DIRECT EVALUATION OF THYROID I-127 AND IODINE OVERLOAD - INVIVO STUDYBY X-RAY-FLUORESCENCE AND INVITRO BY SIMS MICROSCOPY

This review describes the two methods which allow direct estimation of stable iodine (I-127) within thyroid gland either in vivo by X-ray fl uorescence or in vitro by secondary ion mass spectrometry (SIMS) micro scopy on tissue section. Although the measurement of thyroid iodine co ntent (TIC) by X-ray fluorescence has little relevance for routine exp lorations of thyroid function, this is a valuable method for understan ding complex pathophysiological conditions such as the thyroid adaptat ion to iodine overload. On the other hand, SIMS microscopy which is ab le to characterize the functional activity of thyroid tissue by measur ing I-127 concentration within the thyroid follicles, can be used to d etermine the extent to which exogeneous iodine affects the regulation of iodine within the thyroid follicles. Both methods were used to eval uate the quantitative changes in thyroid I-I27 induced by amiodarone i odine overload. TIC measurements shows that hyperthyroidism occured on ly in patients who increased their iodine stores, while the patients w ho developed hypothyroidism had low iodine stores. The SIMS microscopy data obtained in mice demonstrated that the thyroid response to amiod arone is related to dietary iodine intake leading to an increase in lo cal iodine concentration in iodine deficient mice and to a decrease in iodine supplemented mice. This response is specific and different fro m that induced by an iodide overload. These results could explain that hyperthyroidism with high thyroid iodine content occured in areas wit h low thyroid iodine intake and hypothyroidism with low thyroid iodine content in areas with a supplemented iodine diet.