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Serum TBG level   

Overview | Risks | Results

Alternative names:

Serum thyroxine binding globulin; TBG level

Definition:

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 around your body.

How the Test is Performed

Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The area where blood is to be drawn is cleaned with antiseptic, and a tourniquet (a tight elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend, or fill with blood. A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. Once the blood has been collected, the tourniquet is removed to restore circulation, 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.

How to Prepare for the Test

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 valporic 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 practice or demonstrate on a doll. The more familiar your child is with what will happen to him/her, and the purpose for the procedure, the less anxiety he/she will feel.

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the Test is Performed

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.

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