Serum TBG (Thyroxine-Binding Globulin) Level Test

Written by Gretchen Holm | Published on June 1, 2012
Medically Reviewed by George Krucik, MD

What Is a Serum TBG Level Test?

Your thyroid produces T3 and T4, hormones that help regulate body functions. Most of the T3 and T4 in your body binds with proteins that your body produces. The small amounts of T3 and T4 that do not bind with protein are called “free.”

Most of the protein-bound T3 and T4 in your body bind to thyroxine-binding globulin (TBG). The serum TBG level test measures the amount of TBG in your blood. This can help doctors assess thyroid problems.

TBG deficiency usually accompanies an underlying illness. Symptoms are related to this illness rather than to the lack of TBG.

Why Doctors Perform a Serum TBG Level Test

Your doctor may order a serum TBG level test to assess thyroid issues. It can help your doctor diagnose various thyroid disorders including hypothyroidism and hyperthyroidism.

Symptoms of thyroid issues, which may prompt your doctor to order this test, include:

  • constipation
  • difficulty sleeping
  • dry or puffy skin
  • eye problems (such as dryness, irritation, puffiness, bulging)
  • fatigue and weakness
  • hair loss
  • hand tremors
  • increased heart rate
  • sensitivity to cold and light
  • menstrual irregularity
  • weight changes

Preparing for the Serum TBG Level Test

Many different medications and drugs can affect your TBG levels. Some of these are medications that you might take frequently, such as high dosage aspirin and birth control pills containing estrogen. Other medications that can impact your serum TBG level test results include:

  • hormones
  • opiates and opioids
  • other specific medications (including depakote/depakene, Dilantin, phenothiazines, and prednisone) 

Tell your doctor about all medications you take, even those that may seem harmless, such as aspirin and birth control pills. Your doctor will then be able to advise you as to whether you should temporarily stop taking any of these medications before your TBG test.

The Serum TBG Level Test

This test simply involves drawing blood. After collecting your blood sample, your healthcare provider will send it to a laboratory where it will be tested for your TBG level.

Risks

Every blood draw carries some risks. Generally, any complications you experience will be minor and will resolve without treatment. These involve slight bleeding after the procedure and a small bruise at the site where the needle was inserted into your vein. 

More serious complications, such as infection or inflammation of your vein, are rare.

Normal Serum TBG Level Test Results

The normal result range will vary slightly depending on the type of technique the laboratory uses. There are two main types of laboratory techniques used for the serum TBG test: electrophoresis and radioimmunoassay. In electrophoresis, part of your blood (the serum) is placed on specially treated paper or a gel-like substance. An electric current is run through it. The proteins move along the paper or gel and form bands that indicate how much of each protein is in the sample. This can be analyzed by the laboratory. If your sample is tested with electrophoresis, normal results range from 10 to 24 mg per 100 mL.

Radioimmunoassay involves exposing a sample of your blood to an antibody that is programmed to attach to TBG. The antibody has a low-level radioactive isotope attached to it. The laboratory can then measure the amount of radiation in the sample, which indicates the amount of TBG in your blood. Typically, your results for both types of testing are measured in milligrams per 100 milliliters, or mg per 100 mL. If your sample is tested with radioimmunoassay, normal results range from 1.3 to 2.0 mg per 100 mL.

The exact standards for normal results may vary depending on your doctor and lab. Ask your doctor for an explanation if you have concerns.

What Do Abnormal Serum TBG Level Test Results Mean?

High TBG levels do not always indicate that you have a problem. High results on this test are normal in pregnancy and in newborns.

In other cases, high TBG levels may be caused by:

  • acute intermittent porphyria (a rare genetic blood disorder)
  • hypothyroidism (a condition in which your thyroid does not produce enough thyroid hormone)
  • liver disease

Low TBG levels may be caused by:

  • acromegaly (you have too much growth hormone and the body grows disproportionately as a result)
  • acute illness (any time you are sick, your body’s production of thyroid hormones tends to decrease)
  • hyperthyroidism (increased production of thyroid hormones)
  • malnutrition
  • nephrotic syndrome (a group of syndromes caused by kidney damage that include protein in the urine and high cholesterol levels)

Your doctor will probably order more tests to figure out which of these issues is causing abnormal results. 

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