It is well known that the pre‐operative treatment of thyrotoxic patients influences the histology of surgical specimens. However, a description of the exact relationship between the type of pre‐operative treatment and the histology is not available. The aim of the present investigation was to obtain some information on this point. It was found that the histology of thyrotoxic glands varied considerably and that the “classical” picture was seen only in about one third of the cases. Thyroids from patients treated only with iodine showed more variation within the specimen, taller cells, larger nuclei and nucleoli, more marginal vacuolization, and more germinal centres, but less vascularization of the stroma than thyroids from patients treated with sulphur‐containing thy