Thyroid cell growth and function both in vivo and in vitro, are mainly regulated by TSH. Recent studies have shown that growth factors including insulin-like growth factors (IGF-1) have an important role in the control of thyrocyte proliferation and differentiation. The aim of this study was to evaluate the expression of the IGF-1 gene by Northern analysis and the IGF-1 tissue protein by radioimmunoassay in multinodular euthyroid goiters. The study population consisted of 20 patients with multinodular goiter (14 females and 6 males) living in a non endemic geographic area. All patients were euthyroid at the time of surgery and submitted to total or subtotal thyroidectomy. Samples of normal thyroid tissue were obtained from three patients who were operated due to laryngeal carcinomas. The IGF-1 protein content was increased in non toxic multinodular goiter, 22 ng/g vs. 14 in controls (p<0.03), as was IGF-1 gene expression (p<0.05). The increase in the steady state mRNA content correlated with the increase in the protein content (r=0.665; p<0.005). These results suggest that IGF-1 may play a role in proliferation events involved in benign hyperplastic thyroid diseases.