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Your thyroid gland produces T3 and T4, which are hormones that help regulate body functions. Most of the T3 and T4 in your body binds with proteins produced by your body. The small amounts of T3 and T4 that don’t bind with protein are referred to as "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. A low TBG level doesn’t cause symptoms. The illness causing the low TBG level can cause symptoms, however.
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.
The symptoms of thyroid issues may prompt your doctor to order this test. These symptoms can include:
Many different medications and drugs can affect your TBG levels. Some of these are medications that you might frequently take, such as aspirin and birth control pills containing estrogen. Other medications that can affect your serum TBG levels include:
Tell your doctor about all medications you take, even those that may seem harmless, such as aspirin and birth control pills. Your doctor may then advise you to stop taking any of these medications temporarily before your TBG test.
This test simply involves drawing blood. After collecting your blood sample, your doctor will send it to a lab for testing.
Every blood draw carries some risks. The risks you experience will likely be minor and resolve without treatment. These involve:
The normal result range will vary slightly depending on the type of technique the laboratory uses. The two main types of laboratory techniques used for the serum TBG test are electrophoresis and radioimmunoassay. Typically, your results for both types of test are measured in milligrams per 100 milliliters, or mg/100 mL.
During electrophoresis, a lab technician places part of your blood called the serum on specially treated paper or a gel-like substance. An electric current then runs through it. The proteins move along the paper or gel and form bands that indicate how much of each protein is in the sample. A lab can analyze these results. If the lab used electrophoresis to test your sample, then normal results will range from 10 to 24 mg/100 mL.
Radioimmunoassay involves exposing a sample of your blood to an antibody. That antibody will attach to TBG. The antibody has a low-level radioactive isotope attached to it. The lab can then measure the amount of radiation in the sample, which indicates the amount of TBG in your blood. If the lab used radioimmunoassay to test your sample, then normal results will range from 1.3 to 2.0 mg/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.
High TBG levels don’t 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 due to liver disease, a rare genetic blood disorder called "acute intermittent porphyria," or hypothyroidism, which occurs when your thyroid doesn’t produce enough thyroid hormone.
Low TBG levels may be due to:
Your doctor will probably order more tests to figure out which of these issues is causing abnormal results.
If the serum TBG level test indicates that you have an underlying medical issue, talk to your doctor to determine the best treatment plan for you. Your doctor may need to repeat your serum TBG level test to make sure your treatment is effective. Tell your doctor about any changes in your health or medication while you’re on your treatment plan.
Written by: Gretchen Holm
Medically reviewed on: Mar 11, 2016: University of Illinois-Chicago, College of Medicine
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