Serum TBG level

Alternative names
Serum thyroxine binding globulin; TBG level

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, usually on 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 tight elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to 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 band 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 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 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.

Normal Values

Normal values vary somewhat from laboratory to laboratory, and depend on which method is used to evaluate the sample. If electrophoresis is used, results are reported in terms of T4 binding capacity (the binding capacity of the thyroid hormone T4), and normal values may range from 10 mcg/100 ml to 24 mcg/100 ml.

If radioimmunoassay is used to evaluate the sample, then a normal range of 1.3 to 2.0 mcg/100 ml can be expected.

Note: mcg/ml = microgram per milliliter

What abnormal results mean

Other than problems with your thyroid gland, elevated TBG levels may indicate:

  • Pregnancy (TBG levels are normally elevated during pregnancy)  
  • Hypothyroidism  
  • Liver disease  
  • Acute intermittent porphyria  
  • TBG levels are normally elevated in newborns

Decreased TBG levels may indicate:

  • Hyperthyroidism  
  • Acromegaly  
  • stress from surgery  
  • Acute illness  
  • Malnutrition (from the low levels of blood protein)  
  • Nephrotic syndrome

What the risks are

Risks associated with having blood drawn are slight:

  • Excessive bleeding.  
  • Fainting or feeling lightheaded.  
  • Hematoma, or blood accumulating under the skin.  
  • Infection (a slight risk any time the skin is broken).  
  • The person taking the blood may have to make multiple punctures to locate veins.

Special considerations

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.

Johns Hopkins patient information

Last revised: December 5, 2012
by Potos A. Aagen, M.D.

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