endocrine tumor
An endocrine tumor is an abnormal growth that arises in a gland or cell whose normal job is making hormones. Think of a factory whose workers start multiplying out of turn: sometimes the extra workers keep making the product (and flood the market), and sometimes they just take up space without producing anything.
That distinction — whether the tumor secretes hormone or not — drives much of what an endocrine tumor does. A hormone-secreting (functional) tumor announces itself through the symptoms of hormone excess, often while still small, because even a tiny mass can make enough hormone to disturb the body. A non-functional tumor stays silent until it grows large enough to press on nearby structures or is stumbled upon during imaging done for another reason.
Endocrine tumors range from entirely benign to aggressively malignant, and benign ones can still be dangerous purely through their hormone output. They can appear in the pituitary, thyroid, parathyroid, adrenal glands, pancreatic islets, and the scattered hormone cells of the gut and lungs. Some occur sporadically; others are part of inherited syndromes that cause tumors in several glands at once.
A pituitary adenoma is a common endocrine tumor: if it secretes prolactin it causes milk leakage and infertility, but if it is non-functional it may only be found when it grows large enough to push on the optic nerves.
One gland, two very different tumor behaviors.
Benign and malignant are about whether a tumor invades and spreads; functional and non-functional are about whether it secretes hormone. The two axes are independent — a tiny benign adenoma can be devastating if it secretes, while a large non-secreting one may be silent.