Hypothyroidism is most commonly an acquired condition of adult dogs characterized by primary failure of the thyroid gland to produce adequate amounts of thyroxine (T4) and triiodothyronine (T3). Pathogenesis commonly involves immune-mediated destruction of functional thyroid tissue (thyroiditis) or idiopathic thyroid atrophy, which may represent end-stage thyroiditis.³ Well-documented breed associations support a genetic susceptibility, with a selection of commonly affected breeds including English setters, Doberman pinschers, Rhodesian ridgebacks, and golden and Labrador retrievers.^4–7 Disruption of other portions of the thyroid axis are rarely reported as causes of hypothyroidism, such as reduced secretion of pituitary thyroid stimulating hormone (TSH) or hypothalamic thyrotropin-releasing hormone.⁸ Congenital hypothyroidism resulting from inherited genetic defects or abnormal thyroid gland development have been rarely reported in dogs.^9–11