The anterior pituitary gland secretes six major hormones: growth hormone, thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin. Each one targets a different organ or tissue, and together they regulate everything from height and metabolism to reproduction and stress response. The anterior pituitary doesn’t act on its own. It takes orders from the hypothalamus, which sends chemical signals down a short network of blood vessels to either ramp up or dial back each hormone’s release.
How the Hypothalamus Controls the Anterior Pituitary
The hypothalamus sits just above the pituitary and acts as its command center. It releases small signaling molecules called “releasing hormones” that travel to the anterior pituitary and trigger specific cell types to produce their respective hormones. Each anterior pituitary hormone has a corresponding releasing hormone. For example, the hypothalamus releases corticotropin-releasing hormone to stimulate ACTH production, thyrotropin-releasing hormone to stimulate TSH, growth hormone-releasing hormone to stimulate growth hormone, and gonadotropin-releasing hormone (GnRH) to stimulate both FSH and LH.
Two hypothalamic signals work in the opposite direction, putting the brakes on hormone production. Dopamine continuously inhibits prolactin release, which makes prolactin unique among the six hormones: its default state is “off” unless something actively removes the dopamine signal. Somatostatin inhibits both growth hormone and, to a lesser extent, TSH. This push-and-pull system keeps hormone levels within a tight range.
The whole system also relies on negative feedback. When a target organ (like the thyroid or adrenal glands) produces enough of its own hormone, that hormone circulates back to the hypothalamus and pituitary, signaling them to reduce output. This closed loop is what prevents hormone levels from spiraling too high or too low under normal conditions.
Growth Hormone
Growth hormone is produced by cells called somatotrophs, which make up the largest proportion of hormone-producing cells in the anterior pituitary. Its primary job is regulating growth during childhood and adolescence, but it remains active throughout life, influencing metabolism and body composition in adults.
Growth hormone works through two main routes. It acts directly on fat tissue, prompting the release of stored fatty acids for energy. It also signals the liver to produce a secondary messenger called insulin-like growth factor 1 (IGF-1), which does much of the actual growth work. IGF-1 drives cell division and protein synthesis, particularly in the growth plates of long bones during childhood. In the growth plates, IGF-1 stimulates cartilage cells to multiply and stack into columns that eventually harden into bone, which is how children gain height. IGF-1 also influences prenatal growth. Defects in IGF-1 production are linked to smaller birth size and growth restriction in the womb.
At rest, blood levels of growth hormone typically sit below 5 ng/mL. When the body is challenged with exercise, sleep, or fasting, levels rise above 7 ng/mL.
Thyroid-Stimulating Hormone
TSH is produced by thyrotroph cells and targets the thyroid gland in the neck. Its sole job is telling the thyroid to produce and release its two hormones, T4 and T3, which set the metabolic rate of virtually every cell in the body. When T4 and T3 levels are adequate, they feed back to the hypothalamus and pituitary to suppress further TSH release. Normal TSH levels in the blood fall between 0.5 and 4.0 mU/L, and this number is one of the most commonly ordered lab tests in medicine because it’s a sensitive indicator of whether the thyroid is working properly.
The feedback loop for TSH is especially precise. Circulating T4 is converted into the more active T3 form by specialized cells in the hypothalamus itself. That locally produced T3 then directly suppresses the gene that makes the releasing hormone for TSH. Under certain conditions like cold exposure or fasting, the brain can override this feedback and reset the threshold, allowing TSH to rise even when thyroid hormone levels haven’t dropped.
Adrenocorticotropic Hormone
ACTH is secreted by corticotroph cells and travels to the adrenal glands, which sit on top of the kidneys. There, it stimulates the outer layer of the adrenal gland to produce cortisol, the body’s primary stress hormone. Cortisol raises blood sugar, suppresses inflammation, and helps the body respond to physical and psychological stress. The adrenal glands also produce small amounts of sex hormones in response to ACTH. Normal plasma ACTH levels range from 10 to 60 pg/mL and follow a daily rhythm, peaking in the early morning and dropping to their lowest point late at night.
Follicle-Stimulating Hormone and Luteinizing Hormone
FSH and LH are both produced by the same cell type, called gonadotrophs, and both are released in response to the same hypothalamic signal: GnRH. What determines whether more FSH or LH gets made is the pulsing pattern of GnRH. Slow, infrequent pulses favor FSH production, while fast, frequent pulses favor LH.
In women, FSH drives the early phase of the menstrual cycle by stimulating ovarian follicles to mature. It also triggers the follicle’s surrounding cells to produce estrogen. As one dominant follicle grows and estrogen levels rise, FSH drops. When estrogen reaches a sustained high level (200 to 300 pg/mL for about 48 hours), the hypothalamus responds with a large GnRH surge that triggers a spike in LH. This LH surge is what causes ovulation. Normal FSH values for women of reproductive age range from 2 to 9 mIU/mL during the follicular and luteal phases, jumping to 4 to 22 mIU/mL at the mid-cycle peak. After menopause, FSH rises above 30 mIU/mL because the ovaries are no longer producing enough estrogen to suppress it.
In men, FSH is essential for sperm production. It works alongside testosterone to maintain both sperm count and sperm quality. LH targets specific cells in the testes that produce testosterone. Normal adult male FSH levels fall between 1 and 7 mIU/mL, while LH ranges from 2 to 9 mIU/mL.
Prolactin
Prolactin is produced by lactotroph cells and is the only anterior pituitary hormone that’s held in check by default. Dopamine from the hypothalamus continuously suppresses prolactin production, and in the absence of pregnancy or breastfeeding, this inhibition keeps prolactin levels low (typically below 20 ng/mL).
During breastfeeding, the physical act of a baby suckling sends a nerve signal from the nipple up the spinal cord to the hypothalamus, which shuts off dopamine release. With the brake removed, prolactin floods the bloodstream and stimulates the milk-producing structures in the breast to synthesize lactose, casein, and fats, the three main components of breast milk. Prolactin also promotes the growth of these milk-producing structures during pregnancy.
Prolactin has a notable side effect on reproduction. High prolactin levels suppress GnRH, which in turn shuts down FSH and LH production. In breastfeeding women, this leads to a temporary halt in menstruation, functioning as a natural form of contraception that helps space pregnancies. In men, abnormally high prolactin (from a pituitary tumor, for example) can suppress sperm production and cause infertility.
What Happens When the Anterior Pituitary Underperforms
When the anterior pituitary fails to produce one or more of its hormones in sufficient quantities, the result is a condition called hypopituitarism. Symptoms depend entirely on which hormones are missing. A deficiency in TSH leads to hypothyroidism, with fatigue, weight gain, and cold sensitivity. ACTH deficiency means the adrenal glands can’t produce enough cortisol, which can cause dangerous drops in blood pressure and blood sugar. FSH and LH deficiency reduces sex drive, causes fatigue, and can lead to infertility in both men and women. Growth hormone deficiency in children stunts height, while in adults it tends to increase body fat and decrease muscle mass. Prolactin deficiency is primarily noticeable in women who are unable to produce breast milk after delivery.
On the opposite end, overproduction of any single hormone causes its own set of problems. Too much growth hormone in adults leads to acromegaly, a condition where the hands, feet, and facial features gradually enlarge. Excess ACTH drives overproduction of cortisol, resulting in Cushing’s disease, which causes rapid weight gain, thin skin, and muscle weakness. Pituitary tumors are the most common cause of hormone overproduction, since a small growth on one cell type can churn out its hormone without responding to normal feedback signals.
Sudden damage to the pituitary tissue, called pituitary apoplexy, can cause a rapid loss of multiple hormones at once. Warning signs include a severe headache, vision changes, confusion, and a drop in blood pressure.