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 your thyroid. It has a variety of functions in your body, like regulating your heart rate, body temperature, and metabolism .
The other main hormone made by your thyroid is T4, which acts as a precursor to T3. Your cells 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 your body .
Tests that measure the amount of T3 in your blood are often used to evaluate the health of your thyroid. Abnormal T3 levels may indicate several issues, like thyroid disorders, pituitary disorders, or cancer. Two common thyroid disorders are :
- 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. These three hormones give doctors a full picture of your thyroid function and help diagnose various thyroid diseases .
You can find out much more about T3 and T4 in our thyroid hormones article here.
Types of T3
Not all of your T3 is identical. There are several different forms of T3 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 your 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 your 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. We’ll explain more about testing these two types of T3 in the next section.
Reverse T3 (rT3) is also a metabolite of T4 but has a slightly different chemical structure than normal T3. Your body will sometimes convert T4 into rT3 instead of T3, especially when dealing with a serious illness .
T3 Test Types
A total T3 test will measure the total amount of T3 in your 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 .
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 .
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 .
For all of these tests, a medical professional will take a blood sample from a vein in your arm.
Total T3 is usually measured in nanograms per deciliter (ng/dL). Free T3 is measured in picograms per milliliter (pg/mL).
Normal T3 Levels
For total T3, the normal range in adults is 71 – 180 ng/dL .
For free T3, the normal range in adults is 2.0 – 4.4 pg/mL .
These ranges, however, can vary greatly depending on the laboratory performing the test. Your doctor will let you know if your results are abnormal or not.
It’s also important to note that normal T3 levels don’t necessarily mean your thyroid is healthy. 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].
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 is :
- 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 .
This post is part of a series about thyroid hormones. Read the other parts to learn about:
- Causes & Health Risks of High T3 Levels
- All You Need to Know About Thyroid Hormones (T3 & T4)
- Top 12 Surprising Factors That Decrease Thyroid Hormones
- Low T3 / Euthyroid Sick Syndrome Symptoms & Causes
- Reverse T3 Test: Low & High Levels + How to Fix them
- TSH Blood Test: Normal Levels & Range
Irregular T3 Levels?
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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.