This study contributes to the understanding of the IGF system and its regulation in cats. Total IGF‐I concentrations in serum were positively associated with the amount of IGF binding in a 150kDa TC, with lower IGF‐I levels associated with relatively more binary IGF‐IGFBP complexes. Furthermore, IGF‐I concentrations were increased at 2–4 weeks after initiation of insulin treatment and higher IGF‐I concentrations at this time‐point were associated with remission from DM. An increase in circulating IGF‐I with treatment was accompanied by a shift to increased amounts of TC compared with binding in lower molecular mass binary complexes. Consistent with this shift, IGFBP‐3 concentrations were also observed to increase. It is therefore recommended that, when using IGF‐I as a marker of GH status in cats with diabetes, the effect of insulin treatment on IGF‐I should be taken into account.