What sort of thyroid test have they done? You need a
comprehensive thyroid test which also tests antibodies (TgAA), this includes testing for T4, free T4, T3, free T3, TgAA, TSH. Most vets do a basic thyroid test (T4, TSH) which doesn't always give the correct information. If antibodies (that destroy the thyroid glands) are present, then the thyroid hormone levels (T4/free T4) can look ok eventhough they are not! If a vet doesn't do the comprehensive test, then they will never know that.
Also, if you get the results from a comprehensive test, please send them to Jean Dodds (a vet in the States that specialises in thyroid) to interpret the results correctly for you! Many vets can't interpret them correctly because they don't know the relationship between antibodies and T4 or the different normal levels for different breeds, ages etc. ; Jean Dodds does and she is very helpful.
Google for her name and lots of info will come up that might be helpful for you.