Hyperthyroidism in cats was first discovered in the late 1970s (Holzworth and others 1980). since then, its incidence has increased from one in 1000 cats seen by veterinarians (between 1978 and 1982) to 21 in 1000 (between 1993 and 1997) (Edinboro and others 2004). Hyperthyroidism is now recognized as the most commonly diagnosed endocrinopathy in small animal practice. to understand why feline hyperthyroidism ostensibly began in 1979 and has steadily increased in incidence, we should explore the possible causes (ie, risk factors) and whether cats began to be exposed to these risk factors in 1979 and whether they have been increasingly exposed since. despite the disease being well described there remains little insight into its causal mechanism; however, peterson (2012) initiated a key debate on the importance of exposure to chemicals with thyroid mimicking properties as important risk factors in FtH. Here, we will expand on this thinking.