Essential thrombocythemia (throm-boe-sie-THEE-me-uh) is an uncommon disorder in which your body produces too many platelets. Platelets are the part of your blood that sticks together to form clots.

This condition may cause you to feel fatigued and lightheaded and to experience headaches and vision changes. It also increases your risk of blood clots.

Essential thrombocythemia is more common in people over age 60, though younger people can develop it too. It's also more common in women.

Essential thrombocythemia is a chronic disease with no cure. If you have a mild form of the disease, you may not need treatment. If you have severe symptoms, you may need medicine that lowers your platelet count, blood thinners or both.

You may not have any noticeable symptoms of essential thrombocythemia. The first indication you have the disorder may be the development of a blood clot. Clots can develop anywhere in your body, but with essential thrombocythemia they occur most often in your brain, hands and feet.

Signs and symptoms depend on where the clot forms. They include:

Less commonly, essential thrombocythemia may cause bleeding, especially if your platelet count is more than 1 million platelets per microliter of blood. Bleeding may take the form of:

Essential thrombocythemia is a type of chronic myeloproliferative disorder. That means your bone marrow, the spongy tissue inside your bones, makes too many of a certain type of cell. In the case of essential thrombocythemia, the bone marrow makes too many cells that create platelets.

It's not clear what causes this to happen. About 90% of people with the disorder have a gene mutation contributing to the disease.

If an underlying condition such as an infection or iron deficiency causes a high platelet count, it's called secondary thrombocytosis. Compared with essential thrombocythemia, secondary thrombocytosis causes less risk of blood clots and bleeding.

Essential thrombocythemia can lead to a variety of potentially life-threatening complications.

If a blood clot occurs in the arteries that supply the brain, it may cause a stroke or a transient ischemic attack (TIA). A TIA (mini-stroke) is a temporary interruption of blood flow to part of the brain.

Signs and symptoms of both a stroke and a TIA develop suddenly and include:

Seek medical attention immediately if you develop signs or symptoms of a stroke.

Less commonly, essential thrombocythemia can cause clots in the arteries that supply blood to your heart. Signs and symptoms of a heart attack include:

Rarely, essential thrombocythemia may progress to these potentially life-threatening diseases:

Acute myelogenous leukemia. This is a type of white blood cell and bone marrow cancer that progresses rapidly.

Myelofibrosis. This progressive disorder results in bone marrow scarring, leading to severe anemia and enlargement of your liver and spleen.

Most women who have essential thrombocythemia have normal, healthy pregnancies. But uncontrolled thrombocythemia can lead to miscarriage and other complications. Your risk of complications may be reduced with regular checkups and medication, so be sure to have your doctor regularly monitor your condition.