Change in insulin-like growth factor type 1 concentration after radioactive iodine treatment in cats with hyperthyroidism

ObjectivesThis study aimed to evaluate changes in serum insulin-like growth factor type 1 (IGF-1) concentrations in cats with hyperthyroidism before and after radioactive iodine (RAI) treatment, as well as investigate the correlation between thyroid volume and serum IGF-1 concentrations.MethodsA total of 13 cats with hyperthyroidism and 14 healthy controls were included. Serum total thyroxine (TT4)/thyroid-stimulating hormone (TSH) and IGF-1/insulin-like growth factor binding protein-3 (IGFBP-3) concentrations were measured using chemiluminescence immunoassay and ELISA, respectively, at presentation and 6 months after RAI treatment. The results were compared with thyroid volume measured using scintigraphy. Data are presented as median (interquartile range [IQR]) and analysed using non-parametric tests.ResultsSerum TT4 concentrations significantly decreased from 9.30 µg/dl (IQR 6.49-12.7) to 2.23 µg/dl (IQR 1.34-2.94) after RAI treatment (<i>P</i> <0.001), while TSH levels increased from 0.021 ng/ml (IQR 0.021-0.021) to 0.125 ng/ml (IQR 0.050-0.257) (<i>P</i> = 0.002). IGF-1 levels significantly increased from 329 ng/ml (IQR 240-479) to 572 ng/ml (IQR 402-1038) after RAI treatment (<i>P</i> = 0.011), while IGFBP-3 levels did not differ. Serum creatinine concentrations significantly increased from 1.3 mg/dl (IQR 1.2-1.6) to 2.0 mg/dl (IQR 1.7-2.3) after RAI treatment (<i>P</i> = 0.006). No correlation was observed between IGF-1 and any variable, except IGFBP-3 (<i>r</i>_s = 0.587; <i>P</i> = 0.039) in the pretreatment group. IGF-1 and body weight were significantly positively correlated after RAI treatment (<i>r</i>_s = 0.696; <i>P</i> = 0.011) but not before. In healthy cats, IGF-1 was negatively correlated with serum TT4 (<i>r</i>_s = -0.627; <i>P</i> = 0.019).Conclusions and relevanceThe increased serum IGF-1 concentrations after RAI treatment may reflect the restoration of anabolic status in cats with hyperthyroidism. In this study population, no correlation was found between thyroid volume and serum IGF-1 concentrations.