The 20 min 99mTc thyroid uptake compares favourably with the more time consuming 24 h 131J uptake because it results in a lower radiation dose to the thyroid gland. The reliability of both methods is nearly the same. In all conditions in which the 131J uptake is altered, a similar deviation of the 99mTc uptake has been found. A single exception are patients treated with antithyroid drugs (together with thyroid hormones), who show frequently an elevated 99mTc uptake (in contrast to the 131J uptake). Prognostic conclusions have been drawn by other autors from the 99mTc uptake in similar cases.