This test, which involves a small amount of a radioactive substance injected intravenously, measures the blood flow to your heart when you’re resting and active.

A thallium stress test is an imaging test that indicates how well blood flows into your heart while you’re exercising or at rest. It’s also called a nuclear stress test.

During the procedure, a small amount of thallium, a radioactive tracer, is administered into a vein on your arm. The tracer is a dye that makes your blood flow visible to a special camera called a gamma camera. This camera can reveal any issues your heart muscle may be having.

Flat metal disks called electrodes are placed on your chest, arms, and legs and connected to an electrocardiogram (EKG) machine that records your heart’s electrical activity.

A doctor may order a thallium test for a variety of reasons, including:

  • if they suspect your heart isn’t getting enough blood flow when it’s under stress — for example, when you exercise
  • if you have chest pain or worsening angina
  • if you’ve had a heart attack
  • to check how well medications are working
  • to determine whether a procedure or surgery was successful
  • to determine whether your heart is healthy enough to start an exercise program

The thallium stress test can show:

  • the size of your heart chambers
  • how effectively your heart pumps (ventricular function)
  • how well your coronary arteries supply your heart with blood, known as myocardial perfusion
  • if your heart muscle is damaged or scarred from previous heart attacks

A thallium stress test is different from an exercise stress test, which only uses electrodes connected to an EKG to record your heart’s electrical activity as you exercise.

A thallium stress test is also different from a stress echocardiography, during which electrodes are connected to an electrocardiograph (ECG) that uses ultrasound to provide images of your heart as you exercise.

A thallium stress test must be done at a hospital, medical center, or doctor’s office. The test usually takes about 3 to 4 hours.

A nurse or healthcare professional inserts an intravenous (IV) line, usually on the inside of your elbow. Thallium is injected through the IV. This radioactive tracer dyes your blood flow so it can be detected by the gamma camera.

The test includes exercise and resting portions, and your heart is photographed during both. The doctor administering your test will determine the order in which these tests are performed. You’ll receive an injection of thallium before each portion.

Resting portion

During this part of the test, you lie down for 15 to 45 minutes while the radioactive tracer works its way through your bloodstream to your heart.

You then lie down on an exam table with your arms above your head, and a gamma camera above you takes pictures.

Exercise portion

In the exercise portion of the test, electrodes connected to an EKG machine are applied to your chest, arms, and legs.

You then walk on a treadmill or pedal an exercise bicycle. Most likely, the doctor will ask you to start slowly and progressively pick up the pace into a jog. You may need to run on an incline to make it more challenging.

If you’re unable to exercise, the doctor will give you a medication that stimulates your heart and makes it beat faster. This simulates how your heart would act during exercise.

The gamma camera records pictures that show the flow of blood through your heart. Your blood pressure and heart rhythm are monitored while you exercise.

Once your heart is working as hard as it can, you’ll receive another thallium injection and wait 15 to 40 minutes for your heart to absorb the tracer. You’ll then resume exercising.

The doctor will compare these pictures with the set of resting images to evaluate how weak or strong the blood flow to your heart is.

Wear comfortable clothes and shoes for exercising.

You’ll probably need to fast after midnight the night before the test or at least four hours before the test. Fasting can prevent you from getting sick during the exercise portion.

Twenty-four hours before the test, you’ll need to avoid all caffeine, including tea, soda, coffee, chocolate — and even decaffeinated coffee and drinks, which have small amounts of caffeine — and certain pain relievers. Drinking caffeine can cause your heart rate to be higher than it normally would be.

The doctor will need to know all medications that you’re taking. This is because some medications — like ones that treat asthma — can interfere with your test results.

Your doctor will also want to know if you’ve taken any erectile dysfunction medication including sildenafil (Viagra), tadalafil (Cialis), or vardenafil (Levitra) 24 hours before the test.

Most people tolerate the thallium stress test very well. Some people may experience headache, nausea, or a racing heart.

The radioactive material is removed from your body through your urine. Drinking a lot of water may help make it leave faster.

There’s a low level of radiation exposure during a thallium stress test. Because even low radiation levels may be harmful, the American Heart Association recommends not taking a thallium stress test if you’re at low risk for a heart attack or have no other symptoms besides chest discomfort.

If you’re pregnant or nursing, you should not have a thallium stress test because it could harm your baby.

Rare complications from the test may include:

Alert the test administrator if you experience any of these symptoms during your test.

Results depend on the reason for the test, how old you are, your history of heart problems, and other medical issues.

Normal results

A normal result means blood flowing through the coronary arteries in your heart is normal.

Abnormal results

Abnormal results may indicate:

  • reduced blood flow to part of your heart caused by narrowing or blockage of one or more arteries that supply your heart muscle
  • scarring of your heart muscle due to a previous heart attack
  • heart disease
  • a too-large heart, indicating other heart complications

The doctor may need to order more tests to determine whether you have a heart condition. The doctor will develop a treatment plan specifically for you, based on the results of this test.

A thallium stress test uses a low amount of radioactive material to provide information about your heart health and detect some heart conditions.

Although this test is generally safe, it’s not recommended if you have a low risk for a heart attack or if you’re pregnant or nursing.