPO responses no different from people without the condition, so the effect appears to scale with severity. Chronically elevated EPO from untreated severe sleep apnea may be one reason the condition is linked to cardiovascular problems.
Tumors and Kidney Conditions
Certain tumors can independently secrete EPO, bypassing the normal oxygen-sensing feedback loop. Renal tumors and kidney cysts are the most recognized culprits, but liver tumors, cerebellar tumors, and uterine fibroids can also produce EPO. In these cases, the elevated red blood cell count is sometimes the first clue that leads to discovering the tumor.
Kidney transplant recipients also sometimes develop elevated counts after surgery, likely because the new kidney produces EPO at a different set point than the body expects. This typically requires monitoring in the months following the procedure.
Medications and Supplements
Beyond testosterone, several other substances raise red blood cell counts. Anabolic steroids, which are chemically related to testosterone, stimulate the same EPO and iron pathways. A class of diabetes medications known as SGLT-2 inhibitors has also been associated with increased red blood cell production, something your prescriber should be monitoring if you take one.
Synthetic EPO itself, the same hormone your kidneys produce naturally, is available as a prescription drug for people with severe anemia (often from kidney failure or chemotherapy). It works by binding to receptors on red blood cell precursors in the bone marrow, triggering them to multiply and mature while also protecting developing cells from dying off prematurely. This is also the substance infamously misused in endurance sports to artificially boost oxygen-carrying capacity.
Polycythemia Vera
Sometimes the bone marrow produces excess red blood cells on its own, without any external trigger. Polycythemia vera is a slow-growing blood cancer in which a genetic mutation causes the marrow to overproduce red blood cells regardless of oxygen levels or EPO signals. Unlike the secondary causes above, the problem here originates in the marrow itself.
What High Red Blood Cell Counts Feel Like
When red blood cell counts climb high enough, blood becomes thicker and flows less easily. This is called hyperviscosity, and it produces a recognizable set of symptoms. The most common are headaches, dizziness, confusion, and a noticeably reddish skin tone, particularly in the face and hands. Poor circulation to the brain is responsible for most of these.
Less commonly, you might experience blurry vision, shortness of breath, chest pain, ringing in the ears, difficulty walking, or unusual bleeding like chronic nosebleeds or bleeding gums. These symptoms tend to develop gradually, so many people don’t connect them to a blood issue until routine lab work reveals the elevated count. If you’re experiencing several of these together, especially persistent headaches with facial redness, it’s worth getting a complete blood count.