A hemoglobin level of 17.9 g/dL is high. Normal hemoglobin ranges from 14.0 to 17.5 g/dL for men and 12.3 to 15.3 g/dL for women, so 17.9 exceeds the upper limit for both sexes. For women, it’s significantly elevated. For men, it’s just above the normal ceiling, but it still crosses the clinical threshold that typically triggers further investigation.
Whether this number is cause for concern depends on what’s driving it. Some causes are temporary and easily corrected, while others require ongoing monitoring or treatment.
How Far Above Normal Is 17.9?
Hemoglobin is generally considered high above 16.5 g/dL in men and above 16.0 g/dL in women. At 17.9, you’re above both of those cutoffs. The World Health Organization’s diagnostic criteria for polycythemia vera, a blood disorder that causes excess red blood cell production, uses a hemoglobin threshold of 16.5 g/dL for men and 16.0 g/dL for women as one of its major criteria. A level of 18.5 g/dL or higher in men is considered so elevated that it can support a diagnosis even without a bone marrow biopsy. At 17.9, you’re approaching that territory but haven’t crossed it.
Your hematocrit, the percentage of your blood made up of red blood cells, typically tracks with hemoglobin. A rough rule of thumb is that hematocrit runs about three times the hemoglobin value, so a hemoglobin of 17.9 often corresponds to a hematocrit around 54%. For context, hematocrit levels above 52% in men and 47% in women are the point where doctors consider more detailed testing.
Common Causes of Elevated Hemoglobin
The simplest explanation is dehydration. When your body is low on fluids, your blood contains less water, which concentrates the red blood cells and makes hemoglobin appear artificially high. Once you rehydrate, the number typically drops back to normal. If you were mildly dehydrated when your blood was drawn, especially after fasting overnight, this alone could push a borderline reading over the line.
Smoking is another frequent cause. Cigarette smoke introduces carbon monoxide into your bloodstream, which binds to hemoglobin and reduces how much oxygen reaches your tissues. Your body compensates by producing more red blood cells. Heavy smokers can develop carboxyhemoglobin levels high enough to drive real increases in hemoglobin concentration.
Living at high altitude has a similar effect. With less oxygen available in the air, your body ramps up red blood cell production to compensate. People living above 5,000 to 8,000 feet often have hemoglobin levels that would be flagged as high at sea level.
Sleep apnea deserves special mention because it’s common and frequently undiagnosed. When you repeatedly stop breathing during sleep, your oxygen levels drop overnight, and your body responds by making more red blood cells. If you snore heavily, wake up tired, or have been told you stop breathing in your sleep, this could be the link.
Testosterone use, whether prescribed for low testosterone or used for other reasons, directly stimulates red blood cell production and is a well-known cause of elevated hemoglobin. Certain kidney conditions, including narrowing of the blood vessels that supply the kidneys, can also trigger excess production of the hormone that tells your bone marrow to make more red blood cells.
When the Cause Is a Blood Disorder
Polycythemia vera is a condition where the bone marrow produces too many red blood cells on its own, independent of any outside trigger. It’s caused by a specific genetic mutation (called JAK2) that develops in blood-forming cells. It’s not inherited from your parents; it’s acquired during your lifetime. Polycythemia vera is relatively uncommon, but a hemoglobin of 17.9 falls squarely within the range where doctors test for it.
To distinguish polycythemia vera from other causes, doctors typically check two things: whether you carry the JAK2 mutation and your erythropoietin (EPO) level, the hormone your kidneys release to signal for more red blood cells. In polycythemia vera, EPO is usually low because the bone marrow is overproducing cells without being told to. In secondary causes like sleep apnea or high altitude, EPO is normal or high because the body is responding to a real oxygen shortage.
Symptoms You Might Notice
Many people with a hemoglobin of 17.9 feel perfectly fine, which is why the finding often comes as a surprise on routine bloodwork. But thicker blood doesn’t flow as easily, and when hemoglobin climbs high enough, it can affect circulation throughout the body.
The most common symptoms are headaches, dizziness, and a flushed or reddish skin tone, particularly in the face. Some people notice blurry vision, shortness of breath, or a general feeling of mental fog. Less commonly, elevated hemoglobin can cause chest pain, ringing in the ears, nosebleeds, or bleeding gums. Itching after a warm bath or shower is a classic symptom associated specifically with polycythemia vera, though it doesn’t happen to everyone.
The absence of symptoms doesn’t mean the level is harmless. Thick blood increases the risk of blood clots, which can lead to serious complications including stroke, heart attack, and clots in the lungs. This is the primary reason elevated hemoglobin gets taken seriously even when you feel well.
What Happens Next
If your hemoglobin comes back at 17.9, your doctor will likely want to repeat the test, ideally when you’re well hydrated, to confirm it wasn’t a one-time artifact. If the elevation persists, the workup usually involves checking oxygen levels in your blood (to rule out a lung or breathing-related cause), EPO levels, and the JAK2 mutation.
If the cause turns out to be something identifiable like smoking, sleep apnea, or testosterone therapy, addressing that root cause is usually enough to bring hemoglobin back down. Quitting smoking, treating sleep apnea with a CPAP machine, or adjusting testosterone dosing can all normalize levels over time.
For polycythemia vera or cases where the hemoglobin stays stubbornly high, the most common treatment is therapeutic phlebotomy, which is essentially a controlled blood draw similar to donating blood. Removing blood reduces the concentration of red blood cells and lowers the risk of clotting. The goal is typically to keep hematocrit below 45%. How often you need this varies; some people go every few weeks initially, then less frequently as levels stabilize.
Lifestyle adjustments also help. Staying well hydrated keeps blood from becoming more concentrated. Maintaining a healthy weight and managing blood pressure both reduce cardiovascular strain. If you live at very high altitude and have persistent elevation without another explanation, your doctor may discuss whether relocation is realistic or whether periodic phlebotomy makes more sense.
Temporary vs. Persistent Elevation
A single reading of 17.9 is not a diagnosis of anything. It’s a data point that needs context. If you were dehydrated, had just exercised intensely, or recently traveled to a high-altitude location, a repeat test under normal conditions may come back completely normal. Even living in a moderately elevated city can nudge hemoglobin higher than sea-level ranges without any underlying problem.
What matters more than one number is the trend. If your hemoglobin has been gradually climbing over several blood tests, or if it stays above the normal range on repeated checks, that pattern carries more diagnostic weight than any single result. Pulling up your previous lab results and looking at how your hemoglobin has changed over time gives your doctor a much clearer picture than the 17.9 alone.