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TBG level - blood

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

Alternative Names

Serum thyroxine binding globulin; TBG level; Serum TBG level

How the test is performed

A blood sample is needed. For information on how this is done, see: Venipuncture

The sample is then taken to the laboratory where it is examined using special tests such as electrophoresis or radioimmunoassay.

How to prepare for the test

Certain drugs and medicines can affect test results. Your doctor may tell you to temporarily stop taking a certain medicine before the test. Never stop taking any medicine without first talking to your doctor.

The following drugs can increase TBG levels:

  • Estrogens, found in birth control pills and estrogen replacement therapy
  • Heroin
  • Methadone
  • Phenothiazines

The following drugs 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

Certain medical conditions may also affect TBG levels. For example, TBG results may be increased in people with acute intermittent porphyria, HIV, or severe liver disease. They may be reduced in people with kidney failure or liver disease.

How the test will feel

When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed

This test may be done to diagnose problems with your thyroid, including thyroid disorders such as hypothyroidism .

Normal Values

If electrophoresis is used, normal values may range from 10 mg/100 mL - 24 mg/100 mL.

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

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

The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.

What abnormal results mean

Increased TBG levels may be due to:

Note: TBG levels are normally high in newborns.

Decreased TBG levels may be due to:

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)

Updated: 6/15/2012

Frank A. Greco, MD, PhD, Director, Biophysical Laboratory, The Lahey Clinic, Burlington, MA. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.


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