Abstract
BackgroundSerum symmetric dimethylarginine (SDMA) is a sensitive renal biomarker for detecting early chronic kidney disease (CKD) in nonhyperthyroid cats, but knowledge regarding its performance in hyperthyroid cats remains limited.ObjectivesTo determine the relationship between serum SDMA, creatinine and total thyroxine (TT4) concentrations in hyperthyroid cats before (T0) and 3 months after (T1) receiving a PO fixed dose of radioiodine.AnimalsEighty client‐owned hyperthyroid cats.MethodsProspective cohort study. Serum TT4, and SDMA, creatinine concentrations, and urine specific gravity were measured at T0 and T1. Nonparametric tests were used to determine the relationship among SDMA, and creatinine and TT4 concentrations. Agreement between SDMA and creatinine regarding CKD staging at both time points was assessed using Goodman and Kruskal's gamma statistic.ResultsMean serum SDMA concentration increased after treatment of hyperthyroidism. However, 21 of 75 cats experienced a decrease in SDMA between T0 and T1, whereas creatinine decreased in only 2 cats. A moderate correlation between SDMA and creatinine was seen at T1 (r = 0.53; P < .001) but not at T0 (r = 0.13; P = .25). Where assessable at T1, poor agreement was observed between SDMA and creatinine and CKD stage (Goodman and Kruskal's gamma 0.20; P = .29).Conclusions and clinical importanceDiscordant outcomes between SDMA and creatinine after radioiodine treatment in cats with hyperthyroidism suggest extrarenal factors may interfere with the reliability of SDMA to adequately reflect renal function. As a result, SDMA should not be interpreted in isolation in hyperthyroid cats treated with radioiodine.
Highlights
Hyperthyroidism and chronic kidney disease (CKD) are common in older cats
Renal azotemia was defined as a serum creatinine concentration of ≥1.6 mg/dL (≥140 μmol/L) or a serum symmetric dimethylarginine (SDMA) concentration of ≥18 μg/dL with concurrent urine specific gravity (USG)
Consistent with the study hypothesis and recently published results,[18] mean SDMA concentration increased significantly as hyperthyroidism resolved. This is presumably because serum SDMA concentration correlates with GFR16,29 and an increase in serum SDMA concentration is expected as the effects of hyperthyroidism on renal perfusion resolve.[5,30,31]
Summary
Hyperthyroidism and chronic kidney disease (CKD) are common in older cats. Previous studies indicate that 10% to 49% of cats with hyperthyroidism have concurrent renal azotemia diagnosed before or after treatment.[1,2,3,4,5,6] Chronic kidney disease has implications for the clinical decision-making process regarding optimal treatment options for hyperthyroidism and may decrease the lifespan of affected cats. Indirect measures of GFR such as serum creatinine and urea nitrogen concentrations are notoriously inaccurate at detecting CKD in hyperthyroid cats. Their inaccuracy is largely attributable to the effects of extrarenal factors such as muscle mass, as well as daily variability in exogenous and endogenous protein loads.[8,9,10,11]. Symmetric dimethylarginine (SDMA) is an indirect biomarker of GFR that is unaffected by muscle mass and reportedly detects CKD with higher sensitivity than creatinine.[11,16] Recently, SDMA has been incorporated as an adjunctive variable in the International Renal Interest Society (IRIS) guidelines for CKD staging and management.[17] the influence of hyperthyroidism on SDMA in cats is yet to be explored in detail. After radioiodine treatment, where renal dysfunction was suspected based on serum creatinine concentration and concurrent suboptimal urine specific gravity (USG), serum SDMA concentration was hypothesized to be concurrently increased
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