Hypothyroidism in a dog after surgery and radiation therapy for a functional thyroid adenocarcinoma

Hypothyroidism was diagnosed in a dog which had undergone unilateral thyroid lobectomy and external beam irradiation (48 Gy in 3 Gy fractions) for a functional cystic thyroid adenocarcinoma. Hypothyroidism became biochemically apparent within 4 months of completion of radiation therapy, and clinically apparent within 7 months. Clinical signs resolved after thyroid hormone supplementation. The potential for alterations in thyroid function should be considered in any animal undergoing radiation therapy in which the thyroid gland is included in the radiation field. This potential may be greater if surgery and radiation are combined.