The big picture: Hemoglobin is an iron-rich protein found in red blood cells that carries oxygen from the lungs to the body's tissues and removes carbon dioxide. Low levels indicate anemia and high levels indicate excessive red blood cell production which can cause blood clots.

Hemoglobin’s main function is to carry oxygen to cells throughout the body.

Hemoglobin is made up of four protein subunits, each containing an iron-containing compound called heme.
When hemoglobin picks up oxygen in the lungs, the iron in the heme binds to the oxygen, forming a compound called oxyhemoglobin.
As the red blood cells circulate through the body, the oxygen is released into the tissues, and the hemoglobin binds to carbon dioxide, which is then transported back to the lungs to be exhaled.

Abnormally low hemoglobin levels indicate anemia. This means your body does not have enough healthy red blood cells to give the tissues and organs enough oxygen.

There are many different types of anemia, and the condition can be caused by a variety of factors, such as blood loss, chronic illness, malnutrition, and bone marrow disorders.
Anemia can cause symptoms such as fatigue, weakness, shortness of breath, pale skin, and dizziness. In severe cases, anemia can lead to heart problems, infections, and other serious complications.

Abnormally high hemoglobin levels mean your bone marrow is making too many red blood cells.

These excess cells thicken your blood, slowing its flow, which may cause serious problems, such as blood clots, heart attack, or stroke.
Some common causes of high hemoglobin levels include dehydration, smoking, living at high altitudes, and taking certain medications such as erythropoietin or testosterone.
High hemoglobin levels can cause symptoms such as dizziness, headache, shortness of breath, and a red or flushed appearance to the skin.