Abstract : Interferons are a large family of glycoproteins known as cytokines or substances released by lymphocytes that interfere with viral replication within host cells and activate the immune system. Nowadays, interferons are used as immuno-modulators to treat many diseases, especially hepatitis. Among their side effects thyroidopathies are the most important. Their frequency varies from a study to another, and may reach 20%. Thyroid disorders may be an increase in thyroid antibodies or an abnormal function. Interferons can reveal or induce thyroid diseases whose mechanisms are still not understood. It seems that cytokines modify the immune system leading to an increase in stimulating or inhibiting antibodies production. A direct thyroid cells cytolysis is also possible. When stimulating antibodies are prevailing, hyperthyroidism is the resulting disease. This last situation is rarer than hypothyroidism resulting from an increase in inhibiting antibodies and/or thyroid cells cytolysis. When thyroid disease occurs under interferon therapy, overt hyper or hypothyroidism should be treated symptomatically without stopping interferons prescribed for a severe disease. However, after stopping interferons, control of thyroid function should be done to check if there is an ad integrum thyroid recovery.