The thyroid gland regulates metabolic rate throughout the body. When hormone production drops, dogs typically gain weight despite normal food intake, become sluggish, seek warm spots, and develop a dull or thinning coat. Skin may darken and thicken, and ear infections may recur. Diagnosis involves a blood panel measuring T4 and TSH levels. Treatment is straightforward: daily synthetic thyroid hormone tablets taken for life. Most dogs respond well within weeks, regaining energy and healthier coats. Regular blood monitoring ensures the dose stays correct as needs can shift with age or weight changes.