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TBG - blood test

Definition

The TBG blood test measures the level of a protein that moves thyroid hormone throughout your body. This protein is called thyroxine binding globulin (TBG).

Alternative Names

Serum thyroxine binding globulin; TBG level; Serum TBG level; Hypothyroidism - TBG; Hyperthyroidism - TBG; Underactive thyroid - TBG; Overactive thyroid - TBG

How the Test is Performed

A blood sample is taken and then sent to a laboratory for testing. Special tests called electrophoresis or radioimmunoassay are used to measure the TBG level in your blood.

How to Prepare for the Test

Certain drugs and medicines can affect test results. Your health care provider may tell you to stop taking a certain medicine for a short time before the test. Never stop taking any medicine without first talking to your provider.

These medicines and drugs can increase TBG level:

  • Estrogens, found in birth control pills and estrogen replacement therapy
  • Heroin
  • Methadone
  • Phenothiazines (certain antipsychotic drugs)

The following medicines can decrease TBG levels:

  • Depakote or depakene (also called valproic acid)
  • Dilantin (also called phenytoin)
  • High doses of salicylates, including aspirin
  • Male hormones, including androgens and testosterone
  • Prednisone

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 slight bruising. This soon goes away.

Why the Test is Performed

This test may be done to diagnose problems with your thyroid.

Normal Results

If electrophoresis is used, normal values may range from 10 milligrams per 100 milliliters (10 mg/100 mL) to 24 mg/100 mL.

If radioimmunoassay is used, a normal range is 1.3 to 2.0 mg/100 mL.

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or may test different samples. Talk to your provider about the meaning of your specific test results.

What Abnormal Results Mean

An increased TBG level may be due to:

Note: TBG levels are normally high in newborns.

Decreased TBG levels may be due to:

Risks

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 of having blood drawn are slight but may include:

  • Excessive bleeding
  • Fainting or feeling lightheaded
  • Hematoma (blood buildup under the skin)
  • Infection (a slight risk any time the skin is broken)

References

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. 13th ed. Philadelphia, PA: Elsevier; 2016:chap 11.

Weiss RE, Refetoff S. Thyroid function testing. In: Jameson JL, De Groot LJ, de Kretser DM, et al, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 78.

Review Date:2/3/2016
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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