SummaryobjectiveTo evaluate the effect of treatment with TSH suppressive dose of levothyroxine In patients with benig nthyroid nodules.designProspective randomized study. Group A (n= 20) patients received levothyroxine and group B (n= 20) patients did not. The dose of levothyroxine was adjusted to obtain an effective suppression of TSH. A clinical, analytical and morphological (with ultrasound) review was performed every 3 months. The mean ± SD follow‐upperiod was 6·10±2·2 months.patientsForty euthyrold women with solitary thyroid nodule on palpation, cold on sclntlgraphy and cytologically benign without contraindication participated.measurementsAt entry: biochemical and hormonal parameters, thyroid sclntigraphy and thyroid ultrasonography. Every 3 months additional determinations of thyroid hormones and TSH levels were carried out, if necessary, to verify effective TSH suppression. Every 6 months thyroid ultrasound imaging was performed.resultsPatients were euthyroid at entry into the study. The mean dose of levothyroxine necessary to obtain TSH suppression was 2·82±0·6μg/kg/day. No significant modification in the thyroid nodule diameter (mean ± SD 2·6±1·2vs2·5±1·2 cm) or in the thyroid nodule volume(10·3±11·9vs10·1±12·2 ml) were observed in group A. In group B the results were similar (2·8±0·9vs2·7±1·8 cm and 9·2±6·4vs9·2±9·5 ml, respectively). No differences were found in either group in the number of nodules that reduced significantly their volume (four and three, respectively).conclusionsThe suppressive therapy with levothyroxine was not effective In reducing nodule sizes in patients w