When I was going through IVF I was diagnosed as having an underactive thyroid and reffered to an endocrinologist. She put me on thyroxine for a year. I was checked every 4 months or so. Then in April I was diagnosed as having a hyperactive thyroid. Turns out I have developed Graves disease, so I was put on another medication, one that is safe with pregnancy as I was pregnant by then.
I'm going in every few weeks as my doc says the disease will go into remission soon, as sycx has said. I will be taken off the medication but there is a good chance the disease will come back after the pregnancy. Then back on the meds!