I'm hypo too. I increase my dose of thyroxine by 25mcg on having a bfp then inform my doctor that this is what I've done, booking for a bloodtest to check levels 2 weeks later. Levels get checked about every 8 weeks I think and once registered with the midwife, or equivalent if not UK based, you should be referred to an endocrine clinic who will monitor you. At least, that's my experience. Never had any issues, but I can't remember whether my dose increased after my initial increase or not.