Hypothyroidism is the most common endocrine disease in dogs. It can be caused by autoimmune destruction (lymphocytic thyroiditis) or idiopathic atrophy of the thyroid gland. These two causes are responsible for over 95% of the hypothyroidism cases in dogs. Signs include decreased appetite, weight gain, hair loss, dry skin/coat, skin that is cold to the touch, recurring skin infections, and lethargy. The dog may also seek out warm places to lie.
The symptoms of hypothyroidism are shared with many other medical conditions; it may not be the first thought when a diagnosis is made. Symptoms may not appear until 75% or more of the gland is non-functional. In less than 10% of hypothyroidism cases, the problem is not with the thyroid gland itself, but with the pituitary gland in the brain. The pituitary gland produces a thyroid stimulating hormone (TSH); without this hormone to signal the thyroid gland to produce its thyroid hormone, the thyroid gland remains inactive. Treatment is with oral thyroid hormone supplementation.
Lack of enough iodine in the diet can produce a form of hypothyroidism; without the proper amount of it, the thyroid gland fails to produce enough thyroid hormone. Myxedema coma is a rare but serious aspect of the disease that is a medical emergency. – Wikipedia