Doege-Potter syndrome is a very rare paraneoplastic syndrome resulting from a tumor in the pleural cavity that produces large amounts of insulin like human growth hormone. There are only a few hundred recorded cases.
As a result of the overproduction of the hormone, blood sugar levels fall, causing hypoglycemia. This also results in the possibility of insulin shock and usually causes neurological symptoms such as problems with behavior and personality.
Removal of the tumor will alleviate the symptoms. Tumors that cause the syndrome tend to be fairly large - over a kilogram is not uncommon. As such the tumor can cause secondary symptoms by pressing against the heart, lungs or blood vessels. However, they also tend to be distinct and unattached to other tissue.
See also pheochromocytoma, a type of tumor that produces insulin.