Thyrotoxicosis occurs more frequently during the post-partum period than at other times in women of childbearing age. Graves' disease and post-partum thyroiditis are two major causes of thyrotoxicosis in this period. The major task lies in differentiation of these two diseases in the post-partum period; since throtoxicosis caused by post-partum thyroiditis usually does not require treatment. The radioiodine uptake is elevated or normal in Graves' disease and low in post-partum thyroiditis, and TSH-receptor antibodies are positive in Graves' and negative in post-partum thyroiditis. Post-partum thyrotoxicosis due to Graves' disease may be treated with radioiodine but it requires radiation safety measurements for infant and is contraindicated if the mother is breast-feeding. Antithyroid drugs are the mainstay of the treatment of post-partum thyrotoxicosis. Recent investigations conclude that neither propylthiouracil nor methimazole cause any alterations in thyroid function and physical and mental development of infants breast-fed by lactating thyrotoxic mothers, and both can be safely administered in moderately high doses during lactation.