The diagnosis of thyroidal autonomy is based mainly on quantitative scintigraphy of the thyroid. The specificity of an echopoor pattern in ultrasound for autonomy is low. Also determination of TSH is only half as sensitive as measurement of uptake of 99mTc-pertechnetate under suppression (TcTU-supp) in diagnosing autonomy. For diagnosis and decision about therapeutical consequences of thyroidal autonomy the quantitative scintigraphy under suppression is recommended; side-effects of suppression by Levothyroxin are low.