NONINVASIVE MEASUREMENT OF THYROIDAL IODINE CONTENT (TIC) BY X-RAY-FLUORESCENCE ANALYSIS (XFA)

The determination of thyroidal iodine content by X-ray fluorescence an alysis is based on the phenomenon that the gamma radiation of Americiu m-241 excites stable iodine atoms to emit a characteristic fluorescenc e radiation which is proportional to the amount of iodine present in t he gland. To study this, a stationary system has been developed which consists of a 11,1 GBq Am-241 source and a high-purity Germanium detec tor with spectrum analysator. Lower limit of detectability of this sys tem corresponds to 0,013 mg per ml of thyroid volume measured sonograp hically; in-vivo precision given as coefficient of variation of duplic ate measurements amounts to 12%. The thyroid is exposed with a radiati on dose of 60 mu Sv per measurement (approximately 5% of a Tc-99m scan ). Studies carried out in 149 volunteers and 173 patients showed a mea n iodine concentration of 0,325 +/- 0,134 mg/ml in healthy persons; de creased iodine concentrations were found in euthyroid goitre patients and hyperthyroid patiens with focal functional autonomy or Graves' dis ease. iodine concentrations correlated negatively with age and goitre volume. The method is well suited for individual follow-up studies exp loring different treatment modalities because of its sensitivity, high reproducibility and low radiation exposure.