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).
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
- 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
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.
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:
- Acute intermittent porphyria (a rare metabolic disorder)
- Hypothyroidism (underactive thyroid)
- Liver disease
- Pregnancy (TBG levels 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 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)
Serum thyroxine binding globulin; TBG level; Serum TBG level; Hypothyroidism - TBG; Hyperthyroidism - TBG; Underactive thyroid - TBG; Overactive thyroid - TBG
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.
Update Date 2/3/2016
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.