Written by Ada’s Medical Knowledge Team
Triiodothyronine (T3) is a hormone produced by the thyroid, a small gland that sits just below the Adam’s apple on the front of the neck.
T3 regulates several vital functions in the body. So if there’s a problem with T3 production, it can cause serious problems. Various tests can analyze T3 and help your doctor diagnose a thyroid condition or follow up on thyroid treatment.
What is T3?
T3 is an important hormone that is produced by the thyroid gland. Together with the other hormones produced by the thyroid (T4, rT3, and calcitonin), T3 is crucial for processes including metabolism, heart rate, growth, and development.
The thyroid only creates a small amount of T3. However, it produces T4 in higher quantities. T4 is inactive, but enzymes in the body can turn it into activated T3 or reverse T3 (rT3.) The conversion into active T3 happens in the kidneys, liver, muscles, and thyroid gland, while enzymes in the brain and placenta can turn T4 into rT3.
Together, these hormones also give feedback to the pituitary gland so that the right amount of thyroid-stimulating hormone (TSH) is produced, keeping the hormonal system in balance. 1 2 3
What is reverse T3?
rT3 is an inactive form of T3, which is structurally almost identical to active T3. It’s normal for the body to produce some rT3, which it rapidly degrades. Usually, rT3 is not clinically useful for determining whether or not a person has thyroid problems. 4
What does T3 do?
When the thyroid gland releases T3 hormone, it causes a general increase in cell metabolism. It also leads to: 3
- Increased heart rate and blood volume
- Heat production
- Extra oxygen delivery to the tissues
- The promotion of skeletal development
- Increase in wakefulness and alertness
- The regulation of reproductive hormones in men and women
- Increased gastrointestinal function
- Regulation of pituitary function
There are 2 forms of T3 present in the body:
- Free T3, which circulates freely in the body and isn’t attached to anything.
- Bound T3, which is bound to a protein.
Only the free form of T3 affects bodily processes. 5 6
What is a T3 blood test?
A T3 blood test can help assess your thyroid function. When your doctor suspects a thyroid condition, TSH is usually tested first, after which free T3 is checked in combination with free T4.
The results of these tests can help to diagnose thyroid disorders or measure the effectiveness of ongoing thyroid treatment. 5 6
T3 can be measured with various tests that all require a blood sample.
- The free T3, or FT3, test only measures the amount of free T3.
- The total T3 test measures both free and bound T3 in the blood.
- The T3 uptake test, also called T3 resin uptake (T3RU), measures the amount of protein in the blood that carries thyroid hormone.
If T3 needs to be tested, the FT3 test is usually the go-to. The normal range of free T3 levels is between 3.1 and 6.8 pmol/L. 5 6 7
Why are T3 levels important?
T3 blood work is mainly used in diagnosing hyperthyroidism, which is a condition where the thyroid gland is overactive, leading to an excess of thyroid hormone in the body. Your doctor might check your T3 levels if you have abnormal TSH levels or when you are experiencing symptoms of hyperthyroidism such as: 5 8
- Anxiety and irritability
- Intolerance of heat
- Swelling of the thyroid gland
- Racing heart rate
- Weight loss
- Reduced libido
An elevated amount of free or total T3 can be caused by several conditions such as: 9 10
- Graves’ disease
- Thyroid nodules
- Thyroid cancer
- Exposure to environmental chemicals, especially toxic metals
A few medications can also cause a rise in T3 hormone as a side effect. For example, a medication used to treat hypothyroidism that is not regulated well can cause an increase in T3 beyond the normal range. 11
An increase in T3 can also occur naturally during pregnancy. Thyroid hormone levels rise most during the first trimester of the pregnancy and decline afterward. T3 levels usually return to their normal range 1 to 2 months after giving birth. 12
Low T3 and low T3 uptake levels can point to hypothyroidism. This condition means the thyroid is less active than usual, resulting in lower levels of thyroid hormones in the blood. 13
Q: What does T3 mean?
A: T3 stands for triiodothyronine. It’s an important hormone produced by the thyroid gland.
Q: How to increase T3 levels?
A: Your doctor may suggest you take T3 medication or change your diet to increase your T3 levels.
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Armstrong M. et al. (2022). Physiology, Thyroid Function. Accessed on July 4, 2022.
Gomes-Lima C. et al. (2019). Can Reverse T3 Assay Be Employed to Guide T4 vs. T4/T3 Therapy in Hypothyroidism?. Accessed on July 4, 2022.
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Hashemipour S. et al. (2022).Wide spectrum of thyroid function tests in COVID-19: from non-thyroidal illness to hyperthyroxinemia. Accessed on July 4, 2022.
NHS (2019). Symptoms overactive thyroid (hyperthyroidism). Accessed on July 4, 2022.
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De Leo S. et al. (2016). Hyperthyroidism. Accessed on July 4, 2022.
Trohman R. et al. (2018). Amiodarone and thyroid physiology, pathophysiology, diagnosis and management. Accessed on July 4, 2022.
Zhang D. et al. (2019). Trimester-specific reference ranges for thyroid hormones in pregnant women. Accessed on July 7, 2022.
NHS (2021). Diagnosis underactive thyroid (hypothyroidism). Accessed on July 4, 2022.