Not had a dog with a thyroid prob but I have one (born without thyroid gland) and I think it may be the same in dogs as humans that once it is diagnosed - it is usually a 'for life' condition. It normally can be controlled by giving thyroid tablets (in humans at least, I was 3 months old when diagnosed and turned 65 years last week so don't give up hope) and I would think it is easier to treat an underactive thyroid than an overactive.
Hope she is feeling better soon