Autoimmune thyroiditis (AT) is a disease that may be associated with many other autoimmune endocrine and non-endocrine disorders. This disease is mediated by both humoral and cellular mechanisms and it is the result of combined effects of human leukocyte antigen (HLA) class II genes and non-HLA genes polymorphisms. The clinical course of AT is variable and may be characterized by spontaneous remission and by irreversible thyroid insufficiency as the consequence of atrophic and fibrous transformation of the thyroid gland in other cases. In this paper, the AT's etiology and immunological mechanism along with its cytological and histopathological features are reviewed in order to increase our understanding about the mechanism involved in pathogenesis of this disease and to open new directions of investigations that will be useful in a better clinical practice.