The present invention provides a method of diagnosing the existence or risk of hyperthyroidism in a feline comprising measuring the level of expression of one or more biomarkers selected from the group consisting of e.g., IYD, TG, SLC5A5, NIS, TPO, TSHR, DUOX1, DUOX2 (ThOX), TGFB1, CSTD, DCN and SEPP1 and the expression products thereof, in a biological sample from the feline, wherein elevated expression of the one or more biomarkers in the sample relative to a control value for expression in a sample from a normal feline or feline population, or a baseline value from the feline, indicates the existence or risk of hyperthyroidism; a method of treating a feline so diagnosed; and compositions, reagents and kits for carrying out the specified methods.