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High hemoglobin levels are a symptom of many diseases. Here are the seventeen most common reasons you hemoglobin levels may be elevated.

A high hemoglobin count confirms that your blood carries an above-average concentration of the oxygen-carrying compound hemoglobin. That's not quite the same thing as a high red blood cell count, because red blood cells may carry varying amounts of hemoglobin. It's also not the same thing as high iron levels.

It's possible to have high iron levels because of hereditary hemochromatosis or beta-thalassemia but low hemoglobin levels because of red blood cell anemia at the same time. But it's also possible to have high hemoglobin levels even with anemia [1]. 

Hemoglobin A1C (HbA1C) isn't a measurement of total hemoglobin either. It is a measurement of the percentage of red blood cells that are bound to sugar groups. You can have a high HbA1C count but still have low hemoglobin levels because you have both diabetes and anemia. And diabetics can have artificially high HbA1C numbers because they have low hemoglobin or a low red blood cell count.

    Hemoglobin also isn't the same thing as hematocrit. Hematocrit is the ratio of the volume of red blood cells to the total volume of blood. Since red blood cells don't all contain the same amount of hemoglobin, hematocrit doesn't tell you how much hemoglobin you have.

    What does Lab Measurement indicate High Hemoglobin?

    Different doctors apply slightly different definitions for "high" hemoglobin (abbreviated Hb or Hgb).

    In the United States, most doctors would define high hemoglobin counts as anything over 17.5 grams (g) of hemoglobin per deciliter (dl) of blood for men, or 15.5 g/dl for women who are still menstruating. Low, normal, and high hemoglobin levels for children vary by age and gender.

    Most of the rest of the world instead measures hemoglobin in millimoles (mmol) per liter (L). Hemoglobin levels in mmol/L are considerably lower than hemoglobin levels in g/dl. Outside the United States, "normal" hemoglobin values range from 9.6 mmol/L for women to  10.9 mmol/L for men. Babies tend to have higher hemoglobin levels than older children and adults.

    Most data that led to current understandings of "average" hemoglobin levels in fact comes from studies of blood donors, who will be turned down for donation if their hemoglobin levels are too low. [2]

    High Hemoglobin Is a Symptom, Not a Disease

    There aren't any situations in which a doctor orders a hemoglobin test on the basis of a hunch like "I wonder if this patient has high hemoglobin." Hemoglobin measurements come up in the context of a complete blood count (CBC), also known as a whole blood count (WBC) panel. High values of hemoglobin — along with other symptoms — confirm a diagnosis, but there are times that a high hemoglobin level isn't caused by a disease.

    Here are some other factors that can cause elevated hemoglobin counts in people: 

    • Dehydration. In sports with a "weigh-in," such as wrestling and boxing, it's not unusual for athletes to go without food or water so they can compete in a lower weight class against lighter opponents. Sometimes, intentional dehydration can lower blood plasma volume by about 10 percent, and raise hemoglobin levels by about 8 percent. [3] Non-intentional dehydration can have the same effect, of course.
    • Altitude. Mountain air contains less oxygen than low-land air. People who ordinarily live near sea level produce more hemoglobin so that their bodies can cope better with the lower concentrations of oxygen in the air. In athletes, 100 hours at altitudes over 3000 meters (10,000 feet) above sea level increases Hb concentrations by about 1 percent. Training several weeks at a high altitude can increase Hb concentrations by about 3 percent, before the effect is maxed out. When these athletes return to lower elevations, they still have more hemoglobin in their bloodstreams than they would have before, so they can compete more effectively. [4]
    • Time of year. Hemoglobin levels go up about 0.5 g/dl in winter, interestingly, peaking in December [5] before coming down again.
    • Time of day. There can also be variations in hemoglobin saturation with oxygen at night in people who have sickle cell disease [6].
    • "Pink blood." Very high triglyceride levels in a person can change the volume of the blood so that hemoglobin levels appear to be elevated [7]
    • Taking erythropoietin (EPO) to stimulate red blood cell production (either as a treatment for anemia or for athletic performance) will raise hemoglobin levels in athletes (although not usually in people who take EPO because they have kidney disease).  
    • Taking testosterone injections increases EPO production and hemoglobin levels. And in some cases,
    • Laboratory error produces spurious high levels of hemoglobin. If the red blood cell count is off, then Hgb lab measurements will be in error, too. In this case, the blood test will simply have to be repeated.

    Health Problems That Raise Hemoglobin Levels

    There are also health concerns that result in elevated hemoglobin levels. The most common, as well as one of the most dangerous, among these is smoking. Cigarette smoke contains carbon monoxide, which binds to hemoglobin. The body produces more hemoglobin to compensate for the hemoglobin that remains in circulation but that can no longer carry oxygen. [8]  It takes about three months for hemoglobin levels to return to normal after you quit smoking [9].

    Elevated hemoglobin levels also occur in:

    • Aplastic anemia, a failure of the bone marrow to produce red blood cells, and sickle cell disease, a hereditary condition causing misshapen red blood cells, may both increase levels of a kind of hemoglobin known as fetal hemoglobin even into adulthood [10]. Both conditions produce numerous other severe symptoms that make them unlikely to go undiagnosed for very long.
    • COPD and other chronic lung diseases may cause the body to produce more hemoglobin as an adaptation to low oxygen levels [11]. This is a positive compensatory development, as higher hemoglobin permits more phsyical activity.
    • Kidney cancer can stimulate the kidneys to produce more erythropoietin so the bone marrow produces more red blood cells with more hemoglobin, so the cancer can spread [12].
    • Liver cancer interferes with the liver's ability to transform hemoglobin into bilirubin, so hemoglobin stays in the bloodstream.
    • Polycythemia is a condition that causes the overproduction of red blood cells. Hemoglobin levels rise in this case, but because the volume of red blood cells increases.
    • Pulmonary hypertension can increase free hemoglobin in the lungs. This is hemoglobin that is not bound to red blood cells. [13]
    • Type 1 diabetes is often associated with elevated hemoglobin levels, which in turn is associated with increased stress on the kidneys [14].
    • Any or all of the components of metabolic syndrome (elevated cholesterol, elevated blood pressure, prediabetes or type 2 diabetes, obesity, or just increased waistline) is associated with high hemoglobin levels [15]. 

    Remember, elevated hemoglobin is always a symptom, never a disease in itself. However, changes in your hemoglobin lab results are often an important indicator of the progress you are making toward getting back to health, and hemoglobin results can also play a role in the diagnostic process.