How to Tell If Your Dog Is Tired: 7 Key Signs for US Owners

Hypothyroidism is one of the most commonly diagnosed hormonal disorders in dogs, occurring when the thyroid gland produces insufficient levels of thyroid hormones. These hormones play a crucial role in regulating metabolism, and their deficiency leads to a wide range of clinical signs. The condition is most often caused by immune-mediated destruction of the thyroid tissue, a process called lymphocytic thyroiditis, or by idiopathic atrophy of the gland.

Clinical signs of hypothyroidism in dogs develop gradually and can be subtle in the early stages. The most common signs include unexplained weight gain despite normal or reduced food intake, lethargy and exercise intolerance, mental dullness, and cold intolerance. Skin and coat changes are also hallmark signs, including hair loss, particularly over the trunk and tail, dry and flaky skin, and a dull, thin coat. Some dogs develop a tragic facial expression described as a "tragic face" due to thickening of the facial skin, drooping of the eyelids, and a sad or swollen appearance.

Diagnosis is confirmed through blood testing, specifically measuring serum total T4 levels, with additional testing such as free T4 by equilibrium dialysis if initial results are borderline. Treatment consists of daily oral supplementation with synthetic thyroid hormone (levothyroxine), which is highly effective and affordable. Most dogs show significant improvement in energy and behavior within the first few weeks of treatment, and coat and body condition continue to improve over several months. Regular monitoring of thyroid hormone levels is required to ensure the dose remains appropriate as the dog ages.