Thyroid tissue uses iodine to make thyroid hormones (T3 and T4). In order for a radioiodine scan to work, cancerous thyroid tissue must be "hungry" for iodine. Thyroid stimulating hormone (TSH) produced in the pituitary gland is responsible for making thyroid tissue "hungry" for iodine. Once thyroid tissue absorbs the radioactive iodine it will be clearly visible on the scan and can be located for removal. However, thyroid hormone replacement medication tends to lower the activity of the pituitary gland and the amount of naturally produced TSH. So it is necessary to stop thyroid hormone replacement to increase TSH. A problem arises when there is a lack of thyroid hormone replacement causing patients to experience hypothyroidism. This condition is associated with unpleasant physical and emotional symptoms.