Hemoglobin (Hgb) is a protein found within red blood cells. Its primary job is to pick up oxygen in the lungs and transport it to every tissue and organ in the body. Variations in the concentration of this protein, measured as a hemoglobin level, indicate how well the body is performing this function.
What Hemoglobin Is and What It Does
Hemoglobin is a complex protein. The molecule is composed of four protein subunits, each holding a non-protein component called a heme group. The heme group contains a single iron atom, and this iron atom directly and reversibly binds to oxygen. Hemoglobin’s ability to change its shape as oxygen binds allows it to load and unload oxygen depending on the surrounding environment.
The process begins in the lungs, where high oxygen concentration causes the iron atoms to readily bind the gas, forming oxyhemoglobin. As the red blood cells circulate and reach tissues with lower oxygen levels, the hemoglobin releases the oxygen to fuel the cells. Hemoglobin also helps transport a small amount of carbon dioxide waste from the tissues back toward the lungs for exhalation.
Understanding Hemoglobin Testing and Reference Ranges
Hemoglobin levels are routinely measured as part of a complete blood count (CBC). The result is typically reported in grams per deciliter (g/dL), which represents the concentration of the protein in a specific volume of blood. This measurement provides a snapshot of the blood’s oxygen-carrying capacity.
Reference ranges represent the levels considered typical for healthy individuals, though these can vary slightly between laboratories. For adult males, a normal range is often cited as approximately 13.5 to 17.5 g/dL, while for adult females, the range is generally 12.0 to 15.5 g/dL. Testing is performed during routine physicals, before certain medical procedures, or when investigating symptoms like persistent fatigue or shortness of breath.
When Hemoglobin Levels Are Too Low
A low hemoglobin level means the blood has a reduced capacity to deliver sufficient oxygen to the body’s tissues, a condition known as anemia. The resulting lack of oxygen delivery often leads to generalized symptoms such as chronic fatigue, noticeable weakness, and pale skin. A person may also experience shortness of breath or dizziness, particularly during physical activity, as the body struggles to meet the increased oxygen demand.
One of the most frequent causes of low hemoglobin is nutritional deficiency, particularly a lack of iron, which is necessary to form the heme group. Deficiencies in Vitamin B12 and folate can also impair red blood cell production, leading to insufficient hemoglobin levels. Chronic or acute blood loss, such as from heavy menstrual periods or bleeding in the digestive tract from ulcers, also causes hemoglobin to drop by depleting the red blood cell count.
Other causes relate to issues with the production or lifespan of red blood cells themselves. Diseases that affect the bone marrow, such as certain cancers, can reduce the body’s ability to manufacture new red blood cells. Alternatively, conditions like thalassemia or sickle cell disease can cause red blood cells to be prematurely destroyed, leading to a shortage even if production is normal.
When Hemoglobin Levels Are Too High
An elevated hemoglobin level, which is a less common finding, may indicate that the body is attempting to compensate for chronically low oxygen availability. Symptoms associated with high levels can include headaches, dizziness, fatigue, and even easy bruising or joint swelling. The increased concentration of red blood cells makes the blood thicker, which increases the risk of blood clots, potentially leading to complications like stroke or heart attack.
Sometimes, high hemoglobin is a temporary consequence of dehydration, where the reduced blood plasma volume causes the concentration of hemoglobin to appear artificially high. Lifestyle and environmental factors also play a role, such as chronic smoking or living at high altitudes, where the air contains less oxygen. In these situations, the body produces more red blood cells and hemoglobin to capture the limited oxygen available.
In some instances, a high level points to an underlying health condition, such as chronic obstructive pulmonary disease (COPD) or congenital heart disease. These conditions impair oxygen transfer into the bloodstream, prompting the kidneys to release a hormone that stimulates red blood cell production. Rarely, a bone marrow disorder like polycythemia vera causes the marrow to produce an uncontrolled excess of red blood cells, resulting in significantly elevated hemoglobin levels.