Etiology
Primary hypothyroidism is the most commonly occurring type of hypothyroidism in adult dogs. Two variations of hypothyroidism occur – lymphocytic thyroiditis and idiopathic atrophy of the thyroid gland. Both result in the destruction of the thyroid gland.
Lymphocytic thyroiditis is an immune-mediated disorder, involving both the humoral and cell-mediated immune responses. The development of auto-antibodies to thyroglobulin, T4, T3 and other thyroid antigens results in the destruction of thyroid follicular cells through activation of the complement cascade and antibody-dependent cell-mediated cytotoxicity. Once initiated, the destruction of the thyroid gland is progressive, usually resulting in hypothyroidism within three years of onset.
The cause of idiopathic atrophy of the thyroid gland is not known. The syndrome is characterized by the degeneration of follicular cells and the replacement of thyroid tissue with adipose. An inflammatory response is not seen with this condition.