Hyperthyroidism is an excess of the thyroid hormone, thyroxin. Thyroxin determines how quickly your pet burns fuel and makes energy. Because every cell in the body burns fuel, every cell is affected by hyperthyroid disease, but the first sign we often notice is how thin the pet is despite having a ravenous appetite.
Over the last few years, the incidence of hyperthyroid disease has markedly increased in cats (Feline Hyperthyroid Disease). Some believe that hyperthyroid disease is caused, in part, by the chemicals in the plastic liners in canned cat foods. In the few dogs that develop this disease, thyroid cancer may be the cause.
For cats, especially older cats, feline hyperthyroidism is very common; however, there is no breed predisposition. Dogs rarely develop hyperthyroid disease.