hypothalamus
Meaning of hypothalamus: The pituitary is called the master gland, yet it takes its orders from the structure sitting directly above it.
Definition of hypothalamus
The hypothalamus is a small region at the base of the brain, below the thalamus and just above the pituitary gland. Though it weighs only a few grams, it holds the controls for body temperature, hunger, thirst, sleep and the daily clock, and it is the point at which the nervous system takes command of the hormone system.
What does it regulate?
| Function | How it acts |
|---|---|
| Temperature | Compares blood temperature with a set point and triggers sweating or shivering |
| Thirst and water balance | Senses the concentration of blood and releases the hormone that makes kidneys retain water |
| Hunger | Reads hormone signals from fat tissue and the gut, and adjusts appetite |
| Sleep and daily rhythm | Holds the internal clock, reset each day by light reaching the eye |
| Stress and reproduction | Starts the hormone chains that end at the adrenal glands and the gonads |
Each of these works by comparison against an internal reference, which is why the hypothalamus is described as the body's regulator rather than simply another gland of the endocrine system.
How does it command the pituitary?
By two quite separate routes. For the front portion of the pituitary it uses chemistry: it releases small hormones into a private set of blood vessels running the short distance down, and those hormones instruct the pituitary to release or withhold its own. Gonadotropin-releasing hormone works this way, as does the signal that controls growth hormone. For the back portion it uses wiring instead: nerve cells whose bodies sit in the hypothalamus send fibers down into the gland and release their hormones, oxytocin and the water retaining hormone, directly from the fiber endings. The rear pituitary therefore stores and releases hormones it does not manufacture.
What happens when it is damaged?
Because so many chains begin here, injury from a tumor, trauma, surgery, radiation or inflammation rarely produces one isolated symptom. Water balance fails, giving enormous urine output and constant thirst. Temperature regulation becomes unstable. Appetite may collapse or become uncontrollable, sleep patterns break down, and the hormone output of the pituitary falls, affecting growth, reproduction, the thyroid and the stress response together. That pattern of several unrelated systems failing at once is itself the clue that points upstream to this structure.
Used in a sentence
Imaging of the brain was reviewed to evaluate a mass located near the hypothalamus.
Good to know
The pituitary is popularly called the master gland, yet its output is itself directed by the hypothalamus, which makes the hypothalamus the higher level controller of the pair and explains why damage there disturbs several hormone systems at once.
Word origin
Where “hypothalamus” comes from
Formed from hypo-, Greek for under or beneath, and thalamus, from Greek thalamos, meaning inner chamber, describing the structure's location just below the thalamus.