The thyroid gland is an endocrine gland in which the hormones thyroxine and triiodothyronine are synthesized. They are important for the proper growth and development of the organism. The most common thyroid disorders in dogs and cats are hypothyroidism and hyperthyroidism. Hypothyroidism is characterized by a low level of thyroid hormone in the blood due to a decrease in its function. The causes of hypothyroidism are a lack of iodine, T3, TSH or an enzyme involved in hormone synthesis, thyroid dysgenezia, and lymphocytic thyroiditis. Most common symptoms are lethargy, obesity and dermatological changes. The disease is diagnosed on the basis of a clinical examination, hematological and biochemical tests, and an endocrinological test. The concentration of T4 below 15 nmol/L and TSH concentration between 1-3 μg confirm the diagnosis and the disease is treated with levothyroxine. Hyperthyroidism is characterized by a high level of thyroid hormone in the blood and is most often caused by thyroid cancer or thyroid hyperplasia. Symptoms specific to hyperthyroidism are weight loss with increased appetite, restlessness and hyperreactivity, as well as tachycardia and heart murmur. Suspicion is raised in the presence of clinical signs or palpation of enlarged lobes of the gland. The diagnosis is based on determining the concentration of thyroxine in the blood. Therapy is based on antithyroid drugs or a radioactive iodine application and in some cases surgical removal of the thyroid gland is recommended.