Endocrine gland: A gland that secretes a substance (a hormone) into the bloodstream. The endocrine glands are 'glands of internal secretion.' They include the hypothalamus pituitary gland pineal gland thyroid parathyroid glands heart (which makes atrial - natriuretic peptide ) the stomach and intestines islets of Langerhans in the pancreas the adrenal glands the kidney (which makes renin erythropoietin and calcitriol ) fat cells (which make leptin). the testes the ovarian follicle ( estrogens ) and the corpus luteum in the ovary).