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If your thyroid has been removed (during a ), your body can no longer make the thyroid hormone it needs. You will need to take thyroid hormone (levothyroxine) pills to replace the natural hormone. This can help maintain your normal metabolism and possibly lower your risk of the cancer coming back.

How thyroid hormone therapy can help

Normal thyroid function is regulated by the pituitary gland. The pituitary makes thyroid stimulating hormone (TSH). TSH causes the thyroid gland to make thyroid hormone for the body. It also promotes the growth of the thyroid gland, and probably of thyroid cancer cells as well.

In turn, the level of TSH is regulated by how much thyroid hormone is in the blood. If the level of thyroid hormone in the blood is low, the pituitary makes more TSH. If the level of thyroid hormone is high, not as much TSH is needed, so the pituitary makes less of it.

Doctors have learned that by giving higher than normal doses of thyroid hormone, TSH levels can be kept very low. (This is known as thyroid hormone suppression therapy.) This may slow the growth of any remaining thyroid cancer cells and lower the chance of some thyroid cancers (especially high-risk cancers) coming back.

Possible side effects of thyroid hormone therapy

Taking higher than normal levels of thyroid hormone seems to have few short-term side effects, but some doctors have expressed concerns about taking them for long periods of time.

• High levels of thyroid hormone can lead to problems with a rapid or irregular heartbeat.
• Over time, high doses of thyroid hormone can also lead to weak bones (osteoporosis).

Because of this, your doctor might avoid giving you high doses of thyroid hormone unless you have had a differentiated (papillary or follicular) thyroid cancer that has a high risk of coming back.