Autoimmune thyroid diseases (AITDs) are organ-specific autoimmune diseases and affect about 2%-5% of the general population, with a significantly higher prevalence in women (5%-15%) than in men (1-5%).1 The term AITDs refers to a spectrum of phenotypes, from the mere presence of antibodies targeting different thyroid antigens to subclinical and overt thyroid diseases.2 The most frequent AITDs are Graves’ Disease (GD) and Autoimmune Thyroiditis (AIT), respectively. Notably, while opposite dysfunctions occur in GD and AIT, the immunogenetic substrates and pathogenetic mechanisms largely overlap, as demonstrated by a potential spontaneous shift from autoimmune hyperthyroidism to hypothyroidism and, more rarely, vice versa. Moreover, immunogenic destructive thyroiditis presents with an early phase of hyperthyroidism followed by transient (rarely persistent) hypothyroidism.3, 4, 5, 6, 7, 8 In the present study, we will review the biological and clinical aspects of AITDs, focusing on diagnosis and differential diagnosis. Emerging forms of AITDs related to anti-tumor molecular drugs and SARS-CoV-2 infection will also be addressed.9,10