High Hemoglobin: Causes and When It Is Serious

Hemoglobin is the protein in red blood cells that carries oxygen from the lungs to the tissues. Its concentration is one of the basic parameters of a blood count and is usually associated with anemia, that is, a value that is too low. A result above the reference range also needs to be understood, although it causes less worry. Reference ranges differ between laboratories: in men the upper limit is usually around 16–17.5 g/dL, and in women about 15–16 g/dL.

To avoid duplicating content, this text focuses on hemoglobin as a value and on how to read it in women and men, against the background of the whole blood count. We write in more detail about hematocrit in men who are not on testosterone, including the role of SGLT2 drugs and the details of apnea testing, in a separate article on high hematocrit without TRT. Here we mention those threads only as far as needed to understand the whole.

It is worth noting right away that hemoglobin, hematocrit and red cell count (RBC) usually change together, because they measure different aspects of the same phenomenon. If only one of these values is raised while the others are normal, it is often a measurement error or a physiological variant. If all three rise together, we speak of erythrocytosis and it is worth starting to look for the cause.

As with hematocrit, the key question is whether there really are more red blood cells or the blood is just more concentrated. In relative, also called apparent, erythrocytosis the plasma volume shrinks and hemoglobin concentration rises, although the total red cell mass is normal. In absolute erythrocytosis the red cell mass is really increased because the marrow produces more of them.

Relative erythrocytosis usually has transient, easily corrected causes: dehydration (too little fluid, heat, fever, diarrhea), diuretics, heavy sweating, alcohol. It is also associated with stress, hypertension and obesity — it has been linked to so-called Gaisböck syndrome, in which reduced plasma volume and raised hematocrit coexist with hypertension, although the term is rarely used in current practice. The first step is always to repeat the blood count with good hydration.

Absolute erythrocytosis is divided into secondary (the body signals the marrow to make more cells, most often by raising erythropoietin) and primary, that is polycythemia vera, in which the marrow works independently of that signal because of a mutation. Secondary causes are many times more common: hypoxia (smoking, sleep apnea, lung and heart disease, altitude), erythropoietin production by kidney lesions or other tumors, and drugs and substances including androgens and erythropoietin.

Duration is a helpful clue. Hemoglobin that has been raised for years in a stable range, without symptoms, more often reflects a physiological variant, a chronic cause (for example smoking, living at altitude) or a rare congenital form. Hemoglobin that has risen over a few months calls for a quicker search for the cause, because it more often means an acquired process.

Men have on average higher hemoglobin than women, mainly because of the effect of androgens on erythropoiesis and the absence of monthly blood loss. The diagnostic thresholds therefore differ between the sexes. In clinical practice the laboratory reference range is treated as a first filter, and in further assessment thresholds specific to the suspected diagnoses are used.

Two things are worth taking from this. First, the thresholds of 16.5 g/dL (men) and 16 g/dL (women) are not 'normal limits' but values used in the diagnostic criteria of one specific disease, combined with other elements (above all the JAK2 mutation). A result above these values does not mean polycythemia, and a result below does not exclude it entirely, especially when iron deficiency coexists, which can 'mask' the real severity of the disease. Second, the outlook in diagnosed and treated polycythemia vera is long and the main goal of therapy is preventing clots, which shows why there is no reason for panic, but also why it must not be dismissed.

Note that many of these causes concern breathing and blood oxygenation: the body answers 'too little oxygen' by increasing the number of oxygen carriers. The same logic underlies altitude training in athletes, and in pathological conditions it is the source of secondary erythrocytosis. That is why questions about snoring, cough, breathlessness, smoking and place of residence are as important in the workup as the lab results themselves.

Smoking is one of the most common and best documented causes of secondary erythrocytosis. Carbon monoxide from tobacco smoke binds to hemoglobin with an affinity many times higher than oxygen. The resulting carboxyhemoglobin does not carry oxygen, and in addition it shifts the oxygen dissociation curve so that the remaining oxygen is released to tissues less readily. The body reads this as hypoxia and increases erythropoietin production.

The practical conclusion is simple: in a smoker with raised hemoglobin, the first step is to reduce smoking, and only then (if the result persists) further workup. In many cases quitting smoking lowers hemoglobin without any other intervention. It is also worth knowing that nicotine in e-cigarettes or pouches does not produce carboxyhemoglobin to the extent that combustion smoke does, but that does not make it neutral for the circulatory system; that thread goes beyond the scope of this article.

When hemoglobin stays raised after hydration and the obvious causes (smoking, drugs, apnea) do not explain the result, the doctor usually reaches for two tests: serum erythropoietin and the JAK2 mutation. The idea is simple. In polycythemia vera the marrow acts autonomously, so erythropoietin is usually low. In secondary erythrocytosis the stimulus (hypoxia or excess erythropoietin) makes its level normal or raised.

Two practical conclusions follow from this study. First, erythropoietin is a useful but imperfect test: no single threshold is at once fully sensitive and fully specific, so the result is always read together with the JAK2 mutation, the blood count (white cells, platelets) and the clinical picture. Second, low erythropoietin alone does not settle the diagnosis, and a normal level does not exclude polycythemia in rare cases. The final diagnosis, with a bone marrow examination if needed, therefore belongs to the hematologist.

The order matters: we start with the cheapest and simplest things and only then move on to genetic and hematological tests. As a result a good share of people get an answer at the first or second stage and do not have to go through the whole diagnostic path.

This idea is sometimes carried over from the sports context, in which raising the red cell count (for example through altitude training) can indeed improve exercise capacity, as long as it stays within physiology. Outside the physiological range the benefit quickly disappears and a cost appears: blood viscosity. It is no accident that the treatment of polycythemia vera is built on keeping hematocrit below 45%.

Similarly, 'natural ways' to raise hemoglobin, such as iron supplements taken without deficiency, make no sense for a person with a normal or raised result. Iron is replaced only when a deficiency has been confirmed — we discuss that in the entries on ferritin and low MCV.

Remember that the listed symptoms are non-specific and most often have other causes. Their coexistence with high hemoglobin justifies a more urgent assessment but does not settle anything on its own.

High hemoglobin is rarely an urgent problem, but it is always a clue. It shows that either the blood is concentrated, or the body 'thinks' it lacks oxygen, or the marrow is working on its own. The first two situations can usually be clarified with simple questions about water, cigarettes, sleep and drugs; the third is rare, which is exactly why it is good to know the diagnostic path: repeat blood count, erythropoietin, JAK2, hematologist.