Canine Hypothyroidism

Canine hypothyroidism, while a common endocrinopathy in the dog, may be overdiagnosed due to confusion/inconsistencies in establishing a definitive diagnosis. Hypothyroidism most commonly occurs in young to middle-aged dogs with an average age of 7 years. Common clinical signs include lethargy, mental dullness, weight gain, exercise intolerance, alopecia, and obesity. Although rarely necessary, ultrasound of the thyroid glands (by an experienced ultrasonographer) can aid in differentiating dogs with primary hypothyroidism from those with nonthyroidal illness. Thyroid glands of hypothyroid dogs tend to be smaller, less homogeneous, and hypoechoic than those of euthyroid dogs. Levothyroxine is the only hormone that appears necessary for treatment of hypothyroidism. Hyperthyroidism is the most common complication of treatment with levothyroxine, but it is rare in dogs. Clinical signs are similar to those of hyperthyroidism in cats and the diagnosis is confirmed by documenting a substantial elevation of serum T4.