Milo a castrated JRT 3years old has a range of symptoms he is biting at himself especially his legs and feet. His coat has got very thin and on his belly under his arms etc is getting bald. His coat also seems to look more wiry and harsh in places too and doesn't have its normal gleam. He seems to be getting a lot of dark pigmentation on his skin, and I can't get any weight on him even though he is fed a lot more than he is mean't to be for his weight, he is looking quite thin and his back bone sticks out. He does suffer with food allergies which I have him on a food that suits him so I am sure its not that. He has had anemia in the past and occasionally again now his gums are looking pale on and off and then suddenly pink up again. He also doesn't have the normal energy of a JRT. he has his moments but does tire quite quickly.
I was wondering if it could be his thyroid causing the problem, but on reading about it it says dogs get overweight, which he definatly is not.
He is also getting a little grumpy towards boistrous dogs and us if we disturb him when he is resting. His temprement has always been so calm, happy and love everyone and everything.
Could I have your ideas and opinions please as to whether you think it could be thyroid or anything else you may think it is so I can go armed to the vet with suggestions  .
I was going to give him a bath in some malaseb today, but I also have some advocate here for him and I don't know if I can use the two together and if not which I should try first to see if that helps him over the weekend.
Anyone know the answer to that as well please?

Cushings Disease?

It could still be thyroid problems but hyperthyroid rather than the more common hypo. 

I don't know why I didn't think of Cushings as I had a dog with that many years ago.  Milo has the appetite and hair loss etc but he doesn't have the excessive thirst or urination that my girl had.

My flatcoat was diagnosed hypothyroid without having the weight gain etc. Results from labs over here didn't give a definitive diagnosis so he had blood sent to Jean Dodds in the USA. Some itchiness, but certainly not to that degree.  Have the vets done a skin scrape for mites? Demodex can become a problem if they are ill for another reason.