Monoclonal gammopathy with an insulin binding IgG(K) M-component, associated with severe hypoglycaemia

A case is described in which a monoclonal gammopathy was associated with postprandial hypoglycaemia, resulting from insulin binding by the M-component. The monoclonal immunoglobulin possessed a low affinity for insulin, but its binding capacity was very high. Cytotoxic chemotherapy and diazoxide treatment failed to prevent severe hypoglycaemic attacks.