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Definition:

The T3RU test measures the level of proteins that carry thyroid hormone in the blood.



Alternative Names:

Resin T3 uptake; T3 resin uptake; Thyroid hormone-binding ratio



How the test is performed:

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.



How to prepare for the test:

Your health care provider will instruct you, if necessary, to stop taking drugs that may interfere with the test.

Drugs that can increase T3RU values include:

  • Anabolic steroids
  • Heparin
  • Phenytoin
  • Salicylates (high dose)
  • Warfarin

The following can increase thyroxin binding globulin (TBG) levels:

Drugs that can decrease T3RU values 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 as part of an evaluation of thyroid function. Your doctor may order this test if you have signs of a thyroid disorder. Thyroid function is complex and depends on the action of many different hormones, including thyroid-stimulating hormone (TSH), T3, and T4.

This test helps estimate the availability of thyroxin binding globulin (TBG). This is the protein that carries most of the T3 and T4 in the blood. The higher the level of TBG , the lower the value of T3RU. A higher T3RU value means less TBG is available, possibly as a result of hyperthyroidism .



Normal Values:

Normal values range from 24 - 37%.

Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.



What abnormal results mean:

Greater-than-normal levels may indicate:

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 these people.

Other conditions under which the test may be performed:



What the risks are:

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:

AACE Thyroid Task Force. American Association of Clinical Endocrinologists Medical Guidelines for Clinical Practice for the Evaluation and Treatment of Hyperthyroidism and Hypothyroidism. Endocr Pract. 2002;8(6) 459.

Ladenson P, Kim M. Thyroid. In: Goldman L and Ausiello D, eds. Goldman: Cecil Medicine. 23rd ed. Philadelphia, Pa:Saunders; 2007:chap 244.




Review Date: 6/17/2008
Reviewed By: Elizabeth H. Holt, MD, PhD, Assistant Professor of Medicine, Section of Endocrinology and Metabolism, Yale University. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.
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789 Central Avenue, Dover, NH 03820
Phone: (603) 742-5252
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