Talk to Endocrinologist on Thyroid

Dr I want to ask that whether Thyroid is 100% treatable? Because my sister is undertreatment for thyroid for more than 1 year. And some doc says that she has to take medicine for a lifetime (they are just decreasing or increasing med dosage in every visit after seeing the fresh test reports). And she sometimes has leg pain & weakness and Dr also said that she is having some hormonal problem as well. She is going to get married in Nov this year. Please guide me what should we do? & how we should get her treated? So she can recover fully after getting treated in the right direction.