[en] Lymphocytic thyroiditis associated with progressive thyroid atrophy is described in CRHL beagles. Depressed thyroid function was present in many of these dogs, as evidenced by clinical signs of hypothyroidism, elevation of serum cholesterol levels, depressed serum triiodothyronine levels, and alteration of basophils of the antero-medial region and/or unilateral or bilateral obliteration of the thyroid glands by neoplasia. Microscopic changes in the thyroid glands included lymphocytic thyroiditis, thyroid follicular atrophy, adenomatous hyperplasia of follicula cells and C-cells, and follicular cells neoplasia. This disease occurred with no sex predisposition in dogs 2 through 11 years of age. The disease does not appear to be influenced by previous radiation exposure but may be familial. In contrast to thyroid disease in some other beagle colonies, the disease in CRHL beagles more closely resembles atrophic thyroiditis of man rather than human Hashimoto's thyroiditis