Hypothyroidism may be the most common endocrine disorder in adult dogs. As is best currently understood, a majority of cases are caused by autoimmune thyroiditis (AIT), a disorder in which the body’s own immune system attacks the thyroid gland. This causes progressive, irreversible destruction of thyroid gland cells resulting in loss of thyroid hormone production. This disorder has similarities to Hashimoto’s thyroiditis, an important cause of hypothyroidism in people. In dogs with AIT, low circulating concentrations of thyroid hormones are often seen in conjunction with increased autoantibodies against thyroglobulin, a large protein made by thyroid cells. Detection of thyroglobulin autoantibodies (TgAA) is used as a marker for how intensely the immune system is attacking the thyroid gland. Elevated TgAA results are the first marker of an early stage of AIT, long before there is complete loss of thyroid function. Identification of elevated TgAA results with otherwise normal thyroid hormone concentrations is referred to as ‘subclinical thyroiditis.’ Dogs with subclinical thyroiditis are considered at risk of progression to hypothyroidism. It is assumed that while dogs with subclinical thyroiditis have increased TgAA, the rate of progression to hypothyroidism varies and not all animals with increased TgAA will become hypothyroid. The investigators will study dogs that have subclinical thyroiditis to better define what proportion of animals subsequently develop hypothyroidism, and to investigate the amount of time until progression.