Essential thrombocythemia (ET) is a chronic blood cancer in which the bone marrow produces too many platelets. Unlike the temporary rise in platelets you might see after an infection or surgery, ET is a permanent condition driven by a genetic mutation in blood-forming stem cells. It progresses slowly, and median survival exceeds 20 years overall, reaching beyond 30 years for people diagnosed before age 40.
What Happens in the Bone Marrow
Platelets are made by large cells in the bone marrow called megakaryocytes. In ET, a mutation in a stem cell causes these megakaryocytes to multiply and enlarge beyond what the body needs, churning out platelets at an abnormally high rate. A bone marrow biopsy from someone with ET typically shows a crowded landscape of oversized, mature megakaryocytes.
The mutations responsible fall into three main categories. About 60% of ET patients carry a mutation called JAK2 V617F, which locks a growth signal in the “on” position, telling blood cells to keep dividing. Roughly 20% have a CALR mutation, which hijacks the receptor for thrombopoietin (the hormone that regulates platelet production) and forces it to stay active. Around 3% have a mutation in the MPL gene itself, which encodes that same thrombopoietin receptor. A small group of patients, sometimes called “triple negative,” have none of these three detectable mutations.
Symptoms and How ET Feels Day to Day
Many people with ET have no symptoms at all when they’re diagnosed. The condition is often caught incidentally on a routine blood test that shows a persistently elevated platelet count. When symptoms do appear, they tend to fall into two categories: problems from small blood vessel blockages and problems from abnormal bleeding.
The most characteristic symptom is erythromelalgia, a burning pain in the hands and feet accompanied by redness and warmth. It happens when tiny clumps of platelets block the smallest blood vessels in the fingers and toes. In severe cases, this can progress to skin ulcers from poor circulation. Other microvascular symptoms include visual disturbances (sometimes described as ocular migraines), tingling or numbness in the hands and feet, and brief neurological episodes similar to mini-strokes.
Bleeding, when it occurs, is usually mild: nosebleeds, easy bruising, or gastrointestinal bleeding. Serious bleeding is uncommon but can happen in people with extremely high platelet counts, typically above 1,000,000 per microliter. Paradoxically, very high platelet counts can actually impair clotting by depleting a protein called von Willebrand factor, which the body needs to form stable clots. A mildly enlarged spleen is possible, though significant enlargement usually points to a different diagnosis.
How ET Is Diagnosed
Diagnosis requires more than just a high platelet count on a single blood draw. Your doctor will look for a sustained platelet count above 450,000 per microliter, evidence of one of the driver mutations (JAK2, CALR, or MPL), and a bone marrow biopsy showing the characteristic enlarged megakaryocytes. Equally important is ruling out other conditions that can raise platelets, including iron deficiency, chronic inflammation, other blood cancers, and reactive causes like recent surgery.
For triple-negative patients without any of the three common mutations, diagnosis relies more heavily on the bone marrow appearance and on excluding every other possible explanation for the elevated count.
Risk Stratification: Who Needs Treatment
Not everyone with ET needs medication beyond low-dose aspirin. Treatment decisions hinge on your risk of developing blood clots, which is the most serious day-to-day danger of the condition. The revised IPSET-thrombosis system sorts patients into four risk tiers using three variables: age over 60, history of blood clots, and whether you carry the JAK2 mutation.
- Very low risk: Under 60, no prior clots, no JAK2 mutation
- Low risk: Under 60, no prior clots, JAK2 mutation present
- Intermediate risk: Over 60, no prior clots, no JAK2 mutation
- High risk: Over 60 with JAK2 mutation, or any age with a history of blood clots
Cardiovascular risk factors like high blood pressure, diabetes, and smoking also influence the overall picture. Managing these aggressively is part of ET care regardless of your risk category.
Treatment Options
For low-risk patients, the standard approach is low-dose aspirin, which helps prevent the microvascular symptoms like erythromelalgia and reduces clotting risk. Aspirin alone is often enough to control symptoms in younger patients without a clotting history.
High-risk patients generally need platelet-lowering therapy (called cytoreductive treatment) in addition to aspirin. The most common first-line option is hydroxyurea, which has the strongest clinical trial evidence behind it. It works by slowing the overproduction of blood cells in the marrow. Pegylated interferon is an alternative that’s particularly useful for younger patients or those planning a pregnancy, since hydroxyurea must be stopped at least three months before conception. A third option, anagrelide, specifically targets platelet production but needs to be used cautiously in people with heart rhythm problems or heart failure.
For intermediate-risk patients or low-risk patients with bothersome symptoms, doctors typically optimize aspirin and address cardiovascular risk factors first. Platelet-lowering therapy is added only if symptoms persist or if complications develop, such as the acquired von Willebrand syndrome that can come with very high platelet counts.
ET and Pregnancy
ET raises the stakes during pregnancy. The combination of the condition itself and the natural clotting tendency of pregnancy increases the risk of blood clots, preeclampsia, placental problems, and restricted fetal growth. Pregnancies in women with ET are monitored closely with regular ultrasound Doppler scans to check blood flow to the placenta, typically every four weeks.
Low-dose aspirin is continued throughout pregnancy and after delivery. Women at higher risk may also receive daily blood-thinning injections, particularly during the six weeks postpartum when clotting risk peaks. If platelet-lowering medication is needed during pregnancy, interferon is the preferred choice because hydroxyurea can harm a developing baby. Women on hydroxyurea who are planning to conceive are typically switched to interferon at least three months in advance. Estrogen-containing contraceptives are avoided due to their clotting risk; progesterone-only methods or barrier contraception are recommended instead.
Long-Term Outlook
ET is a condition people live with for decades. A Mayo Clinic analysis of over 1,000 patients found a median survival of 20.6 years, with 10-year and 20-year survival rates of 81% and 52%, respectively. For patients diagnosed young, survival approaches that of the general population.
The two long-term complications to be aware of are transformation to myelofibrosis (a more advanced bone marrow condition with scarring) and, much more rarely, transformation to acute leukemia. The 10-year risk of developing myelofibrosis is roughly 4%, rising to about 6% at 15 years. Leukemic transformation at 10 years is less than 1%, though the risk may be slightly higher in JAK2-mutated patients with extremely high platelet counts.
A survival model developed at the Mayo Clinic stratifies patients into four groups based on age and specific blood count markers, with median survivals ranging from 8 years in the highest-risk group to 47 years in the lowest. This wide range underscores that ET is not a single-prognosis disease. Your individual outlook depends heavily on your age at diagnosis, mutation type, and how well thrombotic risk factors are controlled.