Description
The Thyroxine-Binding Globulin (TBG) test measures the level of TBG, a protein that binds and transports thyroid hormones (T4 and T3) in the bloodstream. TBG plays a critical role in thyroid hormone availability, as only unbound (free) hormones can enter tissues and regulate metabolism.
Abnormal TBG levels can affect total thyroid hormone measurements without necessarily indicating true thyroid dysfunction, making this test essential for accurately interpreting thyroid lab results.
Purpose of the TBG Test
- Evaluate whether abnormal total T4 or T3 levels are due to changes in TBG rather than thyroid disease
- Help diagnose congenital or acquired TBG disorders
- Assess thyroid function in complex clinical situations, including pregnancy, liver disease, or estrogen therapy
- Provide context for interpreting total thyroid hormone tests versus free hormone levels
Why TBG Matters
- High TBG Levels may be caused by:
- Pregnancy
- Estrogen therapy (birth control or hormone replacement)
- Genetic variations
- Low TBG Levels may result from:
- Androgen therapy or high testosterone
- Severe illness
- Liver disease or protein deficiency
- Abnormal TBG does not always indicate thyroid disease, but it can mask true thyroid hormone activity if not measured alongside free hormone levels.
Who Should Consider This Test
- Patients with discordant thyroid test results (e.g., high total T4 but normal TSH)
- Individuals on hormone therapy or oral contraceptives
- Women who are pregnant or postpartum
- Patients with liver disease or protein deficiencies
- Those with suspected inherited TBG disorders
Test Details
- Sample Type: Blood
- Fasting Required: No
