Thyroid pathology in canine hypothyroidism

Biopsy or post mortem specimens from the thyroid glands of 7 dogs with clinical hypothyroidism were examined histologically. Six of the 7 cases were diagnosed histologically as lymphocytic thyroiditis which is characterized by widespread destruction and replacement of the gland by an infiltrate of lymphocytes, plasma cells and macrophages. The 7th case was characterized by fibrosis, with minimal inflammatory cell infiltration and might represent an end stage of lymphocytic thyroiditis. Five of the 7 dogs were female and 4 of these animals had shown clinical signs since 2 years of age. Comparisons are made with a previous report of functional lymphocytic thyroiditis in the pet dog and with similar conditions in man and an obese strain of White Leghorn poultry.