The efficacy of suppressing TSH secretion with levothyroxine (L-T(4)) in reducing solitary thyroid nodule growth is still controversial. In this prospective multicenter, randomized, double-blind, placebo-controlled trial, 123 patients with a single palpable benign nodule were included and randomly allocated to an 18-month treatment with L-T(4) or placebo. Individual dose was adjusted to allow a serum TSH level below 0.3 mIU/liter. Clinical and ultrasonographic nodule characteristics were assessed before treatment and 3, 6, 12, and 18 months thereafter. The largest mean nodule size assessed on palpation and largest volume, assessed by ultrasonography, decreased in the L-T(4) group and increased slightly in the placebo group [size, -3.5 +/- 7 mm vs. +0.5 +/- 6 mm (P = 0.006); volume, -0.36 +/- 1.71 ml vs. +0.62 +/- 3.67 ml (P = 0.01), respectively]. The proportion of clinically relevant volume reduction (> or =50%) rose significantly in the L-T(4) group [26.6% vs. 16.9% (P = 0.04)]. The proportion of patients with a reduced number of infraclinical additional nodules was significantly higher in the L-T(4) group [9.4% vs. 0 (P = 0.04)]. It is concluded from this study that suppressive L-T(4) therapy is effective in reducing solitary thyroid nodule volume and improving infraclinical extranodular changes.