TBG (Thyroxine Binding Globulin) Test
The TBG test measures thyroxine-binding globulin, a liver-made protein that carries most of the thyroxine (T4) and triiodothyronine (T3) circulating in the blood. Quest describes it as used to help tell a change in this binding protein apart from true thyroid gland dysfunction.
About this test
Thyroxine-binding globulin, or TBG, is a glycoprotein made by the liver that binds T4 and T3 with high affinity and accounts for most of the blood's thyroxine-binding activity. Because so much circulating thyroid hormone travels bound to TBG, a change in the amount of TBG itself can shift total T4 and T3 results even when the thyroid gland is working normally.
Quest describes this test as used to help distinguish that kind of protein-driven change from an actual thyroid gland problem. Quest also notes that TBG levels can be affected by various diseases, medications, and inherited gene alterations — levels may be elevated with estrogen therapy, including oral contraceptives, pregnancy, or hepatitis, and decreased in cirrhosis, nephritic syndrome, or with androgen use.
Test details
| Quest code | 870 |
|---|---|
| Methodology | Immunoassay |
| Sample type | 1 mL serum |
| Collection | Quest patient service center |
Related tests and panels
Quest resources
Additional details
What this test looks at
The test reports the concentration of thyroxine-binding globulin in a serum sample, analyzed using immunoassay methodology. TBG is the glycoprotein responsible for carrying most of the blood's T4 and T3, so its level affects how much bound hormone is measured on other thyroid tests.
How it's often used alongside other tests
Because TBG changes can move total T4 and T3 results without reflecting true thyroid gland dysfunction, Quest describes this test as helping to tell the two apart. It is typically read alongside T4 or T3 results rather than by itself, so a clinician can judge whether a shift in those numbers comes from the thyroid or from the binding protein.
Order online and a physician order is included — no doctor's visit and no insurance needed. Your sample is collected at a Quest Diagnostics patient service center.
Laboratory results should always be interpreted by a licensed clinician who knows your health history. This page is information about a test, not medical advice.
Frequently asked questions
What does the TBG test measure?
It measures thyroxine-binding globulin, a liver-made protein that carries most of the T4 and T3 circulating in the blood, using an immunoassay method on a serum sample.
How is the TBG sample collected?
The test uses a 1 mL serum sample from a routine blood draw at a Quest Diagnostics patient service center.
Do I need to prepare for a TBG test?
No preparation instructions are listed for this test, though people follow any instructions their ordering clinician provides.
What can affect a TBG result?
Quest notes that TBG levels can be affected by diseases, medications and inherited gene changes — levels may be elevated with estrogen therapy, including oral contraceptives, pregnancy, or hepatitis, and decreased in cirrhosis, nephritic syndrome, or with androgen use.
Why would a clinician order TBG alongside a T4 or T3 test?
Quest describes TBG testing as a way to help distinguish a change caused by the binding protein itself from an actual thyroid gland problem, since a shift in TBG can move total T4 and T3 results without the thyroid gland being at fault.
How much is a TBG test?
$239, paid once at checkout. The price includes the physician's order this test requires and sample collection at a Quest patient service center.