The thyroid gland, located in the neck, produces thyroid hormone thatregulates metabolism and affects every organ in the body. Millions of Americans must take an oral thyroid supplement each day because of decreased thyroid function. This is thought to be the most common endocrine (hormone) disease in dogs, also. (In cats, hypothyroidism is almost unheard of, whereas increased thyroid function, hyperthyroidism, is extremely common.) "Primary hypothyroidism" means that the problem is in the thyroid gland itself, not in the parts of the body that regulate the thyroid gland. Primary hypothyroidism accounts for about 95 percent of cases in dogs. Either the thyroid gland is damaged by inflammation ("thyroiditis"), or the gland atrophies and is replaced by fat. Only rarely in dogs is there a tumor causing destruction of the gland.