The adrenal gland was a part of the endocrine system that regulated the body's stress responses and produced adrenaline.
According to Dr. Phlox, the scent of the Nausicaan adrenal gland gives off a fragrance not unlike the scent of a female Human's sweat after performing exercise. (ENT: "Fight or Flight")
The neural parasites that infested Starfleet Command used their tendrils to stimulate the adrenal glands of their Human victims, generating great strength. (TNG: "Conspiracy")
One of The Doctor's "Hints for Healthful Living" segments on A Briefing with Neelix was titled "'Uncovering the hidden mysteries of the adrenal gland." (VOY: "Investigations")