Hemoglobin (Hb) is a protein within red blood cells responsible for transporting oxygen from the lungs to the body’s tissues and carrying carbon dioxide back to the lungs. A high hemoglobin count, often called polycythemia or erythrocytosis, means there is an abnormally high concentration of this protein in the blood. This elevation indicates an increased number of red blood cells circulating in the bloodstream. Excessive hemoglobin can thicken the blood, affecting its flow and indicating an underlying physiological issue.
The Mechanism Behind High Hemoglobin
The body maintains a highly regulated balance of red blood cell production, a process known as erythropoiesis. This process is largely controlled by the kidneys, which function as the primary oxygen sensors in the body. When the oxygen level in the blood is sensed as low, the kidneys secrete a hormone called erythropoietin (EPO).
Erythropoietin travels to the bone marrow, the site of red blood cell production. The hormone stimulates the bone marrow to accelerate the production and maturation of red blood cells. This increase enhances the blood’s capacity to carry oxygen, correcting the initial deficiency. High hemoglobin levels represent this compensatory response or a malfunction in the production pathway itself.
Secondary Causes Related to Oxygen Deprivation
The most frequent causes of an elevated hemoglobin count are secondary, meaning they result from the body’s attempt to compensate for chronic low oxygen levels, or hypoxia. Living at high altitudes, for instance, naturally exposes the body to air with a lower concentration of oxygen. This reduced oxygen availability triggers the kidneys to release more EPO over several weeks, leading to a sustained increase in red blood cell production to maximize oxygen capture.
Chronic lung conditions prevent the efficient transfer of oxygen into the bloodstream, mimicking the effects of high altitude. Diseases such as Chronic Obstructive Pulmonary Disease (COPD), emphysema, or pulmonary fibrosis diminish the lungs’ ability to oxygenate the blood. The resulting persistent lack of oxygen signals the kidneys to produce more red blood cells, elevating the hemoglobin level.
Severe obstructive sleep apnea (OSA) also causes recurrent drops in blood oxygen saturation due to repeated pauses in breathing during sleep. These nocturnal oxygen deprivation episodes stimulate the EPO-mediated response, resulting in a higher hemoglobin count. Heavy smoking or chronic exposure to carbon monoxide causes a similar effect because carbon monoxide binds to hemoglobin more readily than oxygen, functionally reducing oxygen-carrying capacity. The body interprets this as a lack of oxygen and increases red blood cell production to compensate.
Primary and Disease-Related Causes
Not all causes of high hemoglobin relate to compensating for low oxygen; some stem from internal production errors or changes in blood volume. Polycythemia Vera (PV) is a primary bone marrow disorder where hematopoietic stem cells become hypersensitive to EPO or grow independently. This condition results in the bone marrow overproducing red blood cells autonomously, meaning the high hemoglobin level is a direct result of the marrow’s malfunction, not a reaction to low oxygen.
Certain kidney conditions and tumors can lead to an inappropriately high hemoglobin count. Renal cell carcinoma, a type of kidney cancer, or benign kidney cysts may produce excessive amounts of erythropoietin. This is a form of secondary polycythemia where the kidney tissue over-secretes the hormone, regardless of the body’s oxygen needs.
A temporary and non-disease-related cause is severe dehydration, which leads to a condition sometimes called relative polycythemia. Dehydration reduces the total volume of plasma, the fluid component of blood. While the absolute number of red blood cells remains unchanged, the concentration of hemoglobin within the reduced fluid volume appears higher on a blood test. Rehydration resolves this form of elevated count.
Recognizing Signs and Medical Evaluation
An elevated hemoglobin count can cause various symptoms due to the increased viscosity, or thickness, of the blood. Common signs include persistent headaches, dizziness, and fatigue. Some individuals may notice flushed or reddened skin, particularly on the face, and an uncomfortable itching sensation known as pruritus.
If a high hemoglobin count is suspected, a physician orders a Complete Blood Count (CBC) test, which measures the concentration of hemoglobin and other blood components. If the CBC confirms an elevated level, the next steps involve a targeted investigation to determine the underlying cause.
The diagnostic workup often includes measuring the patient’s erythropoietin levels. Low EPO levels alongside high hemoglobin may point toward a primary bone marrow disorder like Polycythemia Vera. Conversely, high EPO levels suggest the body is reacting to chronic hypoxia or that a tumor is inappropriately secreting the hormone. Further testing, such as oxygen saturation checks or genetic testing for the JAK2 mutation associated with PV, helps the doctor pinpoint the cause for appropriate management.