Quick Facts
- Sample: Blood
- Fasting: No special preparation required
- Turnaround: 3-5 days (may take longer due to weather, holidays, or lab delays; some labs are currently experiencing delays that could add 2-10 days, as this test is only set up on Wednesdays from 6-12 PST)
What This Test Measures
This test measures Thyroxine-Binding Globulin (TBG), a protein found in your blood that transports thyroid hormone. TBG is not a thyroid hormone itself and this test does not measure thyroid hormone levels directly. Instead, it looks at the protein responsible for carrying thyroid hormone through your bloodstream.
Measuring TBG can help a healthcare provider understand whether changes in thyroid hormone levels are related to the hormone itself or to changes in the amount of TBG available for binding. This distinction matters because shifts in TBG levels can affect total hormone measurements without reflecting an actual change in thyroid function.
Benefits
- Measures TBG, the protein that transports thyroid hormone
- Helps distinguish true hormone changes from binding protein changes
- Supports evaluation of unexplained thyroid hormone test results
- Can help identify hereditary TBG deficiency or excess
- Provides useful context for a broader thyroid work-up
Who Is This Test For?
- People with unexplained or unusual thyroid hormone test results
- Individuals whose provider wants to rule out abnormal TBG levels
- People being evaluated for a possible hereditary TBG condition
- Individuals taking estrogens, tamoxifen, or perphenazine with unclear thyroid results
- People with liver disease being evaluated for thyroid hormone changes
- Individuals undergoing a broader thyroid disease work-up
How It Works
- Order Your Test Online
- Visit a Local Lab
- Get Results Online
FAQ
1. What is the purpose of this thyroid test?
This test measures Thyroxine-Binding Globulin (TBG), the protein responsible for transporting thyroid hormone in your blood. It helps a healthcare provider determine whether unusual thyroid hormone levels are due to the hormone itself or to changes in TBG. This test does not directly evaluate overall thyroid function.
2. Who would benefit from this test?
This test may benefit people whose thyroid hormone results are difficult to explain, or whose provider wants to check for abnormal TBG levels. It may also be useful for those with a suspected hereditary TBG condition, or those taking medications known to affect TBG, such as estrogens or tamoxifen. It can also support a broader work-up of thyroid disease.
3. When should I order this test?
You may consider this test if your thyroid hormone results seem inconsistent with your clinical presentation, or if your provider wants to investigate a possible TBG abnormality. It may also be considered as part of a broader evaluation of thyroid disease. This test does not diagnose a condition on its own.
4. How does this test differ from other thyroid tests?
This test measures a binding protein, TBG, rather than a thyroid hormone. Unlike tests that measure thyroid hormones directly, this test evaluates the protein that carries thyroid hormone through the blood. It is not interchangeable with thyroid hormone or thyroid antibody testing, and it does not directly measure thyroid function.
5. Do I need to fast before this test?
No special preparation is required for this test.
6. How quickly will I get results?
Results are typically available in 3-5 days, though this may take longer due to weather, holidays, or lab delays. Some labs are currently experiencing delays that could add 2-10 days to the normal turnaround, as this test is only set up on Wednesdays from 6-12 PST.
7. How do I interpret the results?
The meaning of your TBG result depends on your individual health history, medications, and other thyroid test results. TBG levels can be affected by factors such as pregnancy, estrogens, tamoxifen, liver disease, and certain other medications. This result is often interpreted alongside other thyroid measurements rather than on its own. It should not be treated as a direct measure of thyroid hormone levels or thyroid function. Please discuss your results with a qualified healthcare provider.
Disclaimer: Reference ranges may vary by laboratory. Listed ranges are general guidelines and may differ from those used by the performing lab. Always consult your healthcare provider for interpretation.
8. What should I do if my results are abnormal?
If your results are outside the expected range, review them with a qualified healthcare provider. They can help determine what the result means in the context of your overall health and whether further evaluation is needed. This test result alone does not provide a complete clinical conclusion.
9. How often should I get tested?
Testing frequency should be determined as directed by your healthcare provider based on your individual situation. There is no standard repeat testing schedule for this test.
Pre-test preparation
No special preparation required.
