Quick Facts
- Sample: Blood (serum)
- Fasting required: Not required
- Turn-around time: 3-5 business days. May take longer based on weather, holiday, or lab delays.
Benefits
- Clarifies confusing thyroid results when total hormone levels don't match symptoms
- Prevents misdiagnosis of thyroid disorders by measuring transport proteins
- Essential during pregnancy when TBG levels naturally increase
- Identifies hereditary TBG abnormalities that affect hormone transport
- Avoids unnecessary medication changes by understanding binding capacity
- Convenient direct access without physician referral required
Who Is This Test For?
- Pregnant women experiencing thyroid-related symptoms or abnormal results
- Hormone therapy users (contraceptives, estrogen) with inconsistent thyroid levels
- Individuals with unexplained total T4/T3 levels despite normal symptoms
- Those with family history of hereditary TBG defects
- Patients with liver disease affecting protein production
- Anyone seeking clarity on confusing thyroid test results
How It Works - Just 3 Steps
- Order online and visit a local lab or schedule home collection
- Simple blood draw requiring only 0.5 mL serum sample
- Receive secure results in 3-5 business days.
FAQ
What does TBG measure? TBG measures the protein that transports thyroid hormones through your bloodstream, affecting total hormone test results.
Why is my total T4 high but I feel normal? High TBG levels can elevate total T4 while free hormones remain normal, especially during pregnancy or estrogen use.
Should I avoid supplements before testing? Yes, avoid biotin-containing supplements for 12 hours before blood draw to prevent test interference.
Can TBG levels change over time? Yes, TBG fluctuates with pregnancy, hormone therapy, liver function, and certain medications like tamoxifen.
Do I need to fast for this test? No fasting required, but inform your healthcare provider of all medications and supplements you're taking.
How do TBG results affect thyroid treatment? TBG results help determine if abnormal total hormone levels require treatment or are due to transport protein changes.
More Details
What is the purpose of this test?
The TBG test measures thyroxine-binding globulin concentration to clarify thyroid hormone transport capacity. Since TBG carries 75% of circulating thyroid hormones, abnormal levels can make total T4 and T3 results misleading. This test distinguishes between true thyroid dysfunction and transport protein abnormalities (Mayo Clinic 2024).
Who would benefit from this test?
Individuals with discrepant thyroid results benefit most from TBG testing. Pregnant women, contraceptive users, and those on hormone replacement therapy often show elevated TBG, affecting total hormone measurements. People with suspected hereditary TBG abnormalities, liver disease, or unexplained thyroid symptoms despite normal free hormone levels should consider this test (LabCorp 2023).
When should I order the Thyroxine-Binding Globulin (TBG) Serum Test?
Order TBG testing when total thyroid hormone levels don't correlate with clinical symptoms or free hormone measurements. Common scenarios include pregnancy with abnormal thyroid results, hormone therapy causing confusing lab values, or suspected hereditary TBG defects. Consider testing if liver disease complicates thyroid assessment (UCSF Health 2024).
How do I interpret the results?
TBG Level: High (>30 mg/L)
- What it means: Increased hormone binding capacity
- Typical action: Evaluate for pregnancy, estrogen use, liver disease
TBG Level: Normal (15–30 mg/L)
- What it means: Adequate transport protein function
- Typical action: Continue routine thyroid monitoring
TBG Level: Low (<15 mg/L)
- What it means: Reduced binding capacity
- Typical action: Check for kidney disease, steroid use, hereditary defects
Normal ranges may vary between laboratories and should always be interpreted alongside TSH, free T4, and free T3 levels.
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.
Pre-test preparation
Avoid biotin supplements for 12 hours before blood draw to prevent immunoassay interference. No fasting required. Inform healthcare providers about all medications, especially hormones, steroids, or thyroid medications. Continue prescribed medications unless specifically instructed otherwise (Mayo Clinic Laboratories 2024).
How often should I get tested?
- Initial abnormal thyroid results
- Suggested interval: Once for clarification
- Pregnancy monitoring
- Suggested interval: Each trimester if indicated
- Hormone therapy changes
- Suggested interval: 3–6 months after adjustment
- Hereditary TBG abnormality
- Suggested interval: Annual or as clinically indicated
Why early detection matters
Accurate TBG measurement prevents thyroid disorder misdiagnosis, avoiding unnecessary treatment and anxiety. Early identification helps distinguish transport protein issues from true thyroid dysfunction, ensuring appropriate care. This is particularly crucial during pregnancy when TBG naturally increases, potentially masking or mimicking thyroid disease (NIH PubMed Central 2017).
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