[L-thyroxine therapy in differentiated thyroid carcinoma: criteria for evaluation of TSH suppression]

Life-time L-thyroxine therapy is obligatory in patients treated for differentiated thyroid carcinoma (DTC) in order to suppress serum TSH. The rationale for that is the TSH stimulation of follicular cells' growth and the presence of TSH receptors on DTC cells. Nevertheless, the exact criteria for TSH suppression in DTC are not specified and are a matter of discussion, stimulated by the recent progress in the evaluation of thyroxine side effects on bone and heart. The aim of the study was the optimalization of the reference range for TSH suppression in DTC patients in order to minimalize the risk of iatrogenic thyrotoxicosis. One hundred and twenty nine patients were randomly chosen among patients treated radically for DTC (116 females and 13 males). Basal and TRH stimulated TSH level, FT4 and FT3 serum level were estimated by microimmunoenzymatic method, while SHBG was estimated by immunofluorimetry. Full suppression (basal TSH < 0.05) was obtained in 64 patients (49%), submaximal suppression (TSH between 0.1 and 0.3 mU/l) was observed in 21 patients (16%). In 29 patients (22%) no suppression was obtained by the applied dose of thyroxine. The risk of iatrogenic hyperthyreosis, as judged by the increase of FT3 or SHBG, was found to be 38% in patients with full suppression and only 5% in patients with submaximal suppression (p < 0.05).

1. Suppression of TSH secretion was achieved in 80% of patients with differentiated thyroid carcinoma. The control of TSH level must be controlled every 3 months in 5 first years of therapy. 2. The optimal serum TSH level for L-thyroxine therapy in asymptomatic patients after radical treatment of differentiated thyroid carcinoma ranges between 0.1 to 0.3 mU/L. This range ensures the expected suppression of TSH with only minimal risk of iatrogenic hyperthyreosis.