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).

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 electrophoresis or radioimmunoassay.

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
• Phenothiazines
• Heroin
• Methadone

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 persons with acute intermittent porphyria, HIV, or severe liver disease. They may be reduced in persons with kidney failure or liver disease.

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.

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

Normal values vary somewhat from laboratory to laboratory, and depend on which method is used to evaluate the sample.

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

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

Increased TBG levels may be due to:

• Acute intermittent porphyria
• Hypothyroidism
• 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:

• Acute illness
• Acromegaly
• Hyperthyroidism
• Malnutrition
• Nephrotic syndrome
• Stress from surgery

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)