The pregnancy is a state of thyroid hyperstimulation, therefore of changes of thyroid hormone values. Results of thyroid hormone measurement have to be analysed in the context of gestation age. Hyperthyroidism, mostly represented by Graves' disease, requires a multidisciplinary management, owing to possible maternal, foetal and neonatal complications. Treatment with antithyroid drugs, is a compromise between the risk of uncontrolled maternal hyperthyroidism and the risk of iatrogenic foetal hypothyroidism. Evaluation of foetal thyroid function considers the titre of thyrotropin receptor antibodies in the mother's blood, the dose of antithyroid drugs to maintain euthyroidism in the mother, and the signs of foetal hyperthyroidism on ultrasound. Maternal hypothyroidism is associated with foetal and maternal morbidity. Untreated or inappropriately treated, it is associated with poorer performances of offspring in intelligence tests. Thyroid autoimmunity is associated with hypofertility, particularly with spontaneous abortion. Screening for thyroid dysfunction during pregnancy, although not systematic, should have broad indications.