As far as I know, if thyroxine levels are 'within normal limits' but TSH (ie, thyroxine stimulating hormone) is high, it means your system is having to really work extra hard to stimulate the thyroid to make enough thyroxine, which means that if your system is under any extra demand (like getting pregnant) then it might not be able to cope. I think that's the way it works, anyway. I think it's great that you're finding out what the issues are now, and hopefully you'll conceive this cycle and be managing the levels from the very start.