Non-functional C-cell adenoma in aged horses.

Thyroid tumours occur in older horses, and most such tumours have been considered to be of follicular epithelial origin. However, their immunohistochemical characterization has not been investigated. The purpose of this study was to confirm a suspicion that most of these tumours are in fact parafollicular cell (C cell)-derived adenomas, and to evaluate their pathogenesis and functional state. Thyroid glands from 38 horses aged 10-29 years were evaluated, all tissue samples being examined histologically, immunohistochemically and ultrastructurally. Nodular tumour masses were found in the thyroids of 12 of 38 horses older than 10 years (31.6%), and in nine of 12 horses older than 20 years (75.0%), regardless of sex or breed. Nodular lesions were composed of solid proliferations of polygonal cells with eosinophilic granular cytoplasm. Immunohistochemically, tumour cells were positive for calcitonin and neuron specific enolase, but negative for thyroglobulin. Ultrastructurally, few if any secretion granules were found in tumour cells. On the basis of these results it was concluded that the nodular lesions were C-cell adenomas, not follicular adenomas. It was suspected that the C-cell adenomas were non-functional and unlikely to lead to calcitonin hypersecretion-related diseases.