The trace elements iodine and selenium are important for thyroid function. Thyroid homeostasis may not only be influenced by excessive or deficient absolute intake of iodine or selenium but also by relative historical intake of these trace elements. In addition, non-nutritive factors such as natural and synthetic exogenous thyroid disrupters affect thyroid metabolism. Finally, in addition to the nutritional and environmental influences, genetic factors influence development and progression of thyroid disorders. Despite this complexity, scientific advances have increased understanding of the factors that influence development and progression of feline hyperthyroidism, and provided information that may help to further improve treatment.