What is a TSI test?

The TSI test measures the level of thyroid-stimulating immunoglobulin (TSI) in your blood. High levels of TSI in the blood can indicate the presence of Graves’ disease, which is an autoimmune disorder that affects the thyroid gland.

If you have Graves’ disease, you’re more likely to develop other autoimmune diseases such as type 1 diabetes or Addison’s disease. Women are 7 to 8 times more likely to develop Graves’ disease than men. Rarely, the TSI test can be used to diagnose other disorders that affect the thyroid, such as Hashimoto’s thyroiditis and toxic multinodular goiter.

Your doctor may order a TSI test if you have signs of hyperthyroidism or if you’re pregnant and have a history of thyroid problems.

The thyroid is an endocrine gland. It’s located at the base of your neck. Your thyroid is responsible for the production of various thyroid hormones that help your body regulate metabolism and other important functions.

Several conditions can cause your thyroid to produce too much of the thyroid hormones T3 and T4. When this occurs, it’s known as hyperthyroidism. Hyperthyroidism can lead to a range of symptoms, including:

  • fatigue
  • weight loss
  • restlessness
  • tremors
  • palpitations

When hyperthyroidism suddenly worsens, it’s known as thyroid storm, which is a life-threatening condition. This occurs when there is a surge of thyroid hormone in the body. Usually, it occurs due to untreated or undertreated hyperthyroidism. This is a medical emergency that requires immediate attention.

“Thyrotoxicosis” is an older term for hyperthyroidism due to any cause.

Graves’ disease is one of the most common causes of hyperthyroidism. If you have Graves’ disease, your immune system mistakenly produces the antibody TSI. TSI mimics thyroid-stimulating hormone (TSH), which is the hormone that signals your thyroid to produce more T3 and T4.

TSI can trigger your thyroid to produce more thyroid hormones than necessary. The presence of TSI antibodies in your blood is an indicator that you may have Graves’ disease.

Learn more: Graves’ disease »

Diagnosis of Graves’ disease

Your doctor will typically order a TSI test if you are showing signs of hyperthyroidism and they suspect that you might have Graves’ disease. Graves’ disease is the most common cause of hyperthyroidism. This test can help clarify the cause of your symptoms when your TSH, T3, and T4 levels are abnormal.

In pregnant women

Your doctor may also perform this test during pregnancy if you have symptoms of hyperthyroidism or a history of thyroid problems. Graves’ hyperthyroidism affects about 2 out of 1000 pregnancies.

If you have Graves’ disease, the TSI in your bloodstream can cross the placenta. Those antibodies can interact with your baby’s thyroid and result in a condition called “transient neonatal Graves’ thyrotoxicosis.” This means that although your baby will be born with Graves’ disease, it’s treatable, temporary, and will pass after the excess TSI leaves your baby’s body.

Diagnosis of other diseases

Other disorders related to abnormal TSI levels include Hashimoto’s thyroiditis and toxic multinodular goiter. Also called chronic lymphocytic thyroiditis, Hashimoto’s thyroiditis is inflammation and swelling of the thyroid gland. It typically decreases the function of the thyroid gland, causing hypothyroidism. In toxic multinodular goiter, your thyroid gland is enlarged and has a number of small, round growths, or nodules, that produce too much thyroid hormone.

This test doesn’t generally require any preparation, such as fasting or stopping medications. However, if your doctor asks you to do so, follow their instructions. They may want to draw blood for other tests that require fasting at the same time as your TSI test.

When you arrive for the procedure, a healthcare provider will take a sample of your blood. They’ll send your blood sample to a laboratory, where it will be tested to determine your TSI level.

Normal results

TSI test results are in the form of a percentage or TSI index. Usually, a TSI index of less than 1.3, or 130 percent, is considered normal. Your doctor might have different standards, so you should ask your doctor if you have any questions or concerns.

It’s possible for you to have an autoimmune disorder despite having a normal TSI test result. If your doctor suspects that the antibodies might develop over time, as is the case with some autoimmune disorders, then repeat testing at a later date may be necessary.

Abnormal results

If you have elevated TSI levels, it might indicate that you have:

  • Graves’ disease
  • hashitoxicosis, which is increased thyroid activity due to inflammation related to Hashimoto’s thyroiditis
  • neonatal thyrotoxicosis, in which your baby has high levels of thyroid hormones at birth because of your high levels of thyroid hormones

With treatment, neonatal thyrotoxicosis in your baby will pass.

If TSI is present in the blood, it’s often an indication of Graves’ disease.

Every blood test has some risks, which include the following:

  • minor pain during and shortly after the procedure
  • slight bleeding after the healthcare provider removes the needle
  • the development of a small bruise in the area of the puncture site
  • an infection in the area of the puncture site, which is rare
  • inflammation of the vein in the area of the puncture site, which is rare