Once you have under active or over active thyroid, will you have it always?

Not if treated!: Hi. We can't cure permanent cases of hypo- or hyperthyroidism, but we certainly can treat them! Treatment for hyperthyroidism usually ultimately results in hypothyroidism, which is super easy, cheap, and effective to treat. So untreated, you would still have abnormal thyroid function, but treated, you'll have normal thyroid function. Good luck!