Comprehensive Diabetes And Thyroid Disorders Pathophysiology, Symptoms, And Management Practice Test

Thyroid-binding globulin acts as the main transporter and reservoir for thyroid hormones in the blood. Most T4 and T3 are bound to proteins, with only a small, physiologically active free fraction circulating. The pituitary senses this free fraction to set TSH and regulate thyroid hormone production. During pregnancy or with estrogen therapy, the liver makes more TBG in response to higher estrogen levels. As TBG increases, more thyroid hormone becomes bound, so the total amount of T4 and T3 in circulation rises. However, the free, active hormone remains tightly regulated and stays within normal limits; the pituitary adjusts TSH as needed to maintain euthyroidism. In short, higher TBG boosts total T4/T3 but does not raise the free hormone levels; TSH may adapt accordingly. The other ideas don’t fit because TBG does not degrade hormones, and an increase in TBG does not raise free hormone levels—the bound pool increases while the active free hormone stays normal.