Incidence and morphology of spontaneous thyroid tumours in different strains of rats.

The incidence and morphology of thyroid neoplasms in different strains and stocks of rats are reported. The frequency of thyroid tumours varied to a great extent between different strains of rats. Low incidences were observed in DA/Han (0.85 per cent) and BDII/Han rats (1.25 per cent), while somewhat higher incidences occurred in Han:WIST rats (7.3 per cent). The highest number of thyroid tumour-bearing rats was found in the Han:SPRD stock (55.5 per cent). Thyroid neoplasms were classified histologically as polymorphofollicular adenomas, papillary adenomas, cystadenomas, polymorphofollicular carcinomas, papillary carcinomas, C-cell adenomas and C-cell carcinomas (medullary carcinomas). C-cell neoplasms were predominant in Han:SPRD rats (93.0 per cent), while the majority of tumour-bearing Han:WIST rats (61.4 per cent) had follicular tumours.