Long-Term Follow-Up of Glomerular and Tubular Kidney Function in Hyperthyroid Cats After Treatment with Radioiodine
Treatment of feline hyperthyroidism may influence different functions of the nephron. The objectives of this study were to evaluate long-term effects of radioiodine (131I) treatment on glomerular and tubular function in hyperthyroid (HT) cats and to evaluate the potentially predictive value of renal parameters for development of chronic kidney disease (CKD).
Inclusion criteria were clinical signs compatible with hyperthyroidism, increased serum total thyroxin (TT4) concentration and increased thyroidal uptake of 99mTcO4-. The exclusion criterion was azotemia before treatment. Antithyroid drugs had to be discontinued at least 3 weeks prior to inclusion. One day before, and 1, 4, 12 and 24 weeks after 131I, serum concentrations of TT4, creatinine and urea, urine specific gravity (USG), urinary protein/creatinine ratio (UPC) and blood pressure (BP) were determined. Glomerular and tubular function were evaluated by glomerular filtration rate (GFR) measured with the plasma exo-iohexol clearance test (van Hoek et al., JVIM 2007;21:950) and by urinary retinol binding protein/creatinine ratio (RBP/c) (van Hoek et al., JIM 2008;329:208), respectively. Results were analysed with Student's t-test and with ANOVA.
Twenty-two cats were included and divided at 24 weeks after 131I in group A (n=16) without evidence of CKD and group B (n=6) with CKD (IRIS stage II). Serum TT4 and GFR decreased significantly until 4 weeks after 131I for the whole group (P<.001) and group A (P<.001) but only until 1 week after 131I in group B (P<.001). Serum creatinine increased significantly from 1 week until 12 weeks after 131I (P<.001) in the whole group, until 24 weeks after 131I in group A (P<.001), and only until 4 weeks after 131I in group B (P<.001). Serum urea and USG did not change significantly after 131I for the whole group (P=.224 and P=.216) or for group A (P=.787 and P=.249) and B (P=.107 and P=.539) separately. There was a significant decrease until 4 weeks after 131I in UPC for the whole group (P<.001), group A (P<.001) and group B (P=.029) and in RBP/c for the whole group (P=.001) and group A (P<.001), but in group B RBP/c (P=.149) did not change significantly over time after 131I. Blood pressure decreased significantly for the whole group from 4 until 24 weeks after 131I (P=.022), but did not change significantly in group A (P=.082) or group B (P=.302) separately. Group A had significant higher serum TT4 concentration (P=.003), USG (P=.001) and GFR (P=.009) before 131Icompared to group B. All cats from group B had a USG < 1.035 before 131I.
This study is the first to show a prolonged effect of 131I treatment on glomerular function measured with GFR and tubular function measured with RBP/c in HT cats maintaining a healthy kidney function, but not in patients developing CKD. Pretreatment serum TT4 concentration, USG and GFR could potentially be predictive parameters for development of CKD after 131I treatment.