Non-invasive measurement of thyroidal iodine content (TIC) by X-ray fluorescence analysis (XFA)

The determination of thyroidal iodine content by X-ray fluorescence analysis is based on the phenomenon that the gamma radiation of Americium-241 excites stable iodine atoms to emit a characteristic fluorescence radiation which is proportional to the amount of iodine present in the 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 detector with spectrum analysator. Lower limit of detectability of this system corresponds to 0.013 mg per ml of thyroid volume measured sonographically; in-vivo precision given as coefficient of variation of duplicate measurements amounts to 12%. The thyroid is exposed with a radiation dose of 60 microSvs per measurement (approximately 5 % of a Tc-99m scan). Studies carried out in 149 volunteers and 173 patients showed a mean iodine concentration of 0.325 +/- 0.134 mg/ml in healthy persons; decreased iodine concentrations were found in euthyroid goitre patients and hyperthyroid patients with focal functional autonomy or Graves' disease. Iodine concentrations correlated negatively with age and goitre volume. The method is well suited for individual follow-up studies exploring different treatment modalities because of its sensitivity, high reproducibility and low radiation exposure.