131I uptake scan A 131I uptake scan is performed to measure the amount of 131I taken up by the thyroid gland. We know that the thyroid gland takes iodine from the blood and uses it to make thyroid hormones. Patients with hypothyroidism usually take up too little iodine, while those with hyperthyroidism take up too much. A dose of radioactive iodine is given to the patient on an empty stomach before the scan, and after a few hours the iodine is concentrated in the thyroid gland and excreted in the urine. The amount of iodine entering the thyroid gland can be measured by “thyroid uptake”. Of course, patients who are taking thyroid medication are unlikely to take up large amounts of iodine because their thyroid gland is shut down and in a non-functional state. In some patients, the thyroid gland can take up iodine normally, but cannot convert it into thyroid hormones. Therefore, the analysis of iodine uptake results must be combined with hematological measurements. Other isotopes such as 99mTc, although also concentrated in the thyroid, do not tell us what we really want to know about iodine uptake, since the production of thyroid hormones depends on iodine uptake. A thyroid nodule that is concentrated in iodine is rarely tumorigenic, which is not certain if it is 99mTc. Therefore, 131I scans are now mostly used. Pregnant women should not have a thyroid scan because 131I affects the thyroid gland of the baby. A thyroid scan can be used to confirm nodules and determine their “cold” or “hot” nature; to determine the size of nodules prior to treatment; for postoperative follow-up of thyroid cancer; and to localize thyroid tissue outside the neck (e.g., under the hyoid bone or in the chest). Ultrasound of the thyroid gland uses high-frequency ultrasound to obtain images of the thyroid gland to confirm nodules, which can tell us whether a nodule is “solid” or “cystic” but not whether it is benign or malignant. Ultrasound provides accurate information about the size of the nodule and can be used to observe changes in size during treatment. Ultrasound is also useful for fine needle aspiration. Thyroid needle aspiration Thyroid needle aspiration can identify benign and malignant “cold nodules” (hot nodules are rarely malignant). It provides information not available from other thyroid tests and has a diagnostic accuracy rate of 75%.