THE HYPOFUNCTIONING THYROID-NODULE - DIAGNOSIS WITH NUCLEAR-MEDICINE TECHNIQUES

Since 25 years now, Tc-99m Pertechnetat has turned out to be the radiopharmaceutical of first choice for thyroid scintigraphy because of its ideal radiation quality, short half life, low radiation exposure and its general availability. Thyroid trapping of Tc-99m Pertechnetat during the first 20 minutes after iv application correlates well enough with thyroidal clearance of iodide. So the determination of early Technetium uptake of the thyroid (TcTU) allows to measure a defined step of iodine metabolism. As compared to Tc-99m, I-131 leads to a radiation exposure of the thyroid higher by factor of 1000; thus the diagnostic application of I-131 only can be accepted for rare indications like dosimetric measurements before I-131 treatment or scintigraphic follow-up of thyroid cancer. For the investigation of additional special indications, I-123 should be used (as for example the diagnosis of iodine organification defects or localization of dystopic thyroid tissue). Today, the gamma camera should be used for thyroid scintigraphy because of its high spatial resolution and the possibility to perform quantitative measurements. Basal measurements of TcTU depend on nutritional iodine intake, whereas TcTU values measured after thyroid hormone suppression are not disturbed by exogenous supply of iodine. Thus, the measurement of TcTU after thyroid hormone suppression is the most sensitive and specific method to study regional function of the thyroid. In primary diagnosis, scintigraphy of the thyroid always is indicated for the investigation of functional activity of palpable thyroid nodules and/or sonographic lesions. In endemic goiter regions, scintigraphy always should be evaluated quantitatively because of the high prevalence of functional autonomy. If necessary, quantitative scintigraphy has to be carried out after thyroid hormone suppression. For follow-up however, scintigraphy mostly can be replaced by sonography.