How Well It Works. Antithyroid medicines do not always start working right away. Symptoms usually get better or go away 1 to 8 weeks after you start taking the medicine. It may take as long as 6 months for your thyroid hormone levels to become normal.Antithyroid medicines work best if you have mild hyperthyroidism, if this is the first time you are being treated for Graves' disease, if you are younger than 50, or if your thyroid gland is only swollen a little bit (small goiter).ntithyroid medicines work best if you have mild hyperthyroidism, if this is the first time you are being treated for Graves' disease, if you are younger than 50, or if your thyroid gland is only swollen a little bit (small goiter).