Neoplasms and pigmentation of thyroid glands in F344 rats exposed to 2,4-diaminoanisole sulfate, a hair dye component.

2,4-Diaminoanisole sulfate was fed at dietary levels of 0.12% (low dose) or 0.5% (high dose) to groups of 50 male and 50 female inbred F344 rats for 78 weeks. By 107 weeks after the initial exposure, 58% of the male rats and 42% of the female rats administered the high dose had thyroid neoplasms, whereas only 7--8% of the controls developed them. Follicular cell carcinomas were the primary type of neoplasm induced. None of the controls had these tumors. The carcinomas, which were papillary, cystic, or solid, were highly invasive but did not metastasize. A brown pigment was present as granules primarily in thyroid follicular cells in all exposed rats. The amount of pigment as determined by an image-analyzing computer revealed that the cross-sectional area occupied by the pigment granules and the optical density of the granules were dose related.