T3RU test
The T3RU test measures the level of proteins that carry thyroid hormone in the blood. This can help your health care provider interpret the results of T3 and T4 blood tests.
T3
Triiodothyronine (T3) is a thyroid hormone. It plays an important role in the body's control of metabolism (the many processes that control the rate...
Read Article Now Book Mark ArticleT4
T4 (thyroxine) is the main hormone produced by the thyroid gland. A laboratory test can be done to measure the amount of free T4 in your blood. Fre...
Read Article Now Book Mark ArticleBecause tests called the free T4 blood test and thyroxine binding globulin (TBG) blood tests are now available, the T3RU test is rarely used these days.
Thyroxine binding globulin
The TBG blood test measures the level of a protein that moves thyroid hormone throughout your body. This protein is called thyroxine-binding globuli...
Read Article Now Book Mark ArticleHow the Test is Performed
A blood sample is needed.
Blood sample
Venipuncture is the collection of blood from a vein. It is most often done for laboratory testing.
Read Article Now Book Mark ArticleHow to Prepare for the Test
Your provider will tell you if you need to stop taking any medicines before the test that may affect your test result. Do not stop taking any medicine without first talking to your provider.
Some medicines that can increase T3RU levels include:
- Anabolic steroids
- Heparin
- Phenytoin
- Salicylates (high dose)
- Warfarin
Some medicines that can decrease T3RU levels include:
- Antithyroid medicines
- Birth control pills
- Estrogen
- Thiazides
- Opioids and methadone
Pregnancy can also decrease T3RU levels.
These conditions can decrease TBG levels (see below section Why the Test is Performed for more about TBG):
- Serious illness
- Kidney disease when protein is lost in the urine (nephrotic syndrome)
Other medicines that bind to protein in the blood can also affect test results.
How the Test will Feel
When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.
Why the Test is Performed
This test is done to check your thyroid function. Thyroid function depends on the action of many different hormones, including thyroid-stimulating hormone (TSH), T3, and T4.
This test helps check the amount of T3 that TBG is able to bind. TBG is a protein that carries most of the T3 and T4 in the blood.
Your provider may recommend a T3RU test if you have signs of a thyroid disorder, including:
-
Overactive thyroid (hyperthyroidism)
Hyperthyroidism
Hyperthyroidism is a condition in which the thyroid gland makes too much thyroid hormone. The condition is often called overactive thyroid.
Read Article Now Book Mark Article -
Underactive thyroid (hypothyroidism)
Hypothyroidism
Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone. This condition is often called underactive thyroid....
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Muscle weakness caused by high levels of thyroid hormone in the blood (thyrotoxic periodic paralysis)
Thyrotoxic periodic paralysis
Thyrotoxic periodic paralysis (TPP) is a condition with episodes of severe muscle weakness. It occurs in people who have high levels of thyroid horm...
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Normal Results
Normal values range from 25% to 38%.
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your provider about the meaning of your specific test results.
What Abnormal Results Mean
Higher-than-normal levels may indicate:
- Kidney failure
Kidney failure
Acute kidney failure is the rapid (less than 2 days) loss of your kidneys' ability to remove waste and help balance fluids and electrolytes in your b...
Read Article Now Book Mark Article - Overactive thyroid (hyperthyroidism)
- Nephrotic syndrome
Nephrotic syndrome
Nephrotic syndrome is a group of symptoms and abnormal test results that include protein in the urine, low blood protein levels in the blood, high ch...
Read Article Now Book Mark Article - Protein malnutrition
Protein malnutrition
Kwashiorkor is a form of malnutrition that occurs when there is not enough protein in the diet.
Read Article Now Book Mark Article - Chronic liver disease
Lower-than-normal levels may indicate:
- Acute hepatitis (liver disease)
- Pregnancy
- Hypothyroidism
- Use of estrogen, opioids or methadone
Abnormal results may also be due to an inherited condition of high TBG levels. Usually thyroid function is normal in people with this condition.
This test may also be done for:
- Chronic thyroiditis (swelling or inflammation of the thyroid gland, including Hashimoto disease)
Chronic thyroiditis
Chronic thyroiditis is caused by a reaction of the immune system against the thyroid gland. It often results in reduced thyroid function (hypothyroi...
Read Article Now Book Mark Article - Drug-induced hypothyroidism
- Graves disease
Graves disease
Graves disease is an autoimmune disorder that leads to an overactive thyroid gland (hyperthyroidism). An autoimmune disorder is a condition that occ...
Read Article Now Book Mark Article - Subacute thyroiditis
Subacute thyroiditis
Subacute thyroiditis is an immune reaction of the thyroid gland that often follows an upper respiratory infection. The thyroid gland is located in th...
Read Article Now Book Mark Article - Thyrotoxic periodic paralysis
- Toxic nodular goiter
Toxic nodular goiter
Toxic nodular goiter involves an enlarged thyroid gland. The gland contains areas that have increased in size and formed nodules. One or more of th...
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Risks
There is little risk involved with having your blood taken. Veins and arteries vary in size from one person to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight, but may include:
- Excessive bleeding
- Fainting or feeling lightheaded
- Multiple punctures to locate veins
- Blood buildup under the skin (hematoma)
Hematoma
A bruise is an area of skin discoloration. A bruise occurs when small blood vessels break and leak their contents into the soft tissue beneath the s...
Read Article Now Book Mark Article - Infection (a slight risk any time the skin is broken)
Reviewed By
Sandeep K. Dhaliwal, MD, board-certified in Diabetes, Endocrinology, and Metabolism, Springfield, VA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
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Salvatore D, Cohen R, Kopp PA, Larsen PR. Thyroid pathophysiology and diagnostic evaluation. In: Melmed S, Auchus RJ, Golfine AB, Koenig RJ, Rosen CJ, eds. Williams Textbook of Endocrinology. 14th ed. Philadelphia, PA: Elsevier; 2020:chap 11.
Wassner AJ, Smith JR. Thyroid development and physiology. In: Kliegman RM, St. Geme JW, Blum NJ, et al, eds. Nelson Textbook of Pediatrics. 22nd ed. Philadelphia, PA: Elsevier; 2025:chap 601.