Background: When the antithyroid drugs were discovered in the early 1940s, they were immediately recognized as a revolutionary new treatment for hyperthyroidism. Although much has been learned about their mechanism of action and clinical utility, they continue to be used today in much the same way as they have been since their introduction. Summary: In 1995, Dr. Clark Sawin gave an address on the history of antithyroid drug development at the 11th International Thyroid Congress in Toronto, Ontario, Canada. In his review, Dr. Sawin recounted the original observations by Drs. Julia and Cosmo Mackenzie and Curt Richter at the Johns Hopkins University School of Medicine, and how their work ultimately led to Dr. Edwin (Ted) B. Astwood's seminal 1943 report on the use of thiourea and thiouracil in the Journal of the American Medical Association. He also described the development of propylthiouracil and methimazole as less toxic alternatives. He concluded his remarks by noting the often-serendipitous pathway of drug development and the role of pharmaceutical companies in the process. Conclusions: Antithyroid drugs remain a cornerstone of thyroid therapeutics. It is informative to review the process by which they came into use, as this is a seminal part of the history of thyroid disease in the 20th century. This knowledge may also spark additional research leading to new pharmacotherapies for patients with hyperthyroidism.