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).

How is the Test 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.

Preparation 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:

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 will the Test 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 is the Test Performed?

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

Normal Results for TBG level - blood

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:

TBG level - blood 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)

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Review Date: 6/15/2012
Reviewed By: 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.
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. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- 2014 A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.
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