The pituitary gland is a pea-sized organ at the base of your brain that produces or releases at least eight major hormones, controlling everything from growth and metabolism to reproduction and water balance. Despite weighing only about half a gram, it acts as a central relay between the brain and the body’s other hormone-producing glands, which is why it’s often called the “master gland.”
Size and Location
The pituitary sits in a small bony pocket called the sella turcica, a depression in the sphenoid bone near the center of the skull base. It measures roughly 13 mm wide, 9 mm deep, and 5 to 10 mm tall in adults. During pregnancy, it can swell to 12 mm in height or more, occasionally large enough to press on nearby structures. The optic chiasm, where the two optic nerves cross, sits just above and in front of the gland, separated by only a few millimeters of tissue. That tight spacing is why pituitary tumors sometimes cause vision problems.
Two Lobes, Two Origins
The pituitary is really two glands fused into one. The front portion, called the anterior lobe, develops from embryonic tissue in the roof of the mouth. The back portion, the posterior lobe, grows downward from brain tissue. A thin intermediate lobe sits between them. This dual origin explains why the two halves look different under a microscope and function in completely different ways: the anterior lobe is packed with hormone-producing cells, while the posterior lobe is mostly a bundle of nerve fibers extending down from the hypothalamus.
Hormones of the Anterior Lobe
The anterior lobe accounts for about 75% of the gland’s weight and produces six key hormones:
- Growth hormone drives bone and muscle growth in children and helps maintain muscle mass, bone density, and fat distribution in adults.
- Thyroid-stimulating hormone (TSH) tells the thyroid gland to produce its own hormones, which set your metabolic rate.
- Adrenocorticotropic hormone (ACTH) signals the adrenal glands to release cortisol, the body’s primary stress hormone.
- Follicle-stimulating hormone (FSH) triggers sperm production in the testes and prepares eggs for release in the ovaries.
- Luteinizing hormone (LH) stimulates testosterone production in males and triggers ovulation and progesterone release in females.
- Prolactin drives breast milk production after childbirth and influences menstrual cycles and fertility.
Each of these hormones targets a specific organ or tissue elsewhere in the body, which is why damage to the pituitary can ripple outward and affect so many systems at once.
Hormones of the Posterior Lobe
The posterior lobe doesn’t actually manufacture hormones. Instead, it stores and releases two hormones that are made by nerve cells in the hypothalamus, the brain region directly above. These hormones travel down long nerve fibers and are held in small storage structures within the posterior lobe until the body needs them.
The first is antidiuretic hormone (ADH), which tells the kidneys to retain water. When ADH levels drop, you produce large volumes of dilute urine, a condition called diabetes insipidus. The second is oxytocin, which triggers uterine contractions during labor and the release of breast milk during nursing. Oxytocin also plays a broader role in social bonding and trust.
How the Hypothalamus Controls the Pituitary
The pituitary doesn’t act on its own. It takes orders from the hypothalamus through a dedicated network of tiny blood vessels called the hypophyseal portal system. The hypothalamus releases signaling chemicals into these vessels, and within seconds they reach the anterior pituitary and either ramp up or shut down hormone production. For example, when you encounter a stressor, the hypothalamus releases a signaling peptide that travels through the portal vessels and binds to cells in the anterior pituitary, triggering ACTH release. ACTH then enters the general bloodstream and reaches the adrenal glands, which pump out cortisol.
This chain of command also runs in reverse. When cortisol levels climb high enough, the hypothalamus and pituitary sense the excess and dial back their signals. This feedback loop keeps hormone levels within a tight range. Similar loops govern thyroid hormones, sex hormones, and growth hormone.
Pituitary Tumors
The most common pituitary disorder is a benign tumor called a pituitary adenoma. These growths are not cancer and often grow slowly, but they can cause problems in two ways. “Functioning” tumors overproduce one of the anterior lobe hormones, leading to recognizable syndromes. A tumor that overproduces growth hormone causes acromegaly in adults, characterized by enlarged hands, feet, and facial features. One that overproduces ACTH leads to Cushing’s disease, marked by weight gain, high blood sugar, and thinning skin. Prolactin-producing tumors, called prolactinomas, are the most common type and can cause unexpected breast milk production, irregular periods, or fertility problems.
“Nonfunctioning” tumors don’t pump out extra hormones, but as they grow they can compress the normal gland tissue and reduce hormone output across the board. Because the optic chiasm is so close, a large tumor can also press on the optic nerves, causing loss of peripheral vision, typically in both eyes.
Hypopituitarism
When the pituitary produces too little of one or more hormones, the condition is called hypopituitarism. It can result from tumors, surgery, radiation, head trauma, or loss of blood supply to the gland. Symptoms depend on which hormones are deficient. Low ACTH means the adrenal glands don’t produce enough cortisol, causing fatigue, low blood pressure, and poor stress tolerance. Low TSH leads to an underactive thyroid with symptoms like weight gain, cold sensitivity, and sluggishness. Low FSH and LH cause low sex hormones, which can show up as missed periods, low libido, or infertility.
Diagnosis typically starts with morning blood tests measuring both the pituitary hormone and the hormone from its target organ. If cortisol is very low (below about 3 to 4 micrograms per deciliter) or comfortably high (above 15 to 16), the result is clear-cut. Values in between require a stimulation test, where doctors provoke the gland and measure how it responds. Growth hormone deficiency uses a similar approach: levels below 3 micrograms per liter during a stimulation test indicate severe deficiency, while levels above 5 are considered normal. Treatment generally involves replacing the missing end-organ hormones (thyroid hormone, cortisol, sex hormones, or growth hormone) rather than the pituitary hormones themselves.