[When is a radioisotope diagnosis indicated in thyroid diseases and when is it superfluous?]

In response to the question when radioactive isotopes (radionuclides) are necessary or superfluous in the diagnostic workup of thyroid disease, the following observations can be made: 1. Radionuclides are almost ubiquitously used for the specific and relevant in-vitro tests for measuring the circulating thyroid hormones (T4 und T3, etc.) and necessitate a very small quantity of serum. They lead in approximately 90% of cases of the correct diagnosis, especially when supported by careful clinical workup. 2. The use of radionuclides is of unquestionable value for scanning the thyroid gland and thus permitting the localization of normal vs. ectopic thyroid tissue, and the recognition of active ("warm" or "hot") vs. inactive ("cold") areas, especially in nodular goiter. 3. As the best screening tests for hypo- or hyperthyroidism are those related to the level of circulating thyroid hormones (see above), thyroid radionuclide uptake tests have lost much of their diagnostic value. However, the measurement of thyroidal radionuclide uptake remains valuable whenever radioiodine treatment is considered and in detailed thyroid function studies, and its also useful in the follow-up of drug-treated hyperthyroid patients. Thyroid uptake measurements may be of particular interest in so-called dynamic tests (e.g. TSH-stimulation, T3-suppression). 4. Finally, attention is drawn to the fact that, according to AITCHISON et al. (1), our capacity to interpret multidimensional information properly is limited. Thus, experience and precise knowledge help to economize the use of radionuclides and permit minimal application with maximal output.