I take Synthyroid also for Hashimoto's and have had an extreme reaction to methylfolate, wondered if thyroid could play a role, making it more difficult. It would be interesting to create a profile of those who have a huge response to tiny dosages. I had a Spectrocell test in 2010 that showed no folate deficiency despite catching other deficiencies that were supported by the genetics testing, such as Vitamin A (I have a hard time converting it). When I took 400 mcg methylfolate I was wiped out for days, in bed for most of one whole day. For me, the side effects take days to subside after a single dose, even just 25 mcg. That does not support the idea of methylfolate being cleared in a matter of hours, or, it suggests that a process has been started that is independent of the methylfolate, or that folic is still in my system and interacting somehow.