Switches on and switches off
The hypothalamus controls the front pituitary with two kinds of small messengers. A releasing hormone tells the pituitary to send out one of its hormones; an inhibiting hormone tells it to hold back. Think of them as the gas pedal and the brake. Most front-pituitary outputs are switched on by a releasing hormone, while a few also have a dedicated brake.
- GHRH (release) and somatostatin (inhibit) together control growth hormone — a clear gas-and-brake pair.
- CRH releases ACTH; TRH releases TSH; GnRH releases both LH and FSH.
- Prolactin is the odd one out: its main control is a brake. The hypothalamus mostly *inhibits* it (with dopamine), so prolactin rises by default when that brake is released.
The six outputs and what they command
Four of the front pituitary's six hormones are tropic hormones — their target is another endocrine gland, which they tell to grow and to make its own hormone. TSH commands the thyroid; ACTH commands the adrenal cortex; LH and FSH command the ovaries and testes. The other two act directly on body tissues rather than on a downstream gland: growth hormone drives growth and metabolism throughout the body, and prolactin drives milk production in the breast.
Hypothalamic factor → Anterior pituitary hormone → Final target ───────────────────────────────────────────────────────────────────── GHRH (+) / Somatostatin (–) → Growth hormone (GH) → liver, bone, muscle Dopamine (–, main control) → Prolactin → mammary gland (milk) CRH (+) → ACTH → adrenal cortex → cortisol TRH (+) → TSH → thyroid → T4 / T3 GnRH (+) → LH → gonads → sex steroids GnRH (+) → FSH → gonads → gametes
Notice a recurring pattern: a small signal from the hypothalamus produces a larger signal from the pituitary, which produces a still larger flood of gland hormone. The chain magnifies as it descends. That magnification is exactly why a small problem at the top — a missing releasing hormone, or a stalk that gets cut — can have such wide effects below.