ectopic hormone secretion
Ectopic means out of place. Ectopic hormone secretion is when a hormone is made somewhere it normally is not — most often by a tumor in a non-endocrine organ that has switched on a gene it should not be using. It is as if a car factory suddenly started printing money: the product is real and biologically active, but it is coming from entirely the wrong source.
Many cancers, especially certain lung tumors, can revert to making hormones their cells of origin would never normally produce. The hormone then acts exactly as the real thing would, but with one telling difference: it ignores the body's feedback control, because the tumor is not part of the normal regulatory loop. So levels stay high no matter how the body tries to switch them off.
This matters clinically in two ways. First, the hormone's effects can be the patient's most prominent or earliest symptom, sometimes appearing before the cancer itself is found. Second, the abnormal source explains test results that would otherwise look paradoxical — such as high cortisol driven by ACTH coming not from the pituitary but from a lung tumor, with no normal suppression.
Small-cell lung cancer often secretes ACTH ectopically, driving the adrenal glands to flood the body with cortisol and producing Cushing's syndrome that does not suppress with high-dose dexamethasone.
Cortisol excess from a lung tumor, not the pituitary.
Ectopic secretion is one cause of paraneoplastic syndromes; the difference is mostly emphasis. Ectopic secretion names the misplaced source of the hormone, while paraneoplastic syndrome names the remote, hormone-driven illness it produces.