From a post I made yesterday in Grunt's IGF-I thread. My take on IGF-1
Originally Posted by CryingEmo
The growth-promoting activity of insulin itself is due to its limited ability to bind to IGF-I receptor. To do this, insulin has to be present in high concentrations because the affinity of the IGF-I receptor for insulin is about 1000x less than the affinity of the insulin receptor for insulin. IGF-II can also productively bind to the IGF-I receptor, albeit with much lower affinity. Thus, all three factors are growth-promoting, and IGF-II and insulin become important when IGF-I is low or absent. Presumably, IGFs can cross over to the insulin receptor in the absence of insulin, although this sort of rescue may not be important in insulin-deficient diabetes.
Although the source material doesn't exactly say ...it appears that the insulin receptor has a weak (rather than strong) binding affinity for IGFs...is it 1000x less...maybe?