What Is a T3 Test?

Medically Reviewed by Sabrina Felson, MD on May 28, 2023
4 min read

Like many of the glands in your endocrine system, the thyroid gland produces various chemicals called hormones that control different bodily functions. This butterfly-shaped gland is located underneath your larynx towards the bottom of the neck and is responsible for regulating your metabolism through the release of hormones into the bloodstream. 

Throughout the day, your body may need different levels of thyroid hormones. The thyroid gland itself cannot detect these requirements but instead relies on your pituitary gland to signal how much thyroid hormone to release. The primary hormones in this loop include triiodothyronine (T3) and tetraiodothyronine (T4). ‌

Doctors can determine the health of your thyroid by performing a T3 test. If you have received an abnormal thyroid-stimulating hormone (TSH) test result, a T3 test is likely used as the next test to confirm that the thyroid is the problem. A combination of different thyroid test results is the best method for your doctor to discover the exact cause of your symptoms.

T3 represents only 5% of all hormones released by the thyroid. The remaining 95% consists of T4, which is then converted to T3 in the pituitary gland and the liver after traveling through your bloodstream. T3 generally exists in two forms: free and bound T3. 

‌Most of the T3 in your blood is attached — or bound — to proteins, and only a small percentage of it is free. A total T3 test measures both bound and free T3, whereas a free T3 test measures just free T3. Either of these tests is usually ordered after a TSH test comes back abnormal. 

A T3 test doesn’t usually require a lot of preparation such as overnight fasting. It’s actually recommended that you continue eating as normal in the days leading up to the test and that you avoid fasting because it can impact the results. 

While research hasn’t shown a link between T3 levels and fasting, it can influence thyroid-stimulating hormone (TSH) levels in your body. TSH is also a hormone often tested alongside T3 and T4. 

Before the test, you need to tell your doctor about any medications you’re taking, as some can impact thyroid hormone levels. Depending on how much they can affect the test results, you may be instructed to continue taking the medications or to stop altogether. Some of the medications that can impact the results of a T3 test include:‌

The procedure is only an ordinary blood test, where blood is taken from your arm using a small needle. It generally takes less than 5 minutes and has little risk involved. You may feel some pain or get some bruising after, but this will normally go away within hours.

High T3 levels. High levels of T3 can indicate hyperthyroidism, caused by Graves’ disease or even by thyroiditis and toxic nodular goiters. In rare cases, high T3 levels can show thyrotoxicosis or thyroid cancer. Hyperthyroidism is often treated with antithyroid medication, radioactive iodine therapy, or thyroidectomy (thyroid surgery).‌

Low T3 levels. Low T3 levels may point to hypothyroidism. Depending on your situation, this can be the result of different medications, recent thyroid surgery, radiation therapy, pregnancy, or iodine deficiency. Another cause of hypothyroidism is Hashimoto’s thyroiditis, which is an autoimmune disorder where your immune system attacks your thyroid gland.

If your doctor thinks your thyroid gland isn’t functioning as it should be, a T3 test can help determine the root cause. Here are some of the reasons that may lead to abnormal T3 test results.

Hypothyroidism. In hypothyroidism — also known as underactive thyroid — your thyroid gland is unable to produce enough hormones. This can have a negative impact on your metabolism, and over time, this condition can lead to heart disease, obesity, and joint pain. Some symptoms of hypothyroidism include:‌

  • A puffy face
  • Muscle weakness
  • A slower heart rate
  • ‌Temperature sensitivity
  • ‌Menstrual irregularities
  • Mood changes and depression
  • Constipation
  • Muscle and joint ache
  • Lack of energy‌‌

Hyperthyroidism. When your thyroid gland produces more thyroid hormones than you need, it is known as hyperthyroidism. This results in a faster metabolic rate — which is often characterized by an accelerated or irregular heartbeat and noticeable weight loss.

The most common disorder linked to an overactive thyroid is known as Graves' disease, but toxic nodular goiters and thyroiditis could also play a role. Some other symptoms that indicate hyperthyroidism include:

  • Anxiety‌
  • Insomnia
  • Frequent bowel movements
  • Temperature sensitivity 
  • Hair thinning
  • Hand tremors
  • Thyrotoxic periodic paralysis (TPP)
  • Increased appetite

On the basis of the results of the T3 test alone, your doctor can't narrow down exactly what is causing abnormalities in your thyroid. So, they would test your T4 and TSH levels as well.

Before the test, remember that some medications or supplements can affect your hormone levels, so always make sure you let your doctor know what you’ve taken recently before you get any of the thyroid tests done.