General Remarks on the Substances Considered

This seventy-ninth volume of IARC Monographs contains evaluations of the carcinogenic risk to humans of 19 chemicals that have produced thyroid tumours in rodents. Unless otherwise stated, the term ‘thyroid tumour’, as used in this volume, refers to neoplasms of thyroid follicular cell origin. These substances have diverse uses. A number are used as drugs, including socalled ‘anti-thyroid’ agents (methimazole, methylthiouracil, propylthiouracil and thiouracil), sedatives (doxylamine succinate and phenobarbital), antifungal agents (griseofulvin), diuretics (spironolactone) and antibacterial agents (sulfamethazine and sulfamethoxazole). The others are or have been used in agriculture as pesticides (amitrole, chlordane, heptachlor, hexachlorobenzene and toxaphene), in foods or cosmetics (kojic acid), in hair dyes (2,4-diaminoanisole) and as industrial chemicals (N,N′-diethylthiourea, ethylenethiourea, and thiourea). Many of these agents have been evaluated previously in IARC Monographs, but some (N,N′-diethylthiourea, doxylamine succinate, kojic acid, methimazole and sulfamethazine) are evaluated for the first time. Since the previous evaluations of these agents in the Monographs series, new data, particularly on mechanisms, have become available. Such data now play an important role in making overall evaluations of carcinogenicity to humans (Vainio et al., 1992). A number of guidelines have been developed for the use of information on the mechanisms of induction of tumours of the kidney, urinary bladder and thyroid gland in rodents in making IARC evaluations (Capen et al., 1999), and these guidelines were used in the present volume.