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Objective—To compare concentrations of urinary iodide (UI) in euthyroid and untreated hyperthyroid cats. Animals—118 euthyroid and 88 hyperthyroid client-owned cats from 2 nonreferral veterinary practices. Procedures—Iodide concentration was measured in 5 urine samples collected every 3 to 12 months from selected cats, and variability of results between euthyroid cats and hyperthyroid cats prior to the diagnosis of hyperthyroidism was evaluated via 1-way ANOVA, after logarithmic transformation of UI concentrations (logUIs). The UI concentration in hyperthyroid cats was measured at diagnosis and 2 to 6 weeks and 3 to 6 months after treatment for hyperthyroidism. The pretreatment logUI in hyperthyroid cats was compared with that in euthyroid cats, taking into account the effects of renal function on UI concentration. Iodine intake was estimated in euthyroid cats following calculation of the volume of daily urine output, with a fixed value for iodine concentration in feces. Results—The variability of UI concentrations did not differ significantly between hyperthyroid (n = 10) and euthyroid (8) cats. The logUI increased 2 to 6 weeks after initiation of treatment in hyperthyroid cats (n = 80) and was lower in azotemic versus nonazotemic cats. Hyperthyroid cats had a lower logUI than euthyroid cats, and there was no evidence of deficient iodine intake in euthyroid cats. Conclusions and Clinical Relevance—The logUI was lower in cats with azotemia and with untreated hyperthyroidism, compared with that in euthyroid cats from the same population. Additional studies are needed to determine whether iodine intake plays a role in the development of hyperthyroidism in cats.