X-ray fluorescence analysis is based on the principal that the electron structure of stable iodine in the thyroid is excited by Americium-241 gamma rays to emit a characteristic fluorescence radiation which is proportional to the amount of iodine present in the gland. A stationary measuring system consisting of a 11.1 GBq Am-241 source and a high-purity Germanium detector with spectrum analyser has been improved by a PC guided method for sonographic definition of the measuring volume. The lower limit of detectibility of the system corresponds to 0.01 mg of Iodine per ml of thyroid volume; the in vivo precision given as coefficient of variation amounts to 15%. The thyroid is exposed with a radiation dose of 6 microSv per measurement. First studies with this improved system carried out in 50 female volunteers between 20 and 40 years of age with normal thyroid volumes resulted in a mean iodine concentration of the thyroid of 0.665 +/- 0.304 mg/ml. The mean iodine excretion in urine was normal with 10.8 +/- 10.4 microg/dl.