Thyroid disorders affect millions worldwide, but they can sometimes be difficult to diagnose. T3 uptake helps doctors calculate your levels of free thyroid hormone, which provides important information on thyroid function. Find out how this test works and what can cause abnormal results.
What is the T3 Uptake Test?
Overview of Thyroid Hormones
Before we talk about the T3 uptake test, we first have to talk about your thyroid hormones.
Your thyroid gland produces two main thyroid hormones, called T3 and T4. These thyroid hormones circulate through your blood and have various important roles in your body, like controlling your heartbeat, body temperature, and metabolism .
Both of these hormones can come in two forms. One is the bound form, where T3 or T4 is attached to a protein. This helps the hormones travel through the blood, but also inactivates them. The other form is free T3 or T4, which is not attached to a protein but does have biological activity .
The large majority of your thyroid hormones are in the bound form, but it’s typically more helpful to measure the free form because it’s the one that has a clinical effect. This is where T3 uptake testing comes in. This test provides a way to indirectly measure your free T4 levels [1, 2].
For a more in-depth breakdown of your thyroid hormones, check out our article here.
T3 Uptake Testing
The T3 uptake test, also called the T3 resin uptake test, indirectly measures the binding capacity of TBG. It does not measure T3, despite the name .
TBG, short for thyroxine-binding globulin, is a protein that can bind to T4 and T3. The T3 uptake test reports the percent of TBG that is bound thyroid hormones: a higher T3 uptake % means more TBG bound to T4 or T3. This test can be helpful when abnormalities in thyroid-binding proteins make total T4 levels misleading .
But T3 uptake % is not very useful by itself. The T3 uptake test must be performed alongside a total T4 test in order to calculate free T4, also called the free T4 index. A high free T4 index may indicate hyperthyroidism while low levels may point to hypothyroidism .
The T3 uptake test requires a blood sample, which a medical professional will usually take from the vein. Nowadays, however, it’s unlikely that your doctor will perform a T3 uptake test. Modern tests can directly measure free T4, making this test obsolete for the most part.
Normal T3 Uptake Levels
For adults, the normal range for T3 uptake is 25-35%. However, this range can vary depending on the lab performing the test .
As mentioned earlier, the T3 uptake % is not clinically useful without a total T4 test as well. The free T4 index can be calculated using T3 uptake and total T4. For reference, the normal range of free T4 index in adults is about 6-11 mcg/dL (this also varies depending on the lab performing the test) .
Causes of Low T3 Uptake
High TBG Levels
One of the main causes of low T3 uptake is a high amount of TBG in the blood. Why is this? Remember that T3 uptake measures the percent of TBG bound to thyroid hormones. If there is an excess amount of TBG, there will be more available to bind, which ultimately decreases the percent of T3 uptake .
Some causes of high TBG include:
- Pregnancy 
- Liver diseases (hepatitis B & C, liver scarring, liver cancer) [6, 7, 8, 9]
- Ulcerative colitis and Crohn’s disease 
- Thyroid cancer 
- Down’s syndrome 
- HIV 
Certain medications can also elevate TBG levels, including:
- Birth control 
- Growth hormone therapy 
- Bazedoxifene (Duavee) and Raloxifene (Evista), used to treat postmenopausal osteoporosis [16, 17]
- Tamoxifen (Nolvadex), used to treat breast cancer 
- Therapy for tuberculosis 
Low Thyroid Hormone Levels (Hypothyroidism)
Another way T3 uptake can be low is if your thyroid hormone levels are low. It makes sense; if you have low amounts of T3 and T4 then there’s less available to bind to TBG, which means a lower percentage of bound TBG. When your thyroid gland doesn’t produce enough thyroid hormone it is called hypothyroidism .
There are many different factors that can lead to hypothyroidism. Some of the most common causes include :
- Hashimoto’s disease, an autoimmune disorder
- Iodine deficiency
- Surgery or radiation to the thyroid or pituitary gland
Causes of High T3 Uptake
Low TBG Levels
If you have less TBG in your blood, there is less TBG available to bind to T3 and T4. This means most of your TBG would be used up by your thyroid hormones, which elevates T3 uptake results .
Some possible causes of low TBG include:
- Diabetes 
- Calorie restriction 
- Vitamin B3 
- Endurance exercise 
- Alcoholism 
- Genetic disorders 
- Polyhalogenated compounds (PHCs), found in cookware, cosmetics, and pesticides 
Certain medications may also decrease TBG levels, including:
- Propranolol (Inderal), used to treat high blood pressure and irregular heartbeat 
- Danazol (Danocrine), used to treat endometriosis and other conditions 
- Carbamazepine (Tegretol), used to treat nerve pain and epilepsy 
- Valproate (Depakote), used to treat seizures and bipolar disorder 
- Phenytoin (Dilantin), used to treat and prevent seizures 
High Thyroid Hormones Levels (Hyperthyroidism)
High levels of T4 and T3 mean more TBG is bound to thyroid hormones, which increases the T3 uptake percent. The most common cause for high thyroid hormone levels is hyperthyroidism, a condition where your thyroid gland releases too much hormone .
The most common causes of hyperthyroidism include :
- Graves’ disease, an autoimmune disease
- Goiter, an enlarged thyroid
- Thyroid tumors
- Inflammation of the thyroid
Effects of Abnormal T3 Uptake
What happens if your T3 uptake results are abnormal? By itself, abnormal T3 uptake does not generally cause any symptoms. Instead, any symptoms will be from an underlying cause, such as thyroid dysfunction or problems with TBG levels .
Additionally, an abnormal T3 uptake may or may not indicate a thyroid disorder. A total T4 test must also be performed to calculate the free T4 index. Once the free T4 index is known, your doctor can determine if there is possible hypothyroidism or hyperthyroidism .
Irregular T3 Levels?
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The T3 uptake test indirectly measures the amount of TBG that is bound to thyroid hormones. This test, in combination with a total T4 test, can help identify thyroid dysfunction. However, this test has been made obsolete by newer types of tests.