Feline hyperthyroidism (FH) was first reported in 1979 and is now recognized as the most common feline endocrinopathy. The pathogenesis of the disease remains unclear despite numerous epidemiological and molecular studies reported since that first case. Whether FH is a new disease, an epidemic of a rare disease, or can be attributed to an older cat population, increased veterinarian and owner awareness, or improved diagnostic testing is not known. Several risk factors for FH have been identified but none has proven to be both necessary and sufficient to cause the disease. The cause of FH is likely mulltifactorial.