T4 (Thyroxine), Total

Total Thyroxine (Total T4) measures the total amount of T4 (bound and unbound) in the blood. Like Total T3, its level is affected by thyroid-binding globulin (TBG) concentrations, making it less reliable than Free T4 for assessing metabolically active hormone levels, especially in situations where binding proteins might be altered (e.g., pregnancy, estrogen therapy).