A pituitary adenoma is a benign tumor that grows in the pituitary gland, a pea-sized organ at the base of your brain that controls hormone production throughout your body. These tumors are remarkably common. A large meta-analysis found that roughly 16.7% of people have one, with most never knowing it. The vast majority are slow-growing, non-cancerous, and many never cause symptoms at all.
How Pituitary Adenomas Are Classified
Pituitary adenomas fall into two broad categories based on whether they produce excess hormones.
Functioning adenomas actively pump out too much of a specific hormone, causing symptoms related to that hormone’s effects on the body. The most common type is a prolactinoma, which overproduces prolactin. Others secrete growth hormone (leading to a condition called acromegaly) or ACTH, the hormone that tells your adrenal glands to make cortisol (leading to Cushing’s disease).
Non-functioning adenomas don’t produce clinically significant amounts of any hormone. Many of these are gonadotroph adenomas, meaning they originate from the cells that normally regulate reproductive hormones, but they don’t secrete enough to cause hormonal symptoms. Instead, non-functioning tumors tend to cause problems only when they grow large enough to press on surrounding structures.
There’s also a middle category worth knowing about: silent adenomas. These tumors show hormone production under a microscope but don’t release enough into the bloodstream to cause symptoms. Silent corticotroph adenomas, for instance, contain ACTH but don’t trigger Cushing’s disease. They’re typically managed the same way as non-functioning tumors.
Size Categories Matter
Tumors smaller than 10 millimeters (1 centimeter) are called microadenomas. Those 10 millimeters or larger are macroadenomas. This distinction is more than academic. Microadenomas often stay small for years and may never need treatment. Macroadenomas are more likely to compress nearby nerves, disrupt normal pituitary function, and eventually require intervention.
Symptoms From Hormone Overproduction
The symptoms of a functioning adenoma depend entirely on which hormone it overproduces.
- Prolactinomas can cause unexpected breast milk production, irregular or absent periods in women, and reduced sex drive or erectile problems in men.
- Growth hormone-secreting tumors cause acromegaly in adults, leading to gradual enlargement of the hands, feet, and facial features, along with joint pain and other metabolic changes.
- ACTH-secreting tumors drive Cushing’s disease, which produces weight gain concentrated in the face and midsection, thin and easily bruised skin, muscle weakness, and high blood sugar. These tumors are usually small. Only about 4 to 10% of ACTH-secreting adenomas are macroadenomas.
Symptoms From Tumor Size
When a pituitary adenoma grows large enough, it can press on the optic chiasm, the point where your optic nerves cross just above the pituitary gland. This compression typically causes a specific pattern of vision loss: you lose peripheral vision on both sides, particularly in the outer (temporal) fields. The mechanism involves the tumor pushing the optic nerve upward against the bony edge of the optic canal, compressing the nerve fibers that carry signals from your peripheral visual field.
Large tumors can also damage the healthy pituitary tissue around them, reducing its ability to produce hormones normally. This condition, called hypopituitarism, can cause fatigue, weight changes, low blood pressure, and problems with fertility or sexual function. Persistent headaches are another common symptom of macroadenomas, caused by pressure within the bony space (called the sella turcica) that houses the pituitary gland.
How Pituitary Adenomas Are Diagnosed
MRI is the gold standard for imaging pituitary tumors. It provides detailed views of the gland and surrounding soft tissue. For initial diagnosis, contrast-enhanced MRI (where a dye is injected to highlight the tumor) is significantly more sensitive than plain MRI, detecting microadenomas about 89% of the time compared to roughly 56% without contrast.
Blood tests are the other essential piece. Any time a pituitary tumor is suspected or found incidentally, a full hormonal workup helps determine whether the tumor is functioning. For suspected prolactinomas, the key test is a serum prolactin level. For growth hormone-secreting tumors, doctors measure a blood marker called IGF-1 and may follow up with an oral glucose tolerance test, since glucose normally suppresses growth hormone and a tumor won’t respond to that signal. Cushing’s disease requires a more layered approach: first confirming that cortisol levels are abnormally high (through urine collection, saliva tests, or a low-dose suppression test), then measuring ACTH to pinpoint the pituitary as the source.
For macroadenomas that aren’t clearly producing excess hormones, the workup also screens for the opposite problem: whether the tumor is shutting down normal pituitary function by checking thyroid hormones, cortisol, and reproductive hormones.
Treatment for Prolactinomas
Prolactinomas are unique among pituitary adenomas because medication is the first-line treatment, not surgery. Dopamine agonists, most commonly cabergoline, lower prolactin levels and shrink the tumor itself. Cabergoline tends to be more effective and better tolerated than the older alternative, bromocriptine.
The results are often striking. In patients treated for macroadenomas over one year, tumors typically shrink to half their original size in about 90% of cases. For those who had vision loss from tumor compression, visual fields return to normal in up to 90% of patients as well. Many people stay on medication long-term, though some can eventually taper off if the tumor shrinks sufficiently and prolactin levels remain stable.
When Surgery Is Needed
For non-functioning macroadenomas causing vision loss or other compression symptoms, and for most growth hormone or ACTH-secreting tumors, surgery is the primary treatment. The standard approach is transsphenoidal surgery, where surgeons reach the pituitary through the nose and sinuses rather than opening the skull. This can be done with an endoscope (a small camera) or a microscope.
Recovery varies, but most follow-up in published studies occurs within six months, with many patients followed for over a year to monitor for recurrence. The most commonly reported complication is a cerebrospinal fluid leak, where the protective fluid surrounding the brain seeps through the surgical site. Some leaks seal on their own, while others require a second procedure to repair. Other potential complications include infection (including meningitis in a small percentage of cases) and nosebleeds requiring treatment.
Monitoring Small, Asymptomatic Tumors
Many pituitary adenomas are discovered by accident during brain imaging done for another reason. These incidentalomas, particularly small non-functioning microadenomas, often don’t need treatment at all. Current consensus guidelines recommend repeat MRI every two to three years. If a microadenoma smaller than 5 millimeters stays stable on successive scans, you and your doctor can reassess whether continued imaging is even necessary.
Hormone levels should be rechecked every one to two years, or sooner if new symptoms develop. Although uncommon, some tumors that initially appear non-functioning can begin disrupting normal pituitary hormone production over time, even without obvious growth on imaging.