A 22-year-old woman presents to her obstetrician/gynecologist for follow-up 8 weeks after the delivery of her baby.There were no complications during her pregnancy or during delivery. On this visit,...

A 22-year-old woman presents to her obstetrician/gynecologist for follow-up 8 weeks after the delivery of her baby.There were no complications during her pregnancy or during delivery. On this visit, her thyroid gland is diffusely, but minimally, enlarged and nontender to palpation. The doctor decides to obtain a thyroid function panel to aid her diagnostic evaluation. In this thyroid function panel, the most sensitive indicator of thyroid function is: