A 30-Year-Old Woman in Her First Trimester of Pregnancy Complains

A 30-year-old woman in her first trimester of pregnancy complains of easy fatigability, heat intolerance, and palpitations. Physical examination shows a slightly enlarged, nontender thyroid gland. Thyroid function studies show an increased total serum thyroxine (T₄) level, but normal serum thyroid-stimulating hormone (TSH) levels. Which of the following best explains the results of these studies?

A) Decreased thyroid-binding globulin (TBG)
B) Increased synthesis of T₄
C) Increased synthesis of triiodothyronine (T₃)
D) Thyroiditis-induced release of T₄
E) Normal finding in pregnancy