Our study was designed to assess the hypothesis that insulin resistance might be present in thyroid dysfunction. Seventy women with different clinical manifestation of autoimmune thyroiditis were compared to twenty-seven female controls matched for age. In all participants we evaluated fasting glucose and insulin levels, and HOMA (homeostasis model assessment). In patients with autoimmune thyroiditis the fasting insulin and the insulin sensitivity assessment parameter HOMA positively correlated with TSH and negatively with triiodthyronine and thyroxine levels. In conclusions, thyroid dysfunction is associated with higher fasting insulin levels and relative insulin resistance that might contribute to the atherosclerotic dyslipidemia and the blood hypertension found in patients with hypothyroidism.