Chapter 3 - Determination of Iodine In Vivo and In Vitro by X-Ray Fluorescence Analysis: Methodology and Applications

Abstract

This chapter reviews the XRF technique, with emphasis on the physical background and clinical applications. Necessary equipment, together with the methodological limitations and benefits, is discussed and a brief guide before starting XRF measurements is also provided. Iodine uptake and excretion can be assessed by 131I measurements or urinary iodine, whereas information on the amount of iodine stored in the thyroid, in the iodine pool, is not as commonly obtained. The iodine pool can be estimated by noninvasive X-ray fluorescence (XRF) analysis. Depending on the choice of equipment, the iodine estimation can be made quantitatively or by imaging the iodine distribution. The technique involves a radioactive source or an X-ray tube for irradiation of the stable iodine in the thyroid, together with a detector that registers the subsequent emission of characteristic X-ray photons. The method is noninvasive and can be used in vitro as well as in vivo. The equipment consists of a radiation source, a detector, electronics, and a spectroscopy program. The radiation dose involved in a measurement is very low. The areas of special interest are monitoring of iodination programs, identification of individuals with a small iodine pool who are at special risk of developing hypothyroidism in iodine deficiency, and also investigations of mechanisms of thyroid disease in iodine overload.