The TBG blood test measures the level of a protein that moves thyroid hormone throughout your body. The protein is called thyroxine binding globulin (TBG).
How the Test is Performed
A blood sample is needed. The sample is sent to a laboratory. There 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 level:
- Estrogens, found in birth control pills and estrogen replacement therapy
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
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, including thyroid disorders such as hypothyroidism.
If electrophoresis is used, normal values may range from 10 milligrams per 100 milliliters (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 doctor about the meaning of your specific test results.
What Abnormal Results Mean
An increased TBG level may be due to:
- Acute intermittent porphyria
- Liver disease
- Pregnancy (TBG levels are normally increased during pregnancy)
Note: TBG levels are normally high in newborns.
Decreased TBG levels may be due to:
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)
Serum thyroxine binding globulin; TBG level; Serum TBG level
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, Kronenberg HM, eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 11.
Update Date 5/10/2014
Updated 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.