EXPRESS: Longitudinal evaluation of discordant symmetric dimethylarginine and creatinine as markers of glomerular filtration rate in radioiodine-treated hyperthyroid cats.

Use SDMA ≥18 µg/dL cutoff in senior cats; values 15-18 µg/dL warrant monitoring before assuming GFR decline.

This prospective longitudinal study evaluated 27 hyperthyroid cats treated with radioactive iodine to assess discordance between symmetric dimethylarginine (SDMA) and creatinine as glomerular filtration rate (GFR) markers. Discordance occurred in 10-75% of cases, with SDMA showing higher discordance with GFR than creatinine, particularly at baseline when cats were hyperthyroid. In euthyroid cats at 12 months, SDMA >14 µg/dL demonstrated superior sensitivity (83%) but poor specificity (50%) for decreased GFR, while SDMA ≥18 µg/dL provided better specificity (71%). Both biomarkers showed similar intra-individual variability (~15%). The study concludes that the SDMA cutoff of ≥18 µg/dL is more appropriate for senior cats, as SDMA values of 15-18 µg/dL may reflect non-renal influences such as hyperthyroidism rather than true GFR reduction. Clinicians should maintain ongoing renal monitoring in these patients.

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