Essential thrombocytosis (also called essential thrombocythemia, or ET) is a chronic blood disorder in which your bone marrow produces too many platelets. Platelets are the blood cells responsible for clotting, and a normal count ranges from about 150,000 to 400,000 per microliter. In ET, that count rises above 450,000 and can climb into the millions. The condition belongs to a group of diseases called myeloproliferative neoplasms, where the bone marrow’s production machinery is stuck in overdrive due to a genetic mutation in blood-forming stem cells.
What Causes the Overproduction
ET is driven by acquired genetic mutations, meaning they develop during your lifetime rather than being inherited from a parent. About 60% of people with ET carry a mutation called JAK2 V617F, which sends constant “grow” signals to the cells that produce platelets. Another roughly 25% carry a mutation in a gene called CALR (calreticulin), which also activates the same growth signaling pathway. A smaller group, about 5%, have mutations in a gene called MPL. In CALR-mutated ET, a single abnormal clone of cells gradually expands in the bone marrow and specifically drives the platelet-producing cells, called megakaryocytes, into overdrive.
A small percentage of ET patients have none of these three mutations and are described as “triple-negative.” Which mutation you carry matters: it influences your symptoms, your risk of blood clots, and how the disease behaves over time.
Symptoms and How It Feels
Many people with ET have no symptoms at all and only discover their high platelet count through a routine blood test. When symptoms do appear, they tend to fall into a few categories.
The most common complaints are vasomotor disturbances: headaches (the single most frequent neurological symptom), lightheadedness, visual changes like sudden blurred or double vision, atypical chest pain, and tingling or numbness in the hands and feet. These happen because excess platelets can temporarily clog tiny blood vessels, briefly interrupting blood flow.
One of the most characteristic symptoms is erythromelalgia, a burning pain in the fingers or toes that can lead to redness and even small ulcers. It results from platelet clumps blocking the tiny arteries at the ends of your extremities. Erythromelalgia typically responds quickly to aspirin, which is one reason it’s considered a hallmark of the disease.
Thrombosis and Bleeding Risks
The central paradox of ET is that it raises your risk of both blood clots and, in some cases, abnormal bleeding. Clots can form in arteries (causing strokes, heart attacks, or blocked blood flow to the limbs) or in veins (deep vein thrombosis, pulmonary embolism, or clots in unusual locations like the veins draining the liver or spleen).
Bleeding becomes a concern mainly when platelet counts climb very high, often above 1 million per microliter. At those levels, the body can develop a condition called acquired von Willebrand syndrome, where a key clotting protein gets consumed by the sheer number of circulating platelets. The result is that despite having far too many platelets, your blood actually has trouble forming stable clots. This is why doctors sometimes avoid aspirin in patients with extremely high counts until they’ve checked for this problem.
How ET Is Diagnosed
Diagnosis starts with a persistently elevated platelet count, typically above 450,000 per microliter on repeated blood draws. But a high platelet count alone isn’t enough, because many common conditions can temporarily push platelets up. Iron-deficiency anemia, infections, inflammatory bowel disease, connective tissue disorders like rheumatoid arthritis, and even recovery from surgery or major bleeding can all cause what’s called reactive (or secondary) thrombocytosis. These causes need to be ruled out first.
Once reactive causes are excluded, doctors look for the genetic mutations associated with ET through a blood test. A bone marrow biopsy is usually performed as well, which in ET typically shows increased numbers of large, mature megakaryocytes. The biopsy also helps distinguish ET from other myeloproliferative neoplasms like polycythemia vera (which overproduces red blood cells) and primary myelofibrosis (which scars the bone marrow). Getting the right diagnosis within this group matters because the prognosis and treatment differ for each.
Risk Stratification: Who Needs Treatment
Not everyone with ET needs the same level of treatment. Doctors use a risk scoring system (the revised IPSET-thrombosis model) that sorts patients into four categories based on three factors: age, history of blood clots, and whether you carry the JAK2 mutation.
- Very low risk: Under 60, no history of clots, no JAK2 mutation
- Low risk: Under 60, no history of clots, but JAK2-positive
- Intermediate risk: Over 60, no history of clots, no JAK2 mutation
- High risk: Over 60, or any prior clot, especially with JAK2 mutation
Your risk category determines how aggressively the disease is managed. Very low risk patients may simply be monitored, with aspirin used only if vasomotor symptoms like headaches or erythromelalgia appear. Low and intermediate risk patients generally take daily low-dose aspirin (81 to 100 mg) to prevent clots. High risk patients need both aspirin and medications that actively lower the platelet count.
Treatment Options
For patients who need platelet-lowering therapy, the first-line medication is hydroxyurea, a drug that slows down cell production in the bone marrow. It’s taken as a daily pill, and most people tolerate it well, though it requires regular blood count monitoring. Hydroxyurea is recommended for anyone over 60 or anyone who has already had a blood clot.
If hydroxyurea isn’t tolerated or stops working well, alternatives include interferon-alpha (given by injection) and anagrelide (a pill that specifically targets platelet production). Interferon-alpha is also the preferred option for younger patients who may be on treatment for decades and for pregnant individuals, since hydroxyurea can harm a developing fetus.
Beyond medications, cardiovascular risk factors like smoking, high blood pressure, high cholesterol, and diabetes play a real role in determining clot risk. Managing these aggressively is part of the overall treatment strategy, not just an afterthought.
ET and Pregnancy
Pregnancy in women with ET carries an increased risk of first-trimester miscarriage and placental complications. The standard approach combines low-dose aspirin with a daily injectable blood thinner, which is continued for six weeks after delivery. If platelet counts are dangerously high during pregnancy, interferon-alpha can be used to bring them down and has been shown to improve live birth rates. Hydroxyurea and anagrelide are both off-limits during pregnancy due to risks to the fetus.
Long-Term Outlook
ET is a chronic condition, but for most people it progresses slowly and life expectancy is close to normal, particularly for those under 60 at diagnosis. The main threats over time are clotting events, which is why risk reduction is the central goal of treatment.
A small percentage of ET patients see their disease transform into a more serious condition. The rate of progression to myelofibrosis, where scar tissue gradually replaces healthy bone marrow, is about 4% at 10 years and 6% at 15 years. Transformation to acute myeloid leukemia is rarer still, though the exact rates are harder to pin down and may be influenced by the type of treatment used. The mutation you carry also shapes your trajectory. CALR-mutated ET tends to have a lower clotting risk and a more indolent course compared to JAK2-mutated disease, which is associated with a higher rate of thrombotic events and a clinical profile that more closely resembles polycythemia vera.
Most people with ET live with the condition for years or decades with good quality of life, provided their clotting risk is managed appropriately. Regular blood counts, awareness of new symptoms like unexplained headaches or changes in vision, and staying on top of cardiovascular health are the practical pillars of living well with ET.