The long-term effects of thyroidectomy depend on how much of the thyroid is removed.
If only part of your thyroid is removed, the remaining portion typically takes over the function of the entire thyroid gland, and you might not need thyroid hormone therapy.
If your entire thyroid is removed, your body can't make thyroid hormone and without replacement you'll develop signs and symptoms of underactive thyroid (hypothyroidism). As a result, you'll need to take a pill every day that contains the synthetic thyroid hormone levothyroxine (Levoxyl, Synthroid, Unithroid).
This hormone replacement is identical to the hormone normally made by your thyroid gland and performs all of the same functions. Your doctor will determine the amount of thyroid hormone replacement you need based on blood tests.