The adrenal gland is a part of the endocrine system that regulates the bodies stress responses and produces 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")
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")