Welcome to the forum Noexcuses and well done for getting all your tests done before you started. I wish I had done the same! I started a topic
about this a while ago (to encourage people to take all possible measurements before they start) but I don't think it has changed the world yet.
Your measurement (like Dr Mosley's) is for total IGF-I (aka IGF1, IGF-1). Most IGF-I in the blood is actually inactive because it is bound to 'binding proteins' in particular IGF-BP3, and it is only the active IGF-I which can pass on the 'grow grow grow' signals to cells and hence be potentially carcinogenic. Active IGF-I is often less than 1% of total IGF-I. So how important is total IGF-I as a figure? I don't think anyone really knows.
Although Dr Mosley did lower his total IGF-I significantly during his initial period of 5:2, we've not seen any confirmation of these results in a trial or indeed anecdotally. It is possible that IGF-I may be reduced more by reduced protein
intake than by the limited fasting intervention of 5:2, although as you know from the book/programme much longer fasting does reduce total IGF-I dramatically (but then it also involves zero protein intake!)
I hope you get on well with 5:2 and if so please let us know here in due course how your bloods have changed after following it! If you felt able, it might be good to keep a food diary or at least have some sort of record of what you have been eating as well as how much - I don't think Dr Mosley did this so it makes it harder to speculate on why his IGF-I went down...