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Although the objective of total thyroidectomy is to remove the entire thyroid gland, due to technical reasons this goal is rarely achieved in most patients. In fact, in about 90% of patients undergoing total thyroidectomy, residual thyroid tissue can still be demonstrated after surgery by a radioactive iodine scan. This residual thyroid tissue is either cancerous or has the potential to become cancerous. In order to prevent recurrence of thyroid cancer after thyroid surgery has been completed, patients must be treated with radioactive iodine to destroy (ablate) the residual thyroid tissue remaining after surgery as well as any cancer cells that may have spread to the regional lymph nodes or other areas of the body. Radioactive iodine therapy is also called radioactive iodine ablation or adjuvant radioactive iodine therapy. Therapeutic radiation uses the isotope Iodine-131 and the doses of radiation are much higher than when Iodine-123 is used for diagnostic scans.

Radioactive iodine therapy is considered as a standard treatment for all patients with differentiated thyroid carcinoma after total thyroidectomy. Several studies have shown that this treatment reduces the risk of recurrence of thyroid cancer and prolongs survival. Postoperative radioactive iodine therapy, however, is not used to treat patients with MTC or anaplastic thyroid carcinoma.

Radioactive iodine therapy is usually started about 6-8 weeks after total thyroidectomy has been completed. Once the thyroid gland has been removed, patients can no longer produce thyroid hormones so they need to take thyroid hormone pills for the rest of their lives to replace the natural thyroid hormones that their body can no longer produce.

Thyroid hormone therapy also suppresses the blood levels of a hormone called thyroid stimulating hormone (TSH) that is produced naturally by the pituitary gland. Suppressing the blood levels of TSH with thyroid hormone pills also reduces the risk of recurrence of the thyroid cancer.

Radioactive iodine therapy uses a radioactive form of iodine called Iodine-131 that is administered either in capsule or liquid form. After swallowing, the iodine 131 circulates in the bloodstream and is preferentially absorbed or taken-up by the residual thyroid tissue and thyroid cancer cells leading to their destruction.

In order for this treatment to be most effective, TSH levels in the bloodstream must be sufficiently high to stimulate the residual thyroid tissue and cancer cells to take-up the radioactive iodine. To ensure that high levels of TSH will be present when radioactive iodine therapy is initiated, several weeks before the start of Iodine-131 therapy thyroid hormone replacement pills are stopped and patients are also placed on an iodine-restricted diet. Examples of foods that contain iodine and can reduce the effectiveness of radioactive iodine therapy include canned strawberries/cherries, milk, cheese, salami, and iodine-enriched table salt. Your doctor will provide you with a complete list of food products that should be avoided several weeks prior to starting radioactive iodine therapy.

Radioactive iodine therapy is not used in pregnant women with differentiated thyroid carcinoma because it can damage or destroy the thyroid gland of the fetus. Be sure to notify your doctor if you are pregnant or suspect that you may be pregnant if you are offered radioactive iodine therapy as part of your treatment for thyroid cancer. Women who are not pregnant should use a reliable contraceptive to prevent from becoming pregnant until they complete their radioactive iodine treatment. Because iodine-131 can be passed from mother's milk to a baby, breast-feeding should stop before radioactive iodine therapy is started and should be discontinued during treatment until your doctor has given you approval to resume breast-feeding.