Why are total thyroxine (T4) levels increased in pregnant women and in women taking oral contraceptives?

Why are total thyroxine (T4) levels increased in pregnant women and in women taking oral contraceptives?

• A. Inappropriate iodine metabolism
• B. Changes in tissue use
• C. Changes in concentration of thyroxine-binding globulin (TBG)
• D. Changes in thyroglobulin synthesis

Answer: (C)

The key idea is that total T4 rises when the amount of thyroid-binding protein in the blood increases. During pregnancy and with estrogen-containing oral contraceptives, the liver makes more thyroxine-binding globulin (TBG). With more TBG available, more T4 becomes bound, so the total T4 measured in serum goes up. The free T4, which is the active portion, remains normal because the body regulates it tightly. So the change in concentration of thyroxine-binding globulin best explains the increase in total T4.

The key idea is that total T4 rises when the amount of thyroid-binding protein in the blood increases. During pregnancy and with estrogen-containing oral contraceptives, the liver makes more thyroxine-binding globulin (TBG). With more TBG available, more T4 becomes bound, so the total T4 measured in serum goes up. The free T4, which is the active portion, remains normal because the body regulates it tightly. So the change in concentration of thyroxine-binding globulin best explains the increase in total T4.