Thyroxine-binding Globulin (TBG), Serum

Thyroxine-binding globulin (TBG) is a protein made by the liver that carries thyroid hormones through the bloodstream. Most thyroid hormone is attached to TBG and is inactive; only a small free portion is active in the body. If TBG levels are too high or too low, the total amount of thyroid hormone measured in a blood test can appear abnormal even when the thyroid is actually working normally. Conditions like pregnancy, estrogen use, or certain medications can raise TBG, while steroids, kidney disease, or liver problems can lower it. Measuring TBG helps doctors determine whether an unusual thyroid test result is due to a change in this carrier protein rather than a true thyroid problem.

When elevated: Elevated TBG increases total T4 and total T3 concentrations, which may falsely suggest hyperthyroidism if free hormone levels are not also assessed. Clinical euthyroidism is typically preserved when TBG elevation is the sole cause of raised total thyroid hormones. When low: Decreased TBG reduces total T4 and total T3 concentrations, potentially mimicking hypothyroidism on standard thyroid panels. Free T4 and TSH are usually normal in isolated TBG deficiency, confirming euthyroid status.

TBG is not a primary performance or training biomarker. It becomes relevant only if standard thyroid tests (TSH, free T4) appear abnormal but clinical thyroid function seems normal—this test then helps clarify whether the issue is thyroid hormone binding capacity rather than true thyroid dysfunction. Athletes with unexplained fatigue or recovery issues may benefit from complete thyroid assessment including TBG if initial results are confusing.