Thyroid autonomy: sensitive detection in vivo and estimation of its functional relevance using quantified...

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 high-resolution quantified scintigraphy were used for an accurate detection of thyroid autonomy among the 236 individuals. Suppression scintigraphy revealed autonomy in 171 patients. delta TSH 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 delta TSH 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 delta TSH in each of the TcUs classes depended on the individual iodine supply.(ABSTRACT TRUNCATED AT 250 WORDS)