Following on from my earlier post, I thought I should mention this topic. There is a really good summary on the stopthethyroidmadness website, link here:
It may be the case that, for some people, inactive forms of folate or B12 simply build up in the blood and cannot be converted to more active forms. This is why Methyl B12 works best for some people, or the more active form of folate supplement Methylfolate. It is also why testing serum (blood) levels of B12 and folate don't give the full picture.
This is another website all about MTHFR, and I have specifically linked here to a podcast about thyroid disorders:
I haven't listened to it yet (I'm away with a flaky internet connection) so if anyone does please let me know if it's any good!