Hi Rene70,
I too have an underactive thyroid and questioned my Thyroid consultant at the hospital about whether this was causing me problems with fertility and he said it wouldn't as long as I am taking my medication, the levels are all regulated and the dosage is correct ( regular blood testing), I was told that as soon as I do get pregnant to tell the Dr as will probably need the dose increased....incidentally I did get pregnant (albeit briefly) last year and when I had my blood tested my levels had dropped by 3 times....!
Hope this helps,