Serum TBG level is a test to evaluate thyroid function by measuring the level of thyroxine binding globulin (TBG) in the blood. Thyroxine binding globulin is a protein that transports thyroid hormone throughout your body.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. 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 evaluated by special laboratory tests, such as electrophoresis or radioimmunoassay.

Several medications and conditions can interfere with TBG test results.

If you take any of the following, your TBG results may be elevated:

• Estrogens, found in oral contraceptives or estrogen replacement therapy
• Phenothiazines
• Heroin or methadone

If you have one of the following conditions, your TBG results may also be elevated:

• Severe liver disease
• Acute intermittent porphyria
• HIV

If you take any of the following, your TBG levels may be reduced:

• High doses of salicylates, such as aspirin
• Male hormones, such as androgens and testosterone
• Depakote or Depakene (also called valproic acid)
• Dilantin (also called phenytoin)
• Prednisone

If you have one of the following conditions, your TBG results may also be reduced:

• Nephrotic syndrome, or kidney failure
• Other conditions that cause low albumin levels, such as liver disease

If you take any of these medications, or have any of these conditions, inform your doctor before the test, since you may need to withhold your medication, or have another test.

If your child is to have this test performed, it may be helpful to explain how the test will feel, and even demonstrate on a doll. Explain the reason for the test. Knowing the "how and why" may reduce the anxiety your child feels.

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.

Your doctor may use this test to identify abnormalities with your thyroid binding globulin, or to assist in diagnosing other thyroid disorders such as hypothyroidism.