Hi,
We need to confirm your diagnosis. Are you mildly hypothyroid or hyperthyroid.
Hypothyroid patients need thyroid supplements whereas hyperthyroid patients need drugs like 
methimazole as you mentioned. Surgery is also an option. It would be more convenient if your TSH levels are known.
Considering you are hyperthyroid and planning for 
pregnancy you can consult with your 
endocrinologist and opt for 
propylthiouracil before and during the first trimester of pregnancy and switch to methimazole in remaining trimesters.
If surgery is warranted (which is decided by your cause of 
hyperthyroidism), then I would suggest going for surgery.
Postoperatively you can take thyroid supplements life long and plan for pregnancy which is a very much safer and simpler option for assuring fetal health.
Hope I have answered your query. Let me know if I can assist you further.
Take care
Regards,
Dr. Saranya Ramadoss, General & Family Physician