Quick Facts
- Sample: Blood
- Fasting: No special preparation required
- Turnaround: 3-5 business days. May take longer due to weather, holidays, or lab delays. Some labs are currently experiencing delays that could add 2-10 business 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 that transports thyroid hormone in the blood. It may help a healthcare provider determine whether unusual thyroid hormone results relate to the hormone itself or changes in TBG. This test does not directly evaluate overall thyroid function.
2. When might I consider ordering this test?
You may consider this test if thyroid hormone results seem inconsistent with your clinical presentation, if your provider suspects a TBG abnormality, or as part of a broader thyroid evaluation. It may also be useful for suspected hereditary TBG conditions or for people taking medications that can affect TBG, such as estrogens or tamoxifen.
3. How does this test differ from other thyroid tests?
This test measures TBG, a binding protein, rather than a thyroid hormone. It evaluates the protein that carries thyroid hormone through the blood and is not interchangeable with thyroid hormone or thyroid antibody testing. It does not directly measure thyroid function.
4. Do I need to fast or prepare before this test?
No special preparation is required.
5. How quickly will I get results?
Results are typically available in 3–5 days. Weather, holidays, or laboratory delays may extend this timeframe. Because this test is only set up on Wednesdays from 6–12 PST, some labs are currently experiencing delays of an additional 2–10 days.
6. How do I interpret the results?
The meaning of your TBG result depends on your health history, medications, and other thyroid test results. TBG levels can be affected by pregnancy, estrogens, tamoxifen, liver disease, and certain other medications. This result is often interpreted alongside other thyroid measurements and should not be treated as a direct measure of thyroid hormone levels or thyroid function.
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.
7. What should I do if my result is outside the expected range?
Review your result with a qualified healthcare provider. They can determine what it means in the context of your overall health and whether further evaluation is needed. This result alone does not provide a complete clinical conclusion.
8. How often should I get tested?
Testing frequency depends on your individual situation. Follow your healthcare provider’s guidance, as there is no standard repeat-testing schedule for this test.
Pre-test preparation
No special preparation required.
