Protein hormone secreted in the pancreas by the islets of Langerhans (7)
I believe the answer is:
insulin
(Other definitions for insulin that I've seen before include "Regulator of blood sugar" , "Hormone that regulates the storage of glycogen in the liver" , "Hormone substance injected by diabetics" , "Hormone - nil in us (anag)" , "Secretion, deficiency of which results in diabetes" .)