I had hyperthyroid during my pregnancy . now my baby is three months old n i still have overactive thyroid. is it true that it will continue forever ?

Overactive thyroid: If you have an overactive thyroid your doctor must put you on medication that will bring your thyroid levels back to normal. Should have little or no effect on your new baby if treated properly. See your obstetrician and your primary doctor.

Possibly: It sounds like you have Graves disease that started during pregnancy. About 20-30% of patients with this will go into remission while taking antithyroid drugs. However, that means that 70-80% of patients will have recurrence of hyperthyroidism when they stop the medication. The options then are to do surgery to remove most of the thyroid, or receive radioactive iodine.