I was first diagnosed with an underactive thyroid when I was 7 years old, I was then diagnosed with Hashimoto's 6 years ago with an antibody reading of 1:102,400 (which is rather high). I've just had my results back with antibodies within normal range...
Is this possible?
My doctor is now sending me to a new endo... I'm trying to be thankful for small mercies and am struggling to do so... 
Interesting question! I had antibodies >1,000 at diagnosis. But they haven't been tested since. GPs don't seem to think it matters as you still need to 'keep taking the tablets'.
Thats what I've been told for routine blood work but as my symptoms are coming back full force I need all angles covered.
Its just so unusual, I asked my doc why and she said straight away that she would like to refer me. it clearly puzzles her too!
I'm now starting to think that my thyroid has stopped working altogether and thats why the antibodies have dropped? No thyroid means no reason for them to hang about?
And if my thyroid has ground to a complete stop would it not make sense to increase my thyroxine or supplement with T3?
All these questions running round in my head - its driving me nuts! Another night of little or no sleep ahead 
I can see where you're coming from with the antibodies not having any reason to exist. Not convinced it makes any difference to the medication issue though. Assuming your dose is high enough... and you're absorbing sufficient thyroxine from your tablets... and all the bits of your body are converting to T3 appropriately, then theoretically all should be fine (yeah right).
My own theory is that if the thyroid is totally dead, then a little T3 should be added. That's what a working thyroid does... mainly T4 with a T3 top up. But trying to get T3 on prescription is like trying to get blood from a stone.
well the letter i got from my last (private) endo said that with high antibodies he'd be surprised if my thyroid was producing anything at all and if it was it would progressively reduce over time...
so I'm guessing that might just be the case... a dead thyroid! so maybe this new endo will do a test to see if that is in fact the case.
How would they test that? scan it?
I think a scan could help, but it's probably not definitive. I had an ultrasound some time after diagnosis. The chap said my thyroid showed clear signs of inflammation. Wish I'd known more about all this then... I'd have asked more questions.
I think the only other test would be to come off medication completely and test T4/T3 after a few weeks. Not suggesting you try this of course! But I don't think there's any other way to be sure.
Have you tried desiccated thyroid? Seems to be the answer for some people.
Antibody tests for Graves and Hashimoto
Diagnosis Hashimotos Based On Antibodies Only
Hashimoto's and antibodies
Hashimotos antibodies 
Vitamin D and Hashimoto antibody load
