pituitary gland

The pituitary gland is a tiny structure, about the size of a pea, that hangs from the underside of the brain just behind the bridge of your nose. Despite its small size, it earns the nickname "the master gland," because the hormones it releases reach almost every corner of the body and switch other glands on and off. Hormones are chemical messages that travel through the bloodstream; where a nerve sends a fast, private signal down a wire, a hormone is more like a message broadcast to the whole body, telling distant organs how to behave.

But the master is not really the boss. Sitting just above it is the hypothalamus, a region of the brain that watches the body's inner state and tells the pituitary what to do, so the pituitary is better thought of as a relay station that turns the brain's instructions into hormone signals. When commanded, the front part of the pituitary sends out hormones that order the thyroid to set your metabolic pace, the adrenal glands to release stress hormones, and the ovaries or testes to drive reproduction; it also pours out growth hormone, which builds the body. The back part stores and releases hormones made in the hypothalamus, including ones that control water balance and trigger labor and milk release.

This makes the pituitary the central hinge of the brain–body axis: the point where thoughts, stress, and the body's needs get converted into chemistry that reaches every tissue. It also runs on feedback, much like a thermostat. When the hormone level of a target gland climbs high enough, that rising level signals back to the pituitary and hypothalamus to ease off, keeping each system within a healthy range. Because so much passes through this one small gland, a problem here can ripple outward into growth, energy, mood, fertility, and the body's response to stress.