60% of celiacs with dh test false negative on the celiac blood panel so there goes that neg. result. Plus they often miss tests in the blood panel --- more often than not. So many people don't end up with a FULL blood panel but that is sort of a moot point here in this case.
If you've been taking steroids in the past 2-3 months before any of the celiac tests (blood or dh biopsy or even endoscopic biopsy) then the result will be a false negative. It's very difficult to get dx'd with dh b/c docs just don't understand/know this. It looks like you were on the steroids when the dh biopsy was done which makes the biopsy a wash. In other words, you still don't know. You might as well not have had the biopsy. Even when a biopsy is taken correctly & you have been actively eating gluten for the proper amount of time & not been on steroids for the proper amount of time ~~~~ the LAB that examines the biopsy has to really know what they're doing & the tests they perform are subjective so there's a ton of room for error!
Since you were on steroids then I'm betting (but can't say for positive) that all those tests for anything autoimmune are junk. Steroids ramp down the immune response which is contradictory to the tests to discover immune problems.
A couple things stand out to me though in your case.
1) DH is insanely itchy. Absolutely drive you out of your mind. crazy, just shoot me & be done with it itchy!!! You didn't really mention anything like that.
2) When ppl with dh take sreroids the steroids work to control the dh & you thing YAY! BUT the moment you go off them the dh comes back quadruple fold & punishes you like you wouldn't believe for using steroids for it & then removing them. You didn't mention either of these things happening.
I'm going to have to say, IMO, it doesn't really sound like you have dh. Not saying it isn't possible but it doesn't seem like it.
As far as a test for Iga -- you can get one but that's up to you. It would be better to go back to the derm in 3 months after you've had NO steroids & get a dh biopsy performed again.
I hope this helps.