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T3RU test
Definition
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. However, because the free T4 blood test is available, the T3RU test is rarely used anymore.
Alternative Names
Resin T3 uptake; T3 resin uptake; Thyroid hormone-binding ratio
How the Test is Performed
A blood sample is needed. For information on how this is done, see: Venipuncture
How to Prepare for the Test
Your health care provider will tell you, if needed, to stop taking drugs that may interfere with the test.
Drugs that can increase T3RU levels include:
- Anabolic steroids
- Heparin
- Phenytoin
- Salicylates (high dose)
- Warfarin
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
- Clofibrate
- Estrogen
- Thiazides
Pregnancy can also decrease T3RU levels.
How the Test Will Feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the Test is Performed
This test is done to check your thyroid function. Thyroid function is complex and 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.
The higher the level of TBG, the lower the level of T3RU. A higher T3RU level means less TBG is available. This may be caused by hyperthyroidism.
Your doctor may order this test if you have signs of a thyroid disorder, including:
- Hyperthyroidism
- Hypothyroidism - primary
- Hypothyroidism - secondary
- Thyrotoxic periodic paralysis
Normal Results
Normal values range from 24 - 37%.
The examples above are common measurements for results of these tests. 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:
- Kidney failure
- Overactive thyroid (hyperthyroidism)
- Nephrotic syndrome
- Protein malnutrition
Lower-than-normal levels may indicate:
- Acute hepatitis
- Pregnancy
- Underactive thyroid (hypothyroidism, primary hypothyroidism, or secondary hypothyroidism)
- Use of estrogen
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 (Hashimoto's disease)
- Drug-induced hypothyroidism
- Graves disease
- Subacute thyroiditis
- Thyrotoxic periodic paralysis
- Toxic nodular goiter
Risks
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)
References
Kim M, Ladenson P. Thyroid. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 233.
Salvatore D, Davies TF, Schlumberger MJ, Hay ID, Larsen PR. Thyroid physiology and diagnostic evaluation of patients with thyroid disorders. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 11.
Reviewed By: Shehzad Topiwala, MD, Chief Consultant Endocrinologist, Premier Medical Associates, The Villages, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.
