Because the free T4 blood test and TBG blood tests are available, the T3RU test is rarely used now.
How the Test is Performed
A blood sample is needed.
How to Prepare for the Test
Your health care provider will tell you if you need to stop taking medicines that may affect the test result.
Drugs that can increase T3RU levels include:
- Anabolic steroids
- Salicylates (high dose)
The following can increase thyroxin binding globulin (TBG) levels:
- Male hormones (androgens)
- Serious illness
- Kidney disease
Drugs that can decrease T3RU levels include:
- Antithyroid medications
- Birth control pills
Pregnancy can also decrease T3RU levels.
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 see how much thyroxin binding globulin (TBG) is available. TBG is a protein that carries most of the T3 and T4 in the blood.
Your doctor may order this test if you have signs of a thyroid disorder, including:
Normal values range from 24 - 37%.
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
What Abnormal Results Mean
Higher-than-normal levels may indicate:
Lower-than-normal levels may indicate:
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:
Veins and arteries vary in size from one patient 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 light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
Resin T3 uptake; T3 resin uptake; Thyroid hormone-binding ratio
Guber HA, Farag AF. Evaluation of endocrine function. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 24.
Salvatore D, Davies TF, Schlumberger MJ, et al. Thyroid physiology and diagnostic evaluation of patients with thyroid disorders. In: Melmed S, Polonsky KS, Larsen PR, et al., eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 11.
Update Date 5/10/2014
Updated by: Brent Wisse, MD, Associate Professor of Medicine, Division of Metabolism, Endocrinology & Nutrition, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.