Retinol binding protein (RBP) is a marker of renal tubular damage that is variably detected in urine of untreated hyperthyroid (HT) cats (van Hoek et al., JIM 2008;329:208-213). No data are available on serum RBP in cats or on the influence of treatment on serum or urinary RBP. In humans, serum RBP concentrations can be significantly lower in hyperthyroidism compared to eu- and hypothyroidism. The objectives of this study were to evaluate serum and urinary RBP levels in HT cats compared to healthy cats (H), and the influence hereon of radioiodine (¹³¹I) treatment.

Inclusion criteria were clinical signs compatible with hyperthyroidism, increased total thyroxin (TT4) serum concentration (nmol/L) and increased thyroidal uptake of ^99mTcO₄^-. Blood was taken by jugular venipuncture and urine by cystocentesis 1 day before and 4, 12 and 24 weeks after ¹³¹I treatment. After centrifugation, samples were aliquoted and stored frozen until assayed. A polyclonal rabbit anti-human RBP antibody was used in a commercial sandwich ELISA validated for RBP assessment in feline urine (van Hoek et al., JIM 2008;329:208-213). Parallelism of serial dilution curves of feline serum samples with trend lines from human RBP standards, indicated adequate recovery of feline RBP in serum. RBP concentrations were expressed as µg/L in serum and as RBP/urinary creatinine (RBP/c, 10^-2µg/mg creatinine) ratio in urine.

Ten HT cats and 8 H cats were included. Mean ± SD of serum RBP, urinary RBP/c ratio and serum TT4 concentration in HT cats before and after treatment and H cats are described below. A linear mixed model was used for statistical analysis.

There was a significant difference between H and HT cats before treatment in urinary RBP/c ratio (P=0.015) and in serum TT4 concentration (P<0.001). Serum RBP did not differ significantly between H cats and HT cats before (P=0.494) or 4 weeks (P=0.335), 12 weeks (P=0.383) or 24 weeks (P=0.835) after treatment. Serum TT4 concentration decreased significantly at all time points after treatment (P<0.001) and urinary RBP/c decreased significantly 4 and 12 weeks after treatment (P=0.003). There was no significant change in serum RBP concentration (P=0.796) before and after treatment. There was a significant correlation in HT cats between serum TT4 concentration and urinary RBP/c at all time points (0.42, P=0.007), but no significant correlation between serum RBP and TT4 (0.03, P=0.858) or urinary RBP/c (-0.16, P=0.333).

Serum RBP from HT cats does not differ statistically significantly from those of H cats and does not change after treatment, supporting the hypothesis that urinary RBP in HT cats is caused by a decreased renal function which is reversible upon treatment with ¹³¹I.