Lymphocyte thyroiditis: in dogs

Primary diseases of the thyroid gland, especially lymphocyte thyroiditis an d follicular atrophy, are the most common lesions associated with clinical hypothyroidism in pet dogs. Lymphocyte thyroiditis resembles Hashimoto's th yroiditis in humans. The morphology of the thyroid lesion and frequent occu rrence of circulating thyroglobulin autoantibodies suggests that lymphocyte thyroiditis is immune-mediated in pet dogs. The disease is familial and pr obably hereditary. Thyroid lesions in 82 pet dogs were evaluated by light m icroscopy. Lymphocyte thyroiditis, found in 5 of the dogs, was characterise d by focal or diffuse infiltration of the thyroid gland by lymphocytes, pla sma cells and macrophages with formation of some lymphoid nodules. The infl ammatory lesions were associated with multifocal areas of degeneration of f ollicular cells (degeneration of individual follicular cells was present, w ith exfoliation into the colloid and interfollicular area). The thyroid fol licles were small and were lined in many areas by epithelial cells with abu ndant eosinophilic cytoplasm, termed Hurthle cells. Follicular atrophy was characterised by a loss of thyroid parenchyma and it s replacement by connective tissue.