Hi Synthroid is thyroid hormone Taking daily pills of thyroid hormone (thyroid hormone therapy) post breast cancer can serve 2 purposes: It can help maintain the body’s normal metabolism (by replacing missing thyroid hormone after surgery). It can help stop any remaining cancer cells from growing (by lowering TSH levels). After a thyroidectomy, the body can no longer make the thyroid hormone it needs, so patients must take thyroid hormone (levothyroxine) pills to replace the loss of the natural hormone. Taking thyroid hormone may also help prevent some thyroid cancers from returning. Normal thyroid function is regulated by the pituitary gland. The pituitary makes a hormone called TSH that causes the thyroid gland to make thyroid hormone for the body. TSH also promotes growth of the thyroid gland and probably of thyroid cancer cells. The level of TSH, in turn, is regulated by how much thyroid hormone is in the blood. If the level of thyroid hormone 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 may slow the growth of any remaining cancer cells and lower the chance of some thyroid cancers (especially high-risk cancers) coming back. 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 the long run, high doses of thyroid hormone can lead to weak bones (osteoporosis). Because of this, high doses of thyroid hormone may be reserved for people with differentiated thyroid cancers who are at high risk of recurrence. Regards Dr De