Insulin is normally the only blood glucose-lowering hormone to circulate in biologically significant amounts; however, in some very rare cases where patients have an inborn inability to form insulin from proinsulin, and in some patients with islet cell tumours, proinsulin and its partially split products are the dominant circulating hypoglycaemic hormones. Both insulin and proinsulin reduce hepatic glucose output and increase glucose uptake by insulin-dependent tissues after binding to specific insulin receptors.
