High hemoglobin means your blood is carrying more oxygen-transporting protein than normal, either because your body is producing too many red blood cells or because your blood plasma volume has dropped, concentrating what’s already there. For men, hemoglobin above 17.5 g/dL is generally considered high; for women, the threshold is around 16.0 g/dL. While it sometimes signals nothing more than dehydration or living at high altitude, persistently elevated levels can thicken your blood and significantly raise your risk of heart attack and stroke.
Normal Ranges and What “High” Actually Means
Hemoglobin is the protein inside red blood cells that binds to oxygen in your lungs and delivers it throughout your body. A standard blood test (CBC) measures its concentration in grams per deciliter. Normal ranges typically fall between 13.5 and 17.5 g/dL for men and 12.0 to 16.0 g/dL for women, though labs may use slightly different cutoffs.
A single high reading doesn’t necessarily mean something is wrong. Dehydration is one of the most common explanations. When your body loses fluid, the liquid portion of your blood shrinks while the number of red blood cells stays the same, making hemoglobin appear artificially concentrated. Once you rehydrate, levels typically return to baseline. This is sometimes called “relative” polycythemia because your body isn’t actually overproducing red blood cells.
Why Your Body Might Overproduce Red Blood Cells
When hemoglobin is genuinely elevated and not just a hydration issue, the causes split into two broad categories: your bone marrow is overproducing on its own, or something external is telling it to ramp up production.
Low Oxygen Triggers
Your kidneys monitor oxygen levels in your blood. When they detect chronic low oxygen, they release a hormone called erythropoietin (EPO), which tells your bone marrow to make more red blood cells. This is a normal compensatory response, and it happens in several common situations. Living at high altitude, where the air contains less oxygen, is the classic example. Chronic lung diseases like COPD or emphysema have the same effect, as do certain heart defects that mix oxygen-rich and oxygen-poor blood.
Smoking is another major driver. Carbon monoxide from cigarettes binds to hemoglobin far more aggressively than oxygen does, effectively reducing how much oxygen each red blood cell can carry. Your body responds by making more cells to compensate. This form of elevated hemoglobin is usually reversible once someone quits.
Bone Marrow Disorders
In rarer cases, the bone marrow starts overproducing red blood cells without any oxygen-related trigger. The most well-known condition in this category is polycythemia vera (PV), a slow-growing blood cancer. Nearly all people with PV carry a specific genetic mutation called JAK2, which causes blood-forming cells to multiply uncontrollably. The bone marrow becomes packed with excess red blood cells, white blood cells, and platelets all at once. PV is uncommon, affecting roughly 1 to 2 people per 100,000 each year, but it requires ongoing treatment.
Medications and Hormones
Testosterone replacement therapy is one of the most frequent medication-related causes of high hemoglobin. Testosterone stimulates red blood cell production, and men on supplemental testosterone need regular blood tests to catch rising levels early. Anabolic steroids used for bodybuilding have a similar effect. EPO injections, sometimes prescribed for anemia related to kidney disease, can also push hemoglobin too high if dosing isn’t carefully monitored.
What High Hemoglobin Feels Like
Mildly elevated hemoglobin often produces no symptoms at all, which is why it’s usually caught on routine blood work. As levels climb higher, though, the blood becomes thicker and moves through vessels less efficiently. This poor circulation most noticeably affects the brain: headaches, dizziness, and difficulty concentrating are among the earliest complaints. Some people describe a foggy or “off” feeling that’s hard to pin down.
Other symptoms that can develop include blurry vision, a flushed or reddish skin tone (especially in the face and hands), shortness of breath, and itching after a warm shower, which is particularly associated with polycythemia vera. More concerning signs include chest pain, unexplained nosebleeds or bleeding gums, and ringing in the ears. None of these symptoms are unique to high hemoglobin, which is part of why the condition is easy to overlook without blood work.
The Real Risk: Blood Clots
The most serious consequence of sustained high hemoglobin is an increased risk of blood clots, particularly in arteries. A large study of over 1.5 million blood donors in Sweden and Denmark quantified this risk clearly. Men with hemoglobin at or above 17.5 g/dL had roughly 3.5 times the risk of heart attack and 2.4 times the risk of ischemic stroke compared to men with normal levels. Women with hemoglobin at or above 16.0 g/dL faced similarly elevated risks: about 3.2 times higher for heart attack and 2.4 times higher for stroke.
The mechanism is straightforward. More red blood cells make the blood more viscous, which slows flow and increases the chance that clots form. In polycythemia vera, the risk is compounded because the condition also activates white blood cells and platelets, creating an even more clot-prone environment. Deep vein thrombosis, pulmonary embolism, and clots in unusual locations like the veins draining the liver or spleen are all recognized complications.
How Doctors Figure Out the Cause
Once a high hemoglobin level is confirmed on repeat testing (to rule out a lab error or temporary dehydration), the diagnostic process focuses on separating bone marrow problems from everything else. Two tests do most of the heavy lifting: a blood test for the JAK2 gene mutation and a measurement of EPO levels.
If the JAK2 mutation is positive and EPO is low, polycythemia vera is the diagnosis. Your bone marrow is overproducing on its own, and it’s suppressing the normal EPO signal because there’s already too much hemoglobin in the blood. If JAK2 is negative and EPO is normal or elevated, the cause is secondary: something outside the bone marrow, like lung disease, smoking, altitude, or a medication, is driving the overproduction. This distinction matters because the treatments are completely different.
Your doctor may also check oxygen saturation levels, order lung function tests, or image the kidneys and liver if secondary causes aren’t obvious from your history.
How High Hemoglobin Is Managed
For secondary causes, treatment targets whatever is driving the overproduction. Quitting smoking, adjusting testosterone doses, treating underlying lung disease, or using supplemental oxygen at night for sleep-related breathing disorders can all bring hemoglobin back into range over weeks to months.
For polycythemia vera and other primary bone marrow conditions, the cornerstone treatment is therapeutic phlebotomy, which is essentially a controlled blood draw similar to donating blood. The goal is to bring the hematocrit (the percentage of blood volume occupied by red blood cells) below 45%. Initially this may mean having blood drawn every few days, with each session removing 250 to 500 mL. For older adults or those with heart disease, smaller volumes of 200 to 300 mL are removed twice weekly. Once levels normalize, most people settle into a maintenance schedule with blood counts checked every 4 to 8 weeks to determine when the next draw is needed.
Some people with polycythemia vera also take a daily low-dose aspirin to reduce clotting risk. Those at higher risk for complications, such as people over 60 or those who have already had a blood clot, may be prescribed additional medications that slow red blood cell production in the bone marrow.
When It’s Temporary and When It’s Not
A one-time high reading after a hard workout, a hot day without enough water, or a recent trip to a mountain town is rarely a concern. The hemoglobin bump from dehydration or brief altitude exposure reverses on its own. Even the elevation caused by smoking gradually resolves after quitting, though it can take several weeks for red blood cell counts to normalize because each red blood cell lives about 120 days.
Persistently elevated hemoglobin, confirmed on more than one blood draw weeks apart, deserves investigation. The higher the level and the longer it stays elevated, the greater the cumulative risk to your cardiovascular system. Early identification of the cause, whether it’s an adjustable medication, an undiagnosed lung condition, or a bone marrow disorder, makes the difference between a manageable finding and a serious complication.