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.
The thyroid gland produces two main thyroid hormones, called T3 and T4. These thyroid hormones circulate through the blood and have various important roles in the body, such as controlling 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 is the free form, which is not attached to a protein but does have biological activity .
The large majority of 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 free T4 levels [1, 2].
For a more in-depth breakdown of thyroid hormones, check out our article here.
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 to 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 a doctor will perform a T3 uptake test. Modern tests can directly measure free T4, making this test obsolete for the most part.
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 adults, the normal range for T3 uptake is about 25-35% .
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) .
A number of factors may cause a low T3 uptake result. Test results should be interpreted by a doctor who can take into account your medical history and the results of other tests.
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 potential 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 may 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 
Another way T3 uptake can be low is if thyroid hormone levels are low. If T3 and T4 levels are low, then there’s less available to bind to TBG, which means a lower percentage of bound TBG. When the 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
A number of factors may cause a high T3 uptake result. Test results should be interpreted by a doctor who can take into account your medical history and the results of other tests.
Less TBG in the blood means there is less TBG available to bind to T3 and T4. This means most of the TBG in the body would be used up by 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 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 the 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
What happens if 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 .