Serum 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).
Serum thyroxine binding globulin; TBG level
How the test is performed
Blood is 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.
The sample is then taken to the laboratory where it is examined using special tests such as
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
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
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
Review Date: 05/10/2010
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.