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T3RU test

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

 

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.

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
  • 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. 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:

  • Hyperthyroidism
  • Hypothyroidism
  • Thyrotoxic periodic paralysis

 

Normal Results

 

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:

  • Kidney failure
  • Overactive thyroid (hyperthyroidism)
  • Nephrotic syndrome
  • Protein malnutrition

Lower-than-normal levels may indicate:

  • Acute hepatitis
  • Pregnancy
  • Underactive thyroid (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 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

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.

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            Review Date: 5/10/2014

            Reviewed 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.

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