MS-0942

A 55-year-old woman comes to her doctor complaining of fatigue, difficulty passing stool, and muscle weakness. Her lab results show:

Free T4 6 pmol/l (9-18 pmol/l)
TSH 7.2 mu/l (0.5-5.5 mu/l)

Based on the probable diagnosis, which of the following tests is most likely to be positive in this patient?