SummaryThe authors summarize their experience after more than 2000 determinations of intrathyroidal iodine (ITI) by means of the X-ray fluorescence methode (XRF) and review the literature. This method explores the storage function of the thyroid, the so-called iodine pool with slow turnover rate, function that has hitherto been neglected because of lack of easily available means. The procedure is inocuous and the detector can readily be mounted on a conventional rectilinear scanner.The pattern of distribution of stable iodine within the gland and its qualifications have contributed to a better understanding of various thyroid disorders. Aside from its possibilities as far as the study of thyroid physiopathology is concerned, the method deserves its place in a routine clinical laboratory. Important information can be obtained, useful for diagnosis, treatment and prognosis in many instances, especially in hyperthyroidism and autoimmune thyroiditis. Altough it will not replace a radionuclide scan in most cases, it can serve as a substitute in those circumstances in which a radionuclide scan is not warranted or impossible to perform.