Effect of insulin treatment on circulating insulin‐like growth factor I and IGF‐binding proteins in cats with diabetes mellitus

BACKGROUND: Insulin‐like growth factor‐I (IGF‐I) is used to screen for acromegaly in diabetic cats. In humans, most circulating IGF‐I forms ternary complexes (TC) with IGF‐binding protein (IGFBP‐3) and an acid‐labile subunit. Compared to humans, the amount of TC in cats is more variable. Insulin‐like growth factor‐I concentrations are reported to increase during insulin treatment, more rapidly in cats achieving remission. OBJECTIVES: To investigate (i) factors associated with circulating IGF‐I concentrations, including IGFBP‐profiles (ii) effect of insulin treatment on IGF‐I concentrations and (iii) IGF‐I as prognostic marker of diabetes mellitus remission. ANIMALS: Thirty‐one privately owned diabetic cats of which 24 were followed 1 year, and 13 healthy cats. METHODS: Prospective study. Serum insulin, IGF‐I, glucose, and fructosamine concentrations were measured. IGF‐binding forms were determined by chromatography in 14 diabetic and 13 healthy cats; and IGF‐I, IGF‐II, IGFBP‐3, and IGFBP‐5 by mass spectrometry in 3 cats achieving remission. RESULTS: Insulin‐like growth factor‐I median (interquartile range) before start of insulin treatment was 300 (160‐556) ng/mL. Insulin‐like growth factor‐I was positively associated with TC (P < .0001) and endogenous insulin (P = .005) and negatively associated with fructosamine (P < .0001). Median IGF‐I was higher 2‐4 weeks after start of insulin treatment compared with baseline (300 versus 670 ng/mL, P = .0001) and predicted future remission (P = .046). In cats that went into remission, the amount of TC and IGFBP‐3 increased, suggesting increase in IGF‐I is dependent on TC formation. CONCLUSIONS: Insulin treatment should be accounted for when interpreting IGF‐I in diabetic cats. Insulin‐like growth factor‐I 2‐4 weeks after initiation of insulin treatment shows promise as prognostic marker for remission in diabetic cats.