Extensive hepatic extraction of insulin from portal venous blood can lead to the production of hypoglycaemia by inhibition of glucose release, without necessarily leading to abnormally high peripheral plasma insulin concentrations (Fig. 20.13). An appropriately low plasma insulin level in the presence of fasting hypoglycaemia does not, therefore, exclude the diagnosis of insulinoma, whereas a single inappropriately high one is highly suggestive of it and demands further investigation.
