Heparin-Induced Thrombocytopenia (HIT): Causes, Symptoms & Treatment

Written by Martha Anne Tudor
Medically Reviewed by Melinda Ratini, MS, DO on May 15, 2025
6 min read

Heparin-induced thrombocytopenia (HIT) is an uncommon but life-threatening condition that can happen after taking the blood-thinner heparin. That's a prescription medicine that helps prevent blood clots.

In some cases, heparin can trigger your immune system and cause a reaction where antibodies form and destroy platelets. Platelets are tiny blood cells in your body that clump together to form clots and stop bleeding. Even though HIT causes your platelet levels to decline, it can raise your chance of dangerous blood clots.

When you have an immune response to heparin, it can cause your platelet count to drop too low, causing a condition called thrombocytopenia.

Heparin molecules bind with a protein to form something called heparin-PF4. When you have heparin-induced thrombocytopenia, your immune system sees heparin-PF4 as an intruder and attacks it, setting off a chain reaction that leads to blood clots.

There are two types of HIT:

HIT I. This is when your platelet levels drop in the first few days of taking heparin but then return to normal. This type doesn't increase your risk of blood clots. It doesn't need treatment.

HIT II. When you hear people talk about HIT, they're usually referring to this type. It's the only serious form of HIT. It happens when your immune system destroys your platelets in response to heparin. That makes your platelet levels drop, increasing your risk of forming serious blood clots.

These aren't medical classifications of HIT, but they're terms to describe when an HIT might occur:

Early-onset HIT

This is when you have HIT symptoms one to three days after you start taking heparin. It happens if you've taken heparin in the last three months, you have HIT antibodies, and then you're exposed to heparin again.

Refractory (persistent) HIT 

With this, your HIT symptoms can last for weeks after you've stopped heparin.

Delayed onset HIT

This is when you don't have symptoms until four to five days after you stop taking heparin.

When HIT happens, it's usually five days to two weeks after your first dose of heparin.

It can cause your body to form dangerous blood clots, such as:

  • Deep-vein thrombosis (DVT), a clot that forms in your leg and travels to your lungs
  • Pulmonary embolism (PE), a clot that travels to your lung from a different part of your body

These things happen in about 50% of people with HIT.

Clots in your legs, lungs, or heart can cause:

Other signs of HIT include:

  • Darkening or bruising of your skin where you get your heparin shot
  • Toes, fingers, nose, or nipples that look black or blue
  • Fever
  • Chills

Less often, HIT can put you at risk for heart attacks or strokes.

HIT isn't common. About 5% of people who take heparin for more than four days develop it. But some factors that increase your risk are:

  • Being female
  • Being older than 40

Some research suggests the type of heparin you take can impact your risk of HIT. But more studies are needed to know for sure.

Even though taking heparin can cause HIT, you still may need to take it as part of your treatment for certain health conditions. Those can include cardiovascular issues and diseases. You might also need it when you have medical procedures such as:

  • Orthopedic surgery
  • Heart surgery, such as cardiopulmonary bypass

Gold standard. The serotonin release assay (SRA) is a blood test considered by experts to be the gold standard for HIT diagnosis. It checks whether you have heparin-dependent platelet antibodies in your blood. But the SRA test is usually done only to confirm a HIT diagnosis after other blood tests indicate you may have the condition.

Other blood tests. These look at several factors, including your platelet count and whether your blood is forming clots. These test results can indicate that you may have HIT. They let your doctor know about your:

  • Platelet count
  • Levels of PF4 antibody in your blood

4Ts Test. Your doctor may use a scoring system called the4Ts test to see if you have symptoms of new blood clots, or if your symptoms look like they could be caused by HIT. The higher your score, the more likely you are to have HIT. The score takes into account:

  • How low your platelet count is
  • How quickly you reacted to heparin
  • If you have signs of clotting
  • Whether something other than heparin is causing your low platelets

Symptoms of HIT can sometimes look similar to those of other health issues, such as a blood clot condition called vaccine-induced immune thrombotic thrombocytopenia (VITT). You can get this after you get some vaccines. Your doctor will rule out other conditions before diagnosing you with HIT.

Your HIT treatment usually starts with stopping heparin and switching to a different blood thinner. Your platelet count will usually start increasing in a few days. In the meantime, your doctor will give you an alternative to heparin to prevent blood clots.

Those may include:

  • Direct thrombin inhibitors (argatroban, bivalirudin, lepirudin), which are given through your veins
  • Fondaparinux(Arixtra), a blood-thinning drug you get as a shot
  • Direct oral anticoagulants medications, such as apixaban (Eliquis), dabigatran (Pradaxa), edoxaban (Savaysa), and rivaroxaban (Xarelto)

In addition to those blood thinners, you may also get a high dose of something known as intravenous immunoglobulin. That's a part of your blood's plasma that can help make your immune system stronger.

Your doctor will keep an eye on your platelet count. After a few days, once it returns to a safe range, they might switch you to another blood thinner, warfarin (Coumadin, Jantoven). You'd take this by mouth.

Until recently, HIT was fatal for about 20% of people with the condition. But advances in treatments have dropped the death rate to 6% to 10%. Seek medical help as fast as possible if you think you have HIT.

If you have HIT,  your doctor may monitor your health and progress following your recovery. Having HIT once doesn't mean you have an intolerance to heparin. The antibodies that caused your original reaction disappear from your body naturally about three months after treatment. You may not have the same reaction again.

To help make sure you don't get HIT again, talk with your doctor before you take heparin or other blood thinners. It's a good idea to share the details of when you had HIT, such as:

  • When it happened
  • Which symptoms and reactions you had

What is the difference between HIT and DIC?

Both HIT and disseminated intravascular coagulation (DIC) are health conditions that involve abnormal clotting and bleeding. But HIT is caused by heparin exposure, and DIC is caused by various underlying illnesses. Another difference is in how they begin. HIT tends to happen more quickly, and DIC happens gradually. Most people with HIT don't have DIC, but severe HIT may trigger DIC.

How to reverse heparin-induced thrombocytopenia

Stop taking heparin and switch to a different blood thinner. Your doctor will decide if you need more treatment.

How long does it take to recover from heparin-induced thrombocytopenia?

Your platelet counts will usually return to normal within a week of stopping heparin. But antibodies responsible for HIT can stay in your blood for several weeks, even after your platelet counts have recovered, increasing your risk of thrombosis.