Changes in thyroid hormone concentrations that are characteristic of hyperthyroidism must be distinguished from physiological changes in thyroid hormone economy that occur in pregnancy, especially in the first trimester. Gestational transient thyrotoxicosis, caused by very high serum hCG concentrations, is typically seen in women with hyperemesis gravidarum and occurs in one to two per 1000 pregnancies. This is generally a self-limiting condition which does not require antithyroid treatment and it is important to differentiate this from other forms of hyperthyroidism which may be associated with adverse pregnancy outcomes if untreated. In women of child-bearing age, Graves disease (autoimmune hyperthyroidism) is the most common, although any type of hyperthyroidism including toxic nodular disease may occur during pregnancy. Since autoimmune disorders like Graves disease usually improve during pregnancy it is rare for this condition to present for the first time during gestation. Importantly post-partum thyroiditis occurs in 510% of women and up to 50% of those affected ultimately develop permanent hypothyroidism. The differential diagnosis between transient thyrotoxicosis of pregnancy and Graves disease can often be made on clinical grounds and the measurement of serum fT3 concentrations and TSH-receptor stimulating antibodies are useful diagnostic tools. Antithyroid drug treatment of hyperthyroidism in pregnant women is controversial because the usual drug carbimazole is occasionally teratogenic; and the alternative propylthiouracil can be hepatotoxic. Fetal hyperthyroidism caused by transplacental passage of TSH-stimulating antibodies can be life-threatening, and needs to be recognised as soon as possible so that treatment of the fetus with antithyroid drugs via the mother can be initiated. This Expert session will provide an overview of the clinical presentation, diagnosis and management of hyperthyroidism in pregnancy through case-based discussion.