[Direct evaluation of thyroid I-127 in an iodine overload situation: in vivo study by X-ray fluorescence and in vitro by analytical ion microscopy].

This review describes the two methods which allow direct estimation of stable iodine (127I) within thyroid gland either in vivo by X-ray fluorescence or in vitro by secondary ion mass spectrometry (SIMS) microscopy on tissue section. Although the measurement of thyroid iodine content (TIC) by X-ray fluorescence has little relevance for routine explorations of thyroid function, this is a valuable method for understanding complex pathophysiological conditions such as the thyroid adaptation to iodine overload. On the other hand, SIMS microscopy which is able to characterize the functional activity of thyroid tissue by measuring 127I concentration within the thyroid follicles, can be used to determine the extent to which exogenous iodine affects the regulation of iodine within the thyroid follicles. Both methods were used to evaluate the quantitative changes in thyroid 127I induced by amiodarone iodine overload. TIC measurements shows that hyperthyroidism occurred only in patients who increased their iodine stores, while the patients who developed hypothyroidism had low iodine stores. The SIMS microscopy data obtained in mice demonstrated that the thyroid response to amiodarone is related to dietary iodine intake leading to an increase in local iodine concentration in iodine deficient mice and to a decrease in iodine supplemented mice. This response is specific and different from that induced by an iodide overload. These results could explain that hyperthyroidism with high thyroid iodine content occurred in areas with low thyroid iodine intake and hypothyroidism with low thyroid iodine content in areas with a supplemented iodine diet.