Below the thalamus is the hypothalamus. It is the thermostat. Or, more accurately, it is the thermostat, the appetite regulator, the thirst regulator, the sleep-wake regulator, the libido regulator, the stress-response regulator, and the master controller of the endocrine system. It is roughly the size of an almond and it runs almost everything that keeps your internal milieu stable.
The hypothalamus accomplishes this through specialized nuclei, each with a circumscribed homeostatic job. The preoptic area regulates temperature. The arcuate nucleus integrates appetite signals. The suprachiasmatic nucleus is the master circadian pacemaker — the cells whose firing rhythm sets the body's twenty-four-hour cycle. The paraventricular nucleus and supraoptic nucleus produce the hormones that the posterior pituitary releases. The lateral hypothalamus contains orexin/hypocretin neurons that stabilize wakefulness; their loss produces narcolepsy.
Two output systems carry hypothalamic commands to the body. The autonomic nervous system takes signals down through the brainstem and spinal cord, dilating pupils, accelerating heart rate, slowing digestion. The endocrine system takes signals through the pituitary, releasing hormones that control the thyroid, the adrenal cortex, the gonads, growth, and water balance.
When the hypothalamus malfunctions, the body's internal environment becomes unstable in ways that are sometimes diffuse and sometimes specific. Hypothalamic tumors can produce diabetes insipidus (failure of vasopressin release, with massive polyuria), precocious puberty (early gonadotropin release), or panhypopituitarism (failure of multiple downstream endocrine outputs). Bulimia and anorexia involve disturbances of appetite regulation that have hypothalamic components alongside cortical and limbic ones.
For our purposes, the most clinically important hypothalamic system is the stress response. The next concept walks through the HPA axis — hypothalamus to pituitary to adrenal — and what happens when chronic stress dysregulates it. Hold the thermostat metaphor in mind. The hypothalamus is small, central, and quietly responsible for almost everything that makes your body a stable internal environment in which your brain can do its work.