What is the function of thyroid-binding globulin and how do pregnancy or estrogen therapy affect free thyroid hormone concentrations?

• A. Thyroid-binding globulin degrades thyroid hormones in circulation.
• B. Thyroid-binding globulin transports thyroid hormones; pregnancy or estrogen therapy increases TBG leading to higher total T4/T3 with normal free hormone levels; TSH may adapt accordingly.
• C. Pregnancy reduces thyroid-binding globulin levels.
• D. Free hormone levels rise when TBG increases.

Answer: (B)