Carbimazole induced atypia in the thyroid gland

Thyroid nuclear atypia associated with carbimazole is well-known. However, there are rare cases described in literature. We report here a case of thyroid fine needle aspiration cytology (FNAC) of a follicular lesion of undetermined significance or nuclear atypia of undetermined significance (FLUS/AUS) observed in a 56-year-old female presenting with Hashimoto's thyroiditis on treatment by carbimazole. In conclusion, thyroid nuclear atypia in the form of anisonucleosis, prominent nucleoli, and hyperchromasia associated with carbimazole treatment should be interpreted with caution in differentiating from malignancy.