Test Summary
Analysis Type
Serum immunoassay
Collection method
In person at the lab; at-home where available
Why test
TBG
?
- Clarify unexpected total T4 or T3 results with normal TSH.
- Interpret thyroid tests during pregnancy or when using estrogen therapy or birth control.
- Investigate mismatched total and free thyroid hormone results.
- Assess possible inherited TBG deficiency or excess.
- Track medicine effects that change binding proteins, like androgens or steroids.
What is
TBG
?
- Clarify unexpected total T4 or T3 results with normal TSH.
- Interpret thyroid tests during pregnancy or when using estrogen therapy or birth control.
- Investigate mismatched total and free thyroid hormone results.
- Assess possible inherited TBG deficiency or excess.
- Track medicine effects that change binding proteins, like androgens or steroids.
What insights will i get from
TBG
?
- High: Often from pregnancy or estrogen therapy; raises total T4/T3, free hormones usually normal.
- Low: Often from androgens, steroids, severe illness, or protein loss; lowers total T4/T3.
- TBG shifts total hormones; it does not show thyroid gland function by itself.
- Not guideline-endorsed; no standardized cutoffs; use your lab range and context.
- Retest after big medication changes or after pregnancy for a clearer picture.
Sample type & collection
- Sample: Blood
- Fasting: No
- Best timing: Morning; if on thyroid meds, draw before daily dose
- Collection: In person at the lab; at-home where available
- Typical volume: 1 mL
- Analysis: Serum immunoassay
- Prep tips: Avoid high-dose biotin for 48–72 hours unless your clinician says otherwise
- Turnaround: 1–3 business days
TBG
is best interpreted with:
- TSH
- Free T4
- Free T3
- Total T4
- Total T3
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TBG
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Limits & interferences with
TBG
:
- Pregnancy increases TBG; totals rise, free levels usually unchanged.
- Estrogen therapy or birth control pills raise TBG.
- Androgens or anabolic steroids lower TBG.
- High-dose glucocorticoids may lower TBG.
- Some medicines, like tamoxifen or methadone, may raise TBG.
- High-dose biotin supplements can distort some immunoassays; pause 48–72 hours.
- Blood draw soon after a thyroid pill may shift related hormone results.
- Special situations (when to confirm or adjust): pregnancy; starting or stopping estrogen or androgen therapy; serious illness; suspected inherited TBG changes.
Questions about
TBG
:
- Do I need to fast? No. Fasting is not required for a TBG test.
- Does an abnormal TBG mean thyroid disease? Not by itself. TBG changes shift total hormones; TSH and free T4 guide thyroid status.
- What happens in pregnancy or with birth control? Estrogen raises TBG, increasing total T4/T3. Free hormones and TSH help interpret results.
- How is TBG different from TSH? TSH is a hormone signal from the brain. TBG is a carrier protein in blood.
Sources:
MedlinePlus — Thyroxine-binding globulin (TBG) test. (2024). https://medlineplus.gov/lab-tests/thyroxine-binding-globulin-tbg-test/
Mayo Clinic Laboratories — Thyroxine-Binding Globulin (TBG), Serum. (2023). https://www.mayocliniclabs.com/test-catalog/Clinical+and+Interpretive/8204
ARUP Consult — Thyroid Function Testing. (2024). https://arupconsult.com/content/thyroid-function-testing
American Thyroid Association — Thyroid Function Tests. (2023). https://www.thyroid.org/thyroid-function-tests/
Medical disclaimer: This content is for informational purposes only and is not medical advice. Always discuss results with a qualified healthcare professional.
