Hemoglobin is a protein in your red blood cells that carries oxygen from your lungs to every tissue in your body and helps transport carbon dioxide back out. It’s what makes your blood red. Normal levels range from 14 to 18 g/dL for men and 12 to 16 g/dL for women, and when those numbers fall too low or climb too high, it usually signals something worth paying attention to.
How Hemoglobin Works
Each hemoglobin molecule is built from four protein chains, two alpha and two beta, bundled together. Nestled inside each chain is a small structure called a heme group: an iron atom held at the center of a ring-shaped molecule. That iron atom is the key player. It’s the part that actually latches onto oxygen.
Because there are four of these iron-containing units per hemoglobin molecule, a single hemoglobin can carry up to four oxygen molecules at once. What makes the system remarkably efficient is a property called cooperative binding. When the first oxygen molecule attaches, it changes the shape of the hemoglobin slightly, making it easier for the second oxygen to bind, then the third, then the fourth. This creates an “S-shaped” loading curve: hemoglobin picks up oxygen slowly at first, then loads up rapidly once it gets started.
The reverse happens in your tissues. Where cells are active and producing carbon dioxide, conditions shift (lower pH, higher CO2) and hemoglobin releases its oxygen more readily. This relationship, discovered by physiologist Christian Bohr in 1904 and called the Bohr effect, means hemoglobin automatically delivers more oxygen to the tissues that need it most, like working muscles, and holds onto it more tightly in the oxygen-rich environment of your lungs.
Normal Hemoglobin Levels
A standard blood test, usually part of a complete blood count (CBC), measures your hemoglobin in grams per deciliter of blood. The reference ranges are:
- Men: 14 to 18 g/dL
- Women: 12 to 16 g/dL
Women of childbearing age typically run lower because of monthly blood loss through menstruation. During pregnancy, blood volume expands significantly, which can dilute hemoglobin levels further. Your doctor may also order a hematocrit test alongside hemoglobin. While hemoglobin measures the protein itself, hematocrit tells you what percentage of your blood volume is made up of red blood cells. The two track closely together and are often reported side by side.
What Low Hemoglobin Feels Like
When hemoglobin drops below normal range, your body isn’t getting enough oxygen delivered to its tissues. The resulting condition is anemia, and it tends to come on gradually enough that people sometimes adjust to it without realizing something is off. Common signs include:
- Persistent tiredness and weakness
- Shortness of breath during activities that used to feel easy
- Pale or yellowish skin
- Dizziness or lightheadedness
- Cold hands and feet
- Headaches
- Irregular or fast heartbeat
The skin color changes can be harder to notice on darker skin tones. Checking the inside of your lower eyelids, gums, or nail beds for unusual paleness is often more reliable.
Common Causes of Low Hemoglobin
Iron deficiency is the most frequent culprit. Your body needs iron to build the heme groups that sit at the core of each hemoglobin molecule, so when iron stores run low, hemoglobin production slows down. Women between 19 and 50 need about 18 mg of dietary iron per day, more than double the 8 mg recommended for men the same age. During pregnancy, the requirement jumps to 27 mg per day.
Deficiencies in folate and vitamin B12 can also cause anemia, though through a different mechanism. These nutrients are essential for producing healthy red blood cells in the bone marrow. Without enough of them, the body makes fewer red blood cells or produces ones that are abnormally large and don’t function well.
Chronic kidney disease is another major cause. The kidneys produce a hormone that signals the bone marrow to make new red blood cells. When kidney function declines, that signal weakens and red blood cell production drops. Heavy menstrual periods, gastrointestinal bleeding, and conditions that destroy red blood cells faster than normal can also drive hemoglobin down.
What Causes High Hemoglobin
Hemoglobin levels above the normal range are less common but still clinically significant. The most straightforward cause is living at high altitude. With less oxygen in the air, your body compensates by producing more red blood cells. Smoking has a similar effect because carbon monoxide from cigarette smoke occupies some of hemoglobin’s oxygen-binding sites, prompting the body to make more hemoglobin to keep up.
A more serious cause is polycythemia vera, a type of blood cancer where a genetic mutation causes the bone marrow to overproduce red blood cells. The blood becomes thicker than normal, which increases the risk of clots. Dehydration can also make hemoglobin appear artificially elevated because there’s less fluid in the blood, concentrating the red blood cells.
Genetic Hemoglobin Disorders
Some people inherit genes that produce structurally abnormal hemoglobin or reduce the amount of normal hemoglobin their body makes. The two most well-known conditions are sickle cell disease and thalassemia.
In sickle cell disease, a change in the gene for the beta chain produces an abnormal form called hemoglobin S. When hemoglobin S releases its oxygen, the molecules can stick together and distort the red blood cell into a rigid, crescent shape. These sickle-shaped cells get stuck in small blood vessels, causing episodes of severe pain and organ damage. People who inherit the sickle cell gene from both parents (homozygous HbSS) typically have hemoglobin levels between 6 and 9 g/dL, well into the anemic range. Those who carry just one copy of the gene generally have normal hemoglobin levels and no symptoms.
Thalassemia involves reduced production of either the alpha or beta chains of hemoglobin. In its severe form (beta-thalassemia major), the body compensates by producing large amounts of fetal hemoglobin, a variant normally present only during pregnancy and early infancy. Fetal hemoglobin binds oxygen more tightly than adult hemoglobin, which is useful in the womb but less ideal after birth. People with severe thalassemia may have fetal hemoglobin levels of 70 to 90%, compared to trace amounts in healthy adults.
How Your Body Recycles Hemoglobin
Red blood cells live for about 120 days. When they reach the end of their lifespan, the spleen and liver break them down. The hemoglobin inside is dismantled in stages. First, the iron is stripped out and sent back to the bone marrow to be reused in new hemoglobin molecules. The remaining heme structure is converted into a green pigment and then into bilirubin, the orange-yellow compound that gives bruises their changing colors and stool its brown tone.
About 80% of the bilirubin in your body comes from this hemoglobin recycling process. The liver processes bilirubin and sends it into the digestive tract for elimination. When the liver can’t keep up, either from disease or from an unusually high rate of red blood cell destruction, bilirubin accumulates in the blood and turns the skin and eyes yellow, a condition known as jaundice.
Nutrients That Support Hemoglobin Production
Iron is the most critical nutrient for maintaining healthy hemoglobin. Good dietary sources include red meat, poultry, fish, beans, lentils, and fortified cereals. Iron from animal sources is absorbed more efficiently than iron from plants, but pairing plant-based iron with vitamin C (like squeezing lemon over lentils) improves absorption significantly.
Folate and B12 play supporting roles. Folate is abundant in leafy greens, citrus fruits, and fortified grains. B12 comes almost exclusively from animal products, which is why people following a strictly plant-based diet need a supplement or fortified foods. Without adequate levels of all three nutrients, the bone marrow can’t produce red blood cells at the rate your body needs, and hemoglobin levels gradually fall.