A pregnant woman in her second trimester complains of heat intolerance and palpitations. Physical examination reveals a slightly enlarged, non-tender thyroid gland, and a normal cardiac exam. The serum thyroxine (T4) level is increased; however, the serum thyroid-stimulating hormone (TSH) is normal. Which of the following best explains the laboratory findings in this patient?
Decreased estrogen Increased free thyroxine (T4) Increased progesterone Increased thyroid-binding globulin (TBG)