In the absence of clinical conditions that reduce serum IGF-1 levels—including diabetes mellitus, thyroid, kidney or liver disease, anorexia nervosa, weight loss >5% in the last three months and body mass index <18.5 kg/m2, and oral estrogens—normal IGF-1 levels should rule out acromegaly, even with basal and nadir GH levels following GH suppression test >0.4 μg/L. This evidence concerns the gene GH1 and diabetes mellitus.