Thyroid autonomy: sensitive detection in vivo and estimation of its functional relevance using quantified high-resolution scintigraphy

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Abstract
This study is concerned with 236 euthyroid individuals living in an area of iodine deficiency, 227 of whom had endemic goitres. In these subjects, autonomy could be suspected owing to an inhomogeneous activity distribution on the thyroid scintigram or a subnormal TSH response to TRH. They complete a total number of 426 investigated individuals. Previously, in 190 separated controls without evidence of autonomy, the reference ranges for the thyroid 99mTc pertechnetate uptake under suppression (TcUs), a measure for the non-suppressible thyroid iodide clearance, and for suppressibility of circumscribed thyroid regions, had been determined. These two parameters obtained by highresolution quantified scintigraphy were used for an accurate detection of thyroid autonomy among the 236 individuals. Suppression scintigraphy revealed autonomy in 171 patients. ΔTSH after TRH was subnormal in 40% of the subjects with abnormal thyroid suppressibility. Prevalence of abnormal suppression was dependent on three factors: patient age, goitre type and estimated thyroid weight. In the total investigated collective, the prevalence of autonomy was 77% in patients with a goitre weight above 50 g. The individuals with abnormal suppression were grouped into four classes of TcUs. In these classes, free thyroxine index (FT4I) and total triiodothyronine (TT3) increased with increasing TcUs, whereas ΔTSH decreased. This finding indicates a continuum of different extents of autonomous thyroid function, whereas in the individual patient, the extent can be determined using the pertechnetate uptake under suppression. In addition, FT4I, TT3 and ΔTSH in each of the TcUs classes depended on the individual iodine supply. It is concluded that, in patients with thyroid autonomy, actual thyroid hormone concentrations and TSH stimulation are determined by two major factors: the extent of autonomy and the individual iodine supply. Therefore, in iodine deficiency, the TRH test may be normal, although autonomy is present. This relationship explains the reduced sensitivity of the TRH test to detect autonomy in iodine-deficient goitres.

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