A 40-year-old patient complains of pain caused by cold and sweet food in the 11,21 teeth, as well as of a cosmetic defect. Examination of the 11, 21 teeth revealed some oval, diagonally arranged enamel defects on the convex part of the vestibular surface of tooth crowns. The defects had smooth, glossy, dense floor. The patient has a history of thyrotoxicosis. What is the most likely provisional diagnosis?