Antiepileptic drugs (AED) are a heterogeneous group of compounds widely used in both adults and children. These drugs are related to various adverse effects involving several organs and endocrinological and metabolic functions. In particular, relevant effects on thyroid function have been described. Subclinical hypothyroidism and alterations in thyroid hormone serum levels are reported in the literature; phenytoin, valproate and carbamazepine, in particular, seem to be involved in these alterations. The aim of this review is to analyse critically the principal alterations in thyroid function caused by AED therapy.