Box 3.6Radioisotopic scanning of the thyroid gland

Uptake of a radioactive isotope by the thyroid gland is used to determine:

• localization of the gland
• approximate size
• overall or regional function of the gland.

Isotopes. ¹²⁵I has a radioactive half-life (t_1/2) of 60 days, ¹³¹I a t_1/2 of 8 days and ¹²³I a t_1/2 of 13 hours. A radioisotope of technetium (^99mpertechnetate) is often used because it has a usefully short radioactive t_1/2 (approximately 6-h) and, unlike radioactive iodine, is not organified in the thyroid gland. Radiation dose to the thyroid is thus low.

• Localization and size. Uptake of isotope in a normal gland and in a patient whose left lobe was surgically removed . Arrow indicates uptake of iodine in the salivary glands.
• Overall function. More rapid and increased uptake of the isotope in primary hyperthyroidism compared with normal thyroid activity and slower, reduced uptake in hypothyroidism .
• Regional function. ‘Hot’ nodule in upper left lobe of the thyroid gland (arrow) showing increased isotope uptake and suppression of uptake in surrounding tissue . ‘Cold’ nodule in lower left lobe (arrow) showing reduced isotope uptake .