The different possible underlying mechanisms and different types of treatment suggest diverse causes of HH. In order to increase the insight in these causes and to be able to determine the best treatment for each patient in the future, the underlying cause(s) will be examined first in this study. In previous studies that assessed beta cell mass, only only pathological assessment of pancreas specimens was performed, because in vivo assessment was impossible. The control group was determined from patients undergoing a (partial) pancreatectomy for other diseases or post-mortem. However, ideally the control group would consist of patients who have had RYGB as well, without developing HH.