A 36-year-old female patient complains about an aesthetic defect of upper frontal teeth. In past history: she undergoes regular check-up in the endocrinological department on account of thyrotoxicosis. Enamel defects appeared several years ago and since that have been gradually expanding. Examination revealed that the 12, 11, 21, 22 teeth had transverse oval enamel defects on the vestibular surfaces. Probing and cold test is painless. Floor of the defects is smooth, glossy and hard. What is the most likely diagnosis?