There is only a small amount of T3 in your body, but it does a whole lot. This thyroid hormone controls your metabolism, heart rate, and body temperature, to name a few. Learn about the different types of T3 tests and what levels are considered normal.
What is T3?
T3, short for triiodothyronine, is a hormone that is produced by the thyroid. It has a variety of functions in the body, such as regulating heart rate, body temperature, and metabolism [1].
The other main hormone made by the thyroid is T4, which acts as a precursor to T3. Cells in the body can convert T4 into T3 as needed. While T4 does have some biological activity, T3 is about four times more active and does most of the work in the body [1].
Tests that measure the amount of T3 in the blood are often used to evaluate the health of the thyroid. Abnormal T3 levels may indicate several issues, like thyroid disorders, pituitary disorders, or cancer. Two common thyroid disorders are [2]:
- Hypothyroidism, where the thyroid does not produce enough thyroid hormones
- Hyperthyroidism, where the thyroid produces too much thyroid hormones
T3 tests are commonly performed alongside T4 and TSH tests. TSH is another hormone that stimulates the release of both T3 and T4. Testing the levels of these three hormones give doctors a full picture of thyroid function, which helps in the diagnosis of various thyroid diseases [2].
Find out much more about T3 and T4 in our thyroid hormones article here.
Types of T3
There are several different forms of T3 in the body that are found in varying concentrations and have different functions.
Bound T3 is the most common type of T3. Over 99% of the T3 in the body is found in this form. As the name suggests, bound T3 is attached to proteins, which help transport it in the bloodstream. Despite being the most common form, bound T3 is inactive [1, 3].
Free T3 (fT3) only accounts for 0.3% of the total amount of T3 in the body. However, this is the active form and is responsible for the biological effects of T3. The downside is that free T3 has a shorter half-life in the blood because it is not bound to proteins [1, 3].
Both types of T3 can be tested, although free T3 is typically more clinically useful.
Reverse T3 (rT3) is also a metabolite of T4 but has a slightly different chemical structure than normal T3. The body will sometimes convert T4 into rT3 instead of T3, especially during serious illness [4].
T3 Test Types
Total T3
A total T3 test will measure the total amount of T3 in the blood, including both bound and free T3. Total T3 levels can vary depending on the amount of binding proteins available, which is influenced by many different factors (like liver disease, malnutrition, and pregnancy). This variability makes total T3 less useful for diagnosing thyroid disorders than free T3 [5].
Free T3
A free T3 test only measures the amount of free T3 in the blood. Typically, the free T3 test is used to evaluate thyroid function because it is unaffected by variations in binding protein levels. However, because the amount of free T3 in the body is so tiny, it can be harder to get an accurate measurement of free T3 compared to total T3 [5].
T3 Uptake
The T3 uptake test (or T3 resin uptake test) doesn’t actually measure T3. Instead, it measures the binding ability of a protein called TBG, which indirectly helps determine free T3 and T4 levels. However, this test is used much less frequently now that free T3 and T4 levels can be directly measured. Learn more in our article on the T3 uptake test [6].
For all of these tests, a medical professional will take a blood sample from a vein in the arm.
Normal T3 Levels
Normal ranges can vary between laboratories due to differences in equipment, techniques, and chemicals used. If your results are outside of the normal range, it may not necessarily mean there is something wrong. However, a normal result also doesn’t mean a particular medical condition is absent. Always talk with your doctor to learn more about your test results.
For total T3, the normal range in adults is about 71 – 180 ng/dL [7].
For free T3, the normal range in adults is about 2.0 – 4.4 pg/mL [8].
Normal T3 levels don’t necessarily mean there are no issues with the thyroid. For example, some people with hypothyroidism have normal T3 levels. This is why T4 and TSH levels are also needed to fully evaluate thyroid function [1, 3].
During Pregnancy
T3 and T4 levels in pregnant women naturally increase because the developing baby uses the mother’s thyroid hormones [9, 10].
Due to their higher thyroid requirements, pregnant women with hypothyroidism usually need to increase their thyroid medication dose by as much as 50%. Even women who previously had normal thyroid function may experience hypothyroidism – or, in rare cases, hyperthyroidism – when they become pregnant [9, 10].
Currently, it’s not entirely clear what normal T3 levels look like during pregnancy. According to one study of 150 pregnancies, the normal range of free T3 in healthy pregnant women may be [11]:
- 1st trimester: 2.9 – 31 pg/mL
- 2nd trimester: 2.7 – 33.4 pg/mL
- 3rd trimester: 2.4 – 36.1 pg/mL
It is especially important for pregnant women to control their thyroid hormone levels. Uncontrolled thyroid dysfunction during pregnancy can lead to complications during childbirth and have negative effects on the baby. Find out more about the pregnancy risks in our article on the health risks of high T3 [12].
Takeaway
T3 tests are used to evaluate the function of your thyroid glands. High levels of T3 may indicate hyperthyroidism while low levels may be a sign of hypothyroidism.
However, T3 levels alone are typically not enough to diagnose a thyroid disorder. T3 tests are usually performed alongside tests that measure T4 and TSH to get a full picture of your thyroid health.
Learn More
This post is part of a series about thyroid hormones. Read the other parts to learn about:
