Triiodothyronine, or T3, is an important thyroid hormone that helps maintain muscle control, brain function and development, heart and digestive functions. High or low T3 levels may indicate an overactive or underactive thyroid.
According to the American Thyroid Association (ATA), T3 blood tests are most useful for diagnosing hyperthyroidism. They are less helpful in diagnosing hypothyroidism.
In this article, we will discuss what T3 is, the normal range for T3 levels, and what the results mean.
The thyroid is a small gland in the neck that produces hormones that regulate how the body stores and uses energy. The main hormone that the thyroid makes is thyroxine (T4). Cells in the body then convert T4 into T3.
Together with T4, T3 is responsible for many bodily functions, including heart rate, body temperature, and metabolism, which influences body weight. There are two forms of T3 in the blood:
Bound T3
This form of T3 is the most abundant. Bound T3 is attached to proteins that help circulate the hormone around the body.
Free T3
This type of T3 is not bound to proteins, and it moves freely around the body. It is less abundant than bound T3 but is the active form of the hormone.
Total T3 tests measure both forms of T3. Too much or too little T3 in the blood can indicate a health condition.
Doctors measure the amount of T3 in nanograms per deciliter (ng/dl) of blood. The University of Rochester Medical Center provide the following ranges for normal T3 levels:
| Test | Normal range |
| Total T3 | 75–195 ng/dl |
| Free T3 | 0.2–0.5 ng/dl |
Many factors influence T3 levels, including age, sex, and some preexisting health conditions, such as liver disease. Pregnancy can also increase total T3 levels. As such, an abnormal level of T3 does not always mean that a person has a thyroid condition.
Doctors will typically order T3 tests in conjunction with other thyroid tests to get a better understanding of how the thyroid is functioning.
A doctor may recommend a T3 test for anyone who has the symptoms of a thyroid condition. Thyroid conditions can cause a wide variety of symptoms, including:
- sweating
- tremors
- sensitivity to heat or cold
- tiredness
- irritability
- mood swings
- unexplained weight loss
- difficulty sleeping
- bulging or irritated eyes
- hair loss
- irregular or slower heartbeat
- shortness of breath
- high blood sugar
- increased frequency of urination
A doctor may also recommend the test for people who have a higher risk of thyroid disease. These people include those with:
- type 1 diabetes
- pernicious anemia
- primary adrenal insufficiency, or Addison’s disease
- a family history of thyroid conditions
Older adults and women who are pregnant or have recently given birth are also more at risk.
T3 tests usually require no preparation. However, some medications, such as birth control pills, can affect the results. Therefore, a person should inform a doctor of any medications that they are taking before undergoing T3 testing. They may need to stop taking them to ensure the accuracy of the test.
During a T3 test, a healthcare professional will select a vein to take blood from — either on the inside of the elbow or the back of the hand — and clean the area around it with antiseptic. They may use an elastic band to restrict the blood flow. Then, they will insert a needle into the vein and draw blood into a syringe.
Some people feel a pricking or scratching sensation during blood tests. At this point, if the healthcare professional used an elastic band, they will remove it. When they have collected enough blood, they will remove the needle and apply a cotton ball or bandage.
T3 testing generally does not cause side effects, but there is a slight risk of throbbing, soreness, or mild bruising at the injection site. Some people might also feel lightheaded during or just after the procedure.
You can also consider at-home thyroid test kits to measure T3 and other thyroid hormone levels.
High T3 levels can indicate several conditions, including hyperthyroidism and thyrotoxicosis (excess circulating hormones). In some cases, they can signify thyroid cancer, but this disease does not often cause abnormal T3 levels.
Thyrotoxicosis and thyroid cancer are rare, so hyperthyroidism is more commonly the cause.
In more than 70% of cases, hyperthyroidism is due to Graves’ disease, which occurs most often in females and can run in families. Nodules and lumps that grow on the thyroid can also cause hyperthyroidism. Doctors refer to this as toxic nodular goiter.
If a doctor believes that someone has hyperthyroidism, they will suggest suitable treatments, such as:
- antithyroid drugs, which reduce the thyroid’s ability to make hormones
- beta-blockers, which block the action of thyroid hormones
- radioactive iodine, which damages thyroid cells in a controlled way
- surgery in which a surgeon removes part or all of the thyroid gland
Low T3 test results can indicate an underactive thyroid, or hypothyroidism. Sometimes, low T3 can also indicate starvation.
According to the ATA, a person can develop an underactive thyroid due to autoimmune disease, or as a result of radiation treatment or thyroid surgery. Sometimes, getting too much or too little iodine also causes symptoms.
Treatment often includes levothyroxine medication, which replaces the hormones that the thyroid should produce. This drug controls the symptoms of hypothyroidism, but it does not cure the condition.
The ATA outline some other thyroid tests a doctor may order, including:
- Thyroid stimulating hormone (TSH): Changes in TSH levels can be an early warning sign of a thyroid condition, even if other thyroid hormone levels are still normal.
- Thyroxine (T4): T4 is the other major hormone that the thyroid produces alongside T3. High or low T4, especially in conjunction with unusual TSH levels, can indicate a thyroid condition.
- Thyroid antibody tests: Antibodies are part of the body’s immune defenses. However, some people develop thyroid conditions when the body mistakenly attacks the thyroid. Antibody tests can help a doctor diagnose this issue.
- Radioactive iodine uptake: This test involves the person swallowing a small amount of iodine. The thyroid uses iodine to make T4, so by scanning the thyroid, doctors can see how well the thyroid is taking up iodine.
Normal T3 levels indicate that someone’s thyroid is producing an adequate amount of T3 hormone. High levels can indicate hyperthyroidism, and low levels can be a sign of hypothyroidism.
However, other factors can also influence T3 levels, including certain medications and pregnancy. Due to this, a doctor may need to run additional tests to determine whether someone has a thyroid condition.
Treatment for thyroid conditions may involve medication and, in some cases, surgery.