What Is Thrombocythaemia and Why Do Platelets Rise?

Thrombocythaemia is a chronic blood cancer in which the bone marrow produces too many platelets, the small cell fragments responsible for clotting. In its primary form, called essential thrombocythaemia (ET), a genetic mutation in a blood-forming stem cell drives platelet counts well above the normal range, sometimes into the millions. Despite the word “cancer,” many people with ET live decades with the condition, and the main medical concern is managing the increased risk of blood clots and, paradoxically, bleeding. The biology behind those risks, and the question of who actually needs treatment, turns out to be more nuanced than a simple platelet number suggests.

Why Platelets Climb in the First Place

In roughly half of ET cases, the culprit is a specific mutation in the JAK2 gene, called JAK2 V617F. This mutation keeps a signaling switch permanently turned on inside blood-forming cells, telling megakaryocytes (the giant bone marrow cells that shed platelets) to multiply faster and release more platelets. Mouse studies show the mutation does not just increase platelet numbers; it changes the platelets themselves, making them stickier and more reactive to the signals that trigger clotting.¹

About a quarter of ET patients instead carry a mutation in the CALR gene, which encodes a protein called calreticulin. All the disease-causing CALR mutations share a common feature: they shift the reading frame in the same region of the gene, producing an abnormal protein tail that latches onto the thrombopoietin receptor (the main growth signal for platelet-producing cells) and switches it on without needing its normal hormone trigger.² A smaller group, roughly 3–5%, carries mutations in the MPL gene, which encodes the thrombopoietin receptor itself. These MPL mutations make the receptor hypersensitive to its growth signal or allow it to fire independently.³

That still leaves about 10–15% of ET patients who test negative for all three driver mutations. These “triple-negative” cases are a diagnostic puzzle. Next-generation sequencing has shown that some of these patients harbor mutations in other genes involved in DNA methylation and chromatin remodeling, such as DNMT3A, TET2, and ASXL1. Finding those mutations matters clinically because triple-negative patients who carry them tend to be older and experience blood clots at a significantly higher rate than those without additional mutations.⁴

Essential Thrombocythaemia Versus Reactive Thrombocytosis

A high platelet count does not automatically mean ET. The vast majority of people with elevated platelets have secondary (reactive) thrombocytosis, meaning something else is driving the marrow to produce more platelets. Common triggers include iron deficiency, active infection, chronic inflammation, surgical removal of the spleen, and underlying cancer.⁵ In reactive cases, the platelets themselves function normally and the count usually falls once the underlying cause is treated.

Distinguishing ET from reactive thrombocytosis can be straightforward when a driver mutation is found, but it gets harder when the mutation screen is negative. A study comparing clinical and laboratory features between the two groups found that patients with ET tended to have higher hemoglobin, larger red cells, and bigger platelets, while those with reactive thrombocytosis were more likely to have higher white blood cell counts, higher body mass index, and an identifiable inflammatory or iron-deficiency trigger.⁶ A bone marrow biopsy remains part of the formal diagnostic workup because the pattern of megakaryocyte growth and the amount of scar tissue (fibrosis) help separate ET from a closely related condition called prefibrotic myelofibrosis.

The Diagnostic Gray Zone

Getting the diagnosis exactly right matters because ET and prefibrotic primary myelofibrosis carry different long-term outlooks, yet they can look very similar under the microscope. The 2016 WHO classification drew a sharper line between the two, defining prefibrotic myelofibrosis by specific bone marrow features plus clinical markers like an enlarged spleen or elevated LDH.⁷ In practice, though, pathologists do not always agree. A European trial that sent the same bone marrow slides to multiple expert reviewers found consensus on the histological classification in only about 63% of cases, with inter-observer agreement ranging from low to moderate. When additional clinical criteria were factored in, over half of the cases either could not be reliably classified or fell into an “unclassifiable” category.⁸

For patients, this means that a diagnosis of ET sometimes comes with a degree of uncertainty, particularly in borderline cases. If your hematologist recommends a second pathology opinion on your biopsy, that is not unusual given the recognized difficulty of this distinction.

What Living With ET Actually Feels Like

Many people with ET are diagnosed incidentally through a routine blood test and feel perfectly fine at that point. But symptom-burden studies tell a more complicated story. In a large registry of ET patients who completed validated symptom questionnaires, the average total symptom score was moderate, but a third of patients scored 20 or higher on a scale where higher numbers mean worse symptoms. Fatigue was the single most burdensome symptom, followed by numbness and tingling in the hands or feet, reduced physical activity, and early fullness after eating. Women consistently reported higher symptom scores than men across nearly every category.⁹

One distinctive symptom is erythromelalgia, a burning pain with visible redness and warmth in the fingers or toes. Research has shown that erythromelalgia in ET is driven by platelet clumps forming in tiny arteries of the skin, damaging the vessel lining. These microthrombi respond well to aspirin, which shuts down the platelet activation, but they do not respond to blood-thinning drugs like warfarin or heparin, because the clots are built from platelets rather than from the fibrin meshwork those drugs target.¹⁰ If you have ET and experience burning pain in your extremities that does not improve with conventional blood thinners, this is the reason.

Blood Clots and Bleeding

ET raises the risk of both arterial clots (strokes, heart attacks, blocked peripheral arteries) and venous clots (deep vein thrombosis, pulmonary embolism). The interplay between excess platelet numbers, altered platelet stickiness, and changes in white blood cells and the vessel wall all contribute to this risk.¹¹

At the other extreme, very high platelet counts can paradoxically cause bleeding. When platelet numbers soar above roughly 1,000–1,500 × 10⁹/L, the excess platelets can mop up a clotting protein called von Willebrand factor, impairing the blood’s ability to form normal clots. However, a recent study comparing ET patients with extreme thrombocytosis to those with lower counts found that the rates of major bleeding were not statistically different between the two groups.¹² The bleeding risk is real but probably less dramatic than older textbook descriptions suggested.

Sorting Out Who Needs Aggressive Treatment

Not every ET patient faces the same clot risk, and the main job of risk stratification is figuring out who benefits from platelet-lowering drugs versus who can get by with aspirin alone or even just observation. The revised IPSET-thrombosis scoring system assigns patients to four tiers based on three factors: whether they have ever had a blood clot, whether they are older than 60, and whether they carry the JAK2 V617F mutation. Someone with none of those features falls into the “very low risk” category; someone with a history of clots, or who is older than 60 and also JAK2-positive, lands in the “high risk” tier. Validation in a cohort of 585 patients confirmed that higher tiers carry meaningfully higher clot rates.¹³

One limitation worth knowing about: a large Spanish registry study confirmed that the IPSET system reliably identifies patients at high risk of arterial clots, but it did not predict venous clots well.¹⁴ Venous thrombosis in ET seems to be driven by partially different biology, and better tools for predicting it are still needed.

How ET Is Treated

Treatment is tiered to match risk. For most patients, low-dose aspirin is the foundation. The standard recommendation is 81–100 mg daily for anyone above the very-low-risk category. Plain (immediate-release) aspirin is preferred over enteric-coated formulations because some ET patients show poor responsiveness to enteric-coated versions. There is also evidence that a single daily dose may not suppress platelet activity for a full 24 hours in many ET patients, and twice-daily dosing has been explored as a solution.¹⁵ One notable exception: low-risk patients with very high platelet counts above 1,000 × 10⁹/L or those with CALR mutations and no other risk factors may do better with observation alone rather than aspirin, because aspirin can worsen bleeding when von Willebrand factor levels are already depleted.

When the clot risk is high enough to warrant platelet-lowering therapy, hydroxyurea has been the standard first-line drug for decades. A landmark trial showed that high-risk ET patients treated with hydroxyurea had a thrombosis rate of about 4%, compared with 24% in the control group that received no cytoreductive treatment.¹⁶ Hydroxyurea is generally well tolerated, though it can cause mouth ulcers, skin changes, and lowered blood counts if the dose is too high. Anagrelide is another option, particularly for patients who cannot tolerate hydroxyurea, and interferon-based therapies (especially pegylated interferon alfa-2b) are being studied as alternatives that may modify the disease at a deeper biological level.

On the newer end of the pipeline, bomedemstat, a drug that works by inhibiting an enzyme involved in how stem cells renew themselves and how megakaryocytes mature, has shown the ability to durably control platelet counts and reduce the burden of driver mutations across JAK2-, CALR-, and MPL-mutated ET.¹⁷ Disease-modifying therapies like this represent a shift from simply keeping the platelet count in check toward actually shrinking the mutant clone.

Pregnancy and ET

ET is one of the few blood cancers that frequently affects women of childbearing age. The median age at diagnosis is 65–70, but a second peak occurs in the thirties and forties, and pregnancy in a woman with ET brings specific concerns. In a study of 121 pregnancies in 52 women with ET, the overall live birth rate was 69%, and about half of the women experienced at least one complication. Spontaneous miscarriage was the most common problem, occurring in about a quarter of all pregnancies. Aspirin use during pregnancy was associated with a roughly 70% reduction in the odds of complications, while a history of prior pregnancy loss raised the odds substantially.¹⁸ Interestingly, platelet counts tend to fall naturally during pregnancy in women with ET, dropping by about 43% on average, and women whose counts fell more had fewer complications.

Despite the elevated complication rate, the overall prognosis is encouraging. The same study found that even a woman who experiences a complication in one pregnancy has a greater than 50% chance of a successful pregnancy by the third attempt. Treatment decisions during pregnancy are complex because most cytoreductive drugs are not considered safe for the fetus. Low-dose aspirin is the mainstay, sometimes combined with low-molecular-weight heparin for women with additional thrombotic risk factors. Interferon alfa is the only platelet-lowering drug generally used during pregnancy when cytoreduction is deemed necessary.¹⁹

ET in Children and Adolescents

Pediatric ET is rare and behaves differently from the adult disease. Only about 25–40% of children with ET carry one of the three classic driver mutations, which means the majority test triple-negative. The diagnostic workup in children needs to include hereditary and familial forms of thrombocytosis, which can mimic ET closely but carry a different prognosis. In the short to medium term, pediatric ET appears more benign than adult ET: thrombotic events are uncommon, and bleeding tends to be minor.²⁰ The challenge is that these children face a lifetime of follow-up, and the very long-term risk of progression is less well characterized simply because large cohorts have not been tracked for enough decades.

Long-Term Progression

Two serious long-term outcomes concern hematologists: transformation into myelofibrosis (a condition where scar tissue gradually replaces normal bone marrow) and transformation into acute myeloid leukemia. Leukemic transformation occurs at a rate of about 1–4% over a typical follow-up period of 7–10 years, with higher risk in older patients, those with very high platelet counts, and those carrying multiple somatic mutations. Certain older chemotherapy agents (alkylating drugs and radiophosphorus) have been linked to increased leukemia risk, while hydroxyurea and anagrelide have not.²¹

Fibrotic progression (ET evolving into post-ET myelofibrosis) is more common than leukemic transformation and is where molecular profiling is starting to earn its keep. A multicenter study of over 1,600 patients developed a molecular risk model showing that patients with JAK2 V617F at a variant allele frequency above 35%, or with CALR type 1 or MPL mutations, had significantly higher rates of fibrotic transformation. In one cohort, the 10-year fibrotic progression rate was about 8% in the high molecular risk group versus roughly 1% in the low molecular risk group, and the gap widened further at 20 years.²² Proteomic research has begun identifying the biological pathways driving that transition, highlighting chronic inflammation, tissue remodeling, and impaired immune surveillance as central features of fibrotic progression.²³

The Role of Broader Genetic Testing

Standard diagnostic testing looks for JAK2, CALR, and MPL mutations. But next-generation sequencing panels that screen dozens of genes at once are becoming more common, particularly for triple-negative patients where the diagnosis is uncertain. In one study of triple-negative patients, about 15% were found to carry pathogenic mutations in genes like DNMT3A, TET2, ASXL1, or SRSF2 when a broader panel was used. Those patients were older on average (mid-sixties versus early forties) and had a thrombotic event rate roughly three to four times higher than the group with no detectable mutations.²⁴ Broader sequencing also helps provide proof that the high platelet count is driven by a true clonal (cancer-related) process, which can be reassuring diagnostically even if the finding does not immediately change treatment.²⁵

For patients, the practical takeaway is that if your initial mutation testing comes back negative for all three drivers and your hematologist is not fully confident in the ET diagnosis, asking about extended sequencing is reasonable. It will not change the diagnosis for everyone, but in a meaningful minority it can clarify both what is happening and how aggressively it needs to be managed.

Symptom Burden Across Risk Levels

There is a common assumption that higher-risk ET means worse symptoms, but the data do not clearly support this. A study from the MOST registry found that patients classified as low-risk ET actually reported higher average symptom scores than those classified as high-risk.²⁶ One possible explanation is that high-risk patients are more likely to be on cytoreductive treatment, which may alleviate some symptoms. Another is that the risk classification captures clot probability, not how someone feels day to day. Either way, the point is important: being told you are “low-risk” does not mean you will feel fine, and treatment decisions should account for symptom burden alongside thrombotic risk.

Fatigue, in particular, deserves special mention because it is the single most reported and most severe symptom across ET patients regardless of risk category. It is also the hardest to treat, since it does not respond reliably to platelet-lowering drugs or aspirin. Exercise, sleep hygiene, and pacing strategies are the most commonly recommended approaches, but research specifically targeting fatigue management in ET remains limited.