TcTU before and after exogeneous TSH suppression in diagnosing thyroid autonomy.

In 104 euthyroid patients with goiter and suspected thyroid autonomy two thyroid scintiscans with quantification of the TcTU were performed before (TcTUo) and after exogeneous TSH suppression (TcTUs). In 15 patients with subnormal TSH before suppression the TcTUs revealed no difference to the TcTUo. In contrast to the TcTUo, in euthyroid patients the TcTUs is not relevantly influenced by individual iodine supply. In euthyroid goitreous patients with normal TSH a TcTUo less than 2.0% resulted in a TcTUs less than 1.1% in all 22 cases and, therefore, in these patients a scintiscan after TSH suppression is not necessary, because no relevant mass of functional autonomous tissue can be expected. In all other patients with normal TSH only the TcTUs is relevant for identification of thyroid autonomy. Autonomy with a TcTUs greater than 1.5% and, consequently, with a suspected higher risk of hyperthyroidism after iodine contamination was found in 19% (17/89) of the goitreous patients with normal TSH.