TPO and autoimmune thyroid disease: GD is an immune system disorder characterized by enlargement of the thyroid gland (goiter) and overproduction of thyroid hormones (hyperthyroidism), while HT is an autoimmune thyroid disease for which the exact etiology remains unclear with, as a typical sign, the infiltration of autoreactive T cells in thyroid tissues and the production of autoantibodies (anti-thyroid peroxidase—TPO and anti-thyroglobulin—TG) associated with destruction of the thyroid follicles, which ultimately results in hypothyroidism (i.e., low serum free T4 and elevated serum TSH levels) [12,55,56].