Etiology

Primary hypothyroidism is the most commonly occurring type of hypothyroidism in adult dogs. Two types of primary hypothyroidism that both result in destruction of the thyroid glands.

1. Lymphocytic thyroiditis
   An immune-mediated disorder characterized by development of auto-antibodies to thyroglobulin, thyroxine (T4), triiodothyronine (T3) and other thyroid antigens. Destruction of thyroid follicular cells through activation of the complement cascade and antibody-dependent cell-mediated cytotoxicity. Progressive destruction of the thyroid gland usually results in hypothyroidism within 3 years of the onset.
2. Idiopathic atrophy of the thyroid gland
   The cause is unknown. The syndrome can be characterized by the degeneration of follicular cells and the replacement of thyroid tissue with adipose. There is no inflammatory response seen in this condition.