Pregnancy is characterised by a physiological increase in bound thyroxine but normal values of free hormone. Human chorionic gonadotrophin (hCG may stimulate the thyroid to produce hyperemesis gravidarum (with mild to moderate hyperthyroidism) or result in high thyroid hormone levels associated with gestational trophoblastic disease. Hyperthyroidism occurring during pregnancy is usually due to Graves' disease and must be treated to prevent congenital anomalies, low birth weight and premature labour. Thionamide drugs should be used with a preference for propylthiouracil (PTU) and continued in low doses up to labour. Breast feeding is possible in patients on low dose PTU. In the management of hypothyroidism during pregnancy thyroxine dose may require to be increased but excess dosage should be avoided because of its unwanted effects on foetal cerebral maturation. Neonatal hyperthyroidism due to transplacental passage of thyroid stimulating antibodies (TsAb) should be checked for in pregnant patients with autoimmune thyroid disease. As antithyroid drugs cross the placenta they may be used as therapy in this condition. Prevention of neonatal goitre is vital. Postpartum development of hyperthyroidism may be due to an exacerbation of pre-existing Graves' disease, development of new Graves' hyperthyroidism or postpartum thyroiditis with transient hyperthyroidism. Differentiation by measurement of TsAb and thyroidal iodine uptake is important because of therapeutic considerations.