The frequent occurrence of thyroid tumours in aged horses

Thyroid tumours have been described as "moderately common" in horses, but diseases associated with them are rare and the actual incidence has not been reported. A survey of thyroids from 29 horses aged 12 to 32 years revealed gross lesions in 11 animals, all older than 17. Most lesions were microfollicular adenomas. There was no evidence that the horses suffered from long-standing iodine deficiency or diffuse hyperplasia. Adenomas were more common than hyperplastic nodules and it is unlikely that the former arose from the latter. One thyroid adenocarcinoma was discovered. Progression from thyroid hyperplasia to adenomas and thence to adenocarcinomas has been reported in rodent species, but this study suggests that such progression does not occur in horses. As evidence of its occurrence in man is also lacking, the horse may be a more appropriate model than the rodent for human thyroid neoplasia.