Thrombocythemia is an uncommon disorder in which your body produces too many platelets (thrombocytes). It's also known as primary or essential thrombocythemia.

Many people with thrombocythemia have no signs or symptoms. Some develop blood clots abnormally or, more rarely, have bleeding episodes, such as nosebleeds, which usually are mild. Thrombocythemia most often occurs in women older than 50.

You may not need treatment for thrombocythemia if you have no signs or symptoms. However, if you have abnormal blood clotting or bleeding, medications and procedures can help you avoid potentially serious complications.

It's possible to have thrombocythemia without developing signs or symptoms. However, the first indication you have the disorder may be the development of a blood clot (thrombus). Although clots can develop anywhere in your body, with thrombocythemia, they most often develop in your brain, hands and feet.

Signs and symptoms depend on where the clot forms.

• Clots to your brain. If a clot obstructs blood flow to your brain, you may experience chronic headache and dizziness. In extreme cases, the clot may cause a transient ischemic attack (TIA) — a temporary interruption of blood flow to part of the brain that destroys no brain cells and causes no permanent disability — or stroke. Signs and symptoms include sudden weakness or numbness of your face, arm or leg, usually on one side of your body, sudden difficulty speaking or understanding speech (aphasia), and sudden blurred, double or decreased vision.
• Clots to your hands and feet. Obstructing blood flow in these tiny blood vessels may cause numbness, redness, and intense burning or throbbing on your palms or soles. The signs and symptoms may worsen with heat or exercise. Diminished blood flow to your feet and toes may cause them to have a patchy redness with burning pain (erythromelalgia).

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

• Nosebleeds
• Bruising
• Bleeding from your mouth or gums
• Bloody stool

When to see a doctor
If you have any of the signs or symptoms of abnormal blood clotting or bleeding, see your doctor.

If you develop signs or symptoms of a TIA or stroke (such as numbness or paralysis on one side of your body) seek medical attention immediately.

Bone marrow — spongy tissue inside your bones — contains stem cells that can become red blood cells, white blood cells or platelets. Platelets travel through your blood vessels. They stick together to form clots to stop bleeding that occurs when you damage a blood vessel, such as when you experience a cut. A normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood.

If you have thrombocythemia, your bone marrow overproduces the cells that form platelets (megakaryocytes), releasing too many platelets into your blood. The overproduced platelets may not function normally, leading to abnormal clotting or bleeding.

The cause of thrombocythemia isn't well understood. Some people with the disorder have a mutation of the Janus kinase 2 (JAK2) gene in their blood cells, but having the mutation doesn't appear to affect the course of the disease, so its exact role is unknown. A rare form of thrombocythemia is inherited. Generally, if you receive a diagnosis of thrombocythemia, the cause isn't known.

In some cases, an underlying condition, such as cancer, removal of the spleen, rheumatoid arthritis or iron deficiency anemia, may cause the overproduction of platelets. Then the disorder is known as thrombocytosis.

The abnormal blood clotting of thrombocythemia can lead to a variety of potentially serious complications, including:

• Pregnancy complications. Uncontrolled thrombocythemia can cause miscarriage, premature delivery, early separation of the placenta from the uterine wall (placental abruption) and slow fetal growth. If you're a woman with thrombocythemia and you become pregnant, your doctor should monitor you carefully throughout your pregnancy.
• Stroke. A clot that obstructs blood flow to your brain can cause you to have a stroke. If you develop signs and symptoms of a stroke, seek prompt medical attention.
• Heart attack. A clot that obstructs blood flow to your heart can cause you to have a heart attack. If you develop signs and symptoms of a heart attack, such as pressure, fullness or a squeezing pain in the center of your chest lasting more than a few minutes; pain extending to your shoulder, arm, back, teeth or jaw; shortness of breath; and sweating or clammy skin, seek prompt medical attention.

Thrombocythemia also can cause hemorrhage with significant blood loss. Some people with thrombocythemia may later develop acute leukemia or myelofibrosis, both of which can be life-threatening:

• Acute leukemia. Acute myelogenous leukemia is a type of blood and bone marrow cancer that progresses rapidly.
• Myelofibrosis. This progressive bone marrow disorder disrupts your body's normal production of blood cells, resulting in bone marrow scarring, severe anemia and enlargement of your liver and spleen.

It's likely that a routine blood test that shows a high platelet count will be your first indication that you may have thrombocythemia. Or you may see your doctor because of blood-clotting or bleeding incidents.

Besides taking your medical history, examining you physically and running tests, your doctor may ask you about factors that could affect your platelets, such as recent medical procedures, blood transfusions or infections. Your doctor may refer you to a doctor who specializes in blood diseases (hematologist).

Because appointments can be brief, and because there's often a lot of ground to cover, it's a good idea to be well prepared for your appointment. Here's some information to help you get ready for your appointment, and what to expect from your doctor.

What you can do

• Be aware of any pre-appointment restrictions. At the time you make the appointment, be sure to ask if there's anything you need to do in advance, such as restrict your diet.
• Write down any symptoms you're experiencing, including any that may seem unrelated to the reason for which you scheduled the appointment.
• Write down your health history, including your history of blood clots or bleeding incidents. Also include any family history of high platelet counts.
• Write down key personal information, including any major stresses or recent life changes.
• Make a list of all medications, as well as any vitamins or supplements, that you're taking. Some medications, such as oral contraceptives, can increase the risk of blood clots in women with thrombocythemia.
• Take a family member or friend along, if possible. Sometimes it can be difficult to take in all the information you hear during an appointment. Someone who accompanies you may remember something that you missed or forgot.
• Write down questions to ask your doctor.

Your time with your doctor is limited, so preparing a list of questions will help you make the most of your time together. List your questions from most important to least important in case time runs out. For thrombocythemia, some basic questions to ask your doctor include:

• What kinds of tests do I need?
• Should I see a specialist? What will that cost, and will my insurance cover seeing a specialist?
• Are there any brochures or other printed material that I can take home with me? What Web sites do you recommend visiting?

In addition to the questions that you've prepared to ask your doctor, don't hesitate to ask questions that arise during your appointment if you don't understand something or need more information.

You may find out you have thrombocythemia through a routine blood test that shows you have a higher than normal platelet count. Or, if you've had an unexpected blood clot or bleeding episode, or your doctor finds that your spleen is enlarged, your doctor may order a complete blood count (CBC) to determine your platelet count. A blood smear — a test in which a small amount of your blood is examined under a microscope — helps determine the condition of your platelets. Other blood tests can determine whether you have genetic factors that can cause a high platelet count.

If your blood count is above 600,000 platelets per microliter of blood, then your doctor will look for an underlying condition. If there's no evident cause of your high platelet count, and if your platelet count remains high over time, then your doctor may request a bone marrow test. There are two types of bone marrow tests, which provide different but complementary information about your blood cells. Often, they're done together.

• Bone marrow aspiration. Your doctor extracts a small amount of your liquid bone marrow through a needle and examines it under a microscope, looking for abnormal cells.
• Bone marrow biopsy. Your doctor takes a sample of solid bone marrow tissue through a needle for examination under a microscope. If you have thrombocythemia, your bone marrow has more than a normal amount of the large cells that make platelets.

Treatment of thrombocythemia depends on your risk of blood-clotting or bleeding episodes. If you're younger than 50, have had no signs or symptoms and have no other risk factors for developing blood clots, such as smoking, you may just need periodic medical checkups. If you're older than 60 and have had previous signs and symptoms of the disease, you're at greater risk of clotting or bleeding, and your doctor likely will prescribe medication or a medical procedure to lower your platelet count.

Medication
If you're at risk of blood clots, your doctor may recommend low-dose aspirin, particularly if you're pregnant. Aspirin makes the platelets less sticky and your blood less likely to form clots with little risk to a fetus. However, if you take aspirin during pregnancy, stop taking it at least one week before delivery to lower your risk of bleeding complications during delivery.

The platelet-lowering drugs most commonly used to treat thrombocythemia include:

• Hydroxyurea (Hydrea, Droxia). This drug suppresses bone marrow's production of blood cells, including platelets. Also used to treat cancers, it's the most commonly prescribed platelet-lowering drug for thrombocythemia. It's sometimes used with aspirin. If you take this drug, your doctor will monitor you and your blood counts closely. There's some evidence that long-term use increases your risk of developing acute myelogenous leukemia, but compared with other drugs of its type, hydroxyurea is thought to have less potential for causing leukemia.
• Anagrelide (Agrylin). Unlike hydroxyurea, anagrelide doesn't kill cells while lowering platelet counts, nor is it associated with increased risk of leukemia. However, it's not considered as effective as hydroxyurea. Side effects may include fluid retention, heart problems, headaches, dizziness, nausea and diarrhea. Anagrelide isn't recommended for older people with pre-existing heart conditions.
• Interferon alfa. Given by injection, this drug is less convenient to administer than hydroxyurea or anagrelide, may be more expensive and may cause less tolerable side effects. Side effects may include flu-like symptoms, confusion, nausea, depression, diarrhea, seizures, irritability and sleepiness.

Plateletpheresis
Used only in emergencies, such as when your blood is clotting dangerously, a medical procedure known as plateletpheresis rapidly lowers platelet count. During the procedure, an intravenous (IV) needle connected to a tube is inserted into one of your blood vessels to remove your blood. A device removes the platelets from your blood. The liquid portion of your blood (plasma) and your red cells are returned to you through an IV line. The effect is temporary.

Take extra care to reduce your risk of developing blood clots if you have thrombocythemia. Healthy lifestyle habits can lower your risk of developing conditions that may contribute to blood clotting, including diabetes, high blood pressure and high blood cholesterol. Take steps to:

• Eat healthy foods. Choose a varied diet rich in whole grains, vegetables and fruits and low in saturated fats. Try to avoid trans fats. Learn about portion control to maintain a normal weight.
• Increase your physical activity. Aim for 30 minutes of moderate physical activity a day. Take a brisk daily walk. Ride your bike. Swim laps. If you can't fit in a long workout, break it up into smaller sessions spread throughout the day.
• Achieve or maintain normal weight. Being overweight or obese increases the pressure in the veins in your pelvis and legs and is a risk factor for conditions, such as high blood pressure, that increase your risk of blood clotting.
• Stop smoking. Smoking affects blood clotting and circulation.

If your thrombocythemia increases your tendency to bleed, take extra precautions to keep from injuring yourself. Follow these suggestions:

• Avoid playing contact sports or engaging in other activities that could be dangerous or could cause you to fall.
• Use a softer toothbrush and waxed floss.
• Avoid shaving cuts. Shave with an electric razor.
• Be cautious with household tasks involving knives, scissors and other sharp tools.