What is a Pituitary Microadenoma?

A pituitary microadenoma is a benign (noncancerous) growth measuring less than 1 centimeter (10 mm) in the pituitary gland. Located at the base of the brain, the pituitary produces hormones that help regulate many essential functions throughout the body. Like other pituitary adenomas, microadenomas typically arise from cells in the anterior (front) portion of the gland. The term “microadenoma” refers specifically to size; pituitary adenomas measuring 1 centimeter or larger are classified as macroadenomas.

The effects of a pituitary microadenoma often depend on whether it produces excess hormones. A functioning microadenoma can release excess amounts of a pituitary hormone and cause conditions such as Cushing disease, acromegaly or hyperprolactinemia. By comparison, a nonfunctioning microadenoma does not cause a hormone-excess syndrome and may produce no noticeable symptoms. Some are found unexpectedly during brain imaging performed for an unrelated medical concern.

Because of their small size, microadenomas are less likely than macroadenomas to press on the optic chiasm or other nearby structures. Nevertheless, hormone-producing tumors can have effects throughout the body, making appropriate endocrine evaluation important even when an adenoma measures only a few millimeters.

Causes and risk factors for pituitary microadenoma

Most pituitary microadenomas occur sporadically, without an identifiable trigger or hereditary explanation. Tumor formation is thought to involve molecular changes that alter the normal controls on pituitary cell growth and division. Why these changes develop in a particular individual is usually not known.

For a small proportion of patients, an inherited predisposition may play a role. Genetic conditions associated with an increased likelihood of developing pituitary adenomas include multiple endocrine neoplasia type 1 (MEN1), familial isolated pituitary adenoma (FIPA) and Carney complex. A family history of pituitary tumors or certain endocrine tumor syndromes may therefore prompt consideration of whether an inherited condition could be involved.

Pituitary microadenoma has not been linked to established behavioral or environmental risk factors. Modifiable factors such as diet, physical activity and other lifestyle habits are not known to cause these tumors or reliably alter the likelihood that they will develop. As a result, there is no proven method for preventing a pituitary microadenoma.

Signs and symptoms of pituitary microadenoma

Many pituitary microadenomas do not cause symptoms, particularly when they are nonfunctioning. When symptoms do develop, they are often related to abnormal hormone production rather than pressure on structures surrounding the pituitary. The specific effects can vary considerably depending on which type of pituitary cell gave rise to the adenoma and which hormone is being produced in excess.

Common signs of a functioning pituitary microadenoma

Hormonal changes can affect reproductive function, metabolism, physical appearance and other body processes. Depending on the hormone involved, signs and symptoms of a pituitary microadenoma may include:

• Menstrual changes or absent menstrual periods
• Infertility
• Breast milk production unrelated to pregnancy or breastfeeding (galactorrhea)
• Reduced libido
• Erectile dysfunction
• Unexplained weight gain
• Easy bruising
• Muscle weakness
• Increased facial roundness
• Enlargement of the hands or feet
• Changes in facial features
• Excessive sweating
• Joint discomfort
• Headaches

The presence of these symptoms does not necessarily indicate a pituitary tumor. Many can result from other medical conditions, and appropriate testing is necessary to determine the cause.

Although it's not considered cancer, pituitary tumors have serious consequences to body function beyond appearance.

Dr. Andre Beer Furlan

Neurosurgeon

How pituitary microadenoma symptoms differ by hormone type

Several distinct endocrine disorders can result from a functioning pituitary adenoma. These include:

• Prolactin excess – A prolactin-secreting microadenoma, or microprolactinoma, can interfere with reproductive hormones. Possible effects include menstrual irregularities, infertility, galactorrhea, reduced libido and erectile dysfunction.
• Adrenocorticotropic hormone (ACTH) excess – Some microadenomas release too much ACTH, stimulating the adrenal glands to produce excessive cortisol. The resulting condition, Cushing disease, can cause weight gain, easy bruising, muscle weakness, high blood pressure and changes in glucose metabolism, among other effects.
• Growth hormone excess – Excess growth hormone can lead to acromegaly in adults. Changes may include enlargement of the hands and feet, increasingly prominent facial features, excessive sweating, joint problems and metabolic abnormalities.

Other functioning pituitary adenomas are possible but less common. The pattern of symptoms and results of laboratory testing can help physicians determine which hormone system is affected.

How pituitary microadenoma is diagnosed

A pituitary microadenoma may come to medical attention in two very different ways. Some are identified during an evaluation for unexplained hormonal symptoms, while others appear unexpectedly on brain imaging obtained for another medical reason. An imaging finding alone does not necessarily establish whether a microadenoma is clinically significant, so the healthcare team will generally evaluate both the tumor itself and pituitary hormone function. Diagnostic testing may include:

• Physical and clinical evaluation – Findings such as menstrual changes, characteristic changes in physical appearance, abnormal blood pressure, changes in body composition or sexual dysfunction can provide important clues about whether excess pituitary hormone production is occurring.
• Blood tests – Laboratory studies can measure prolactin, growth hormone-related markers, cortisol-related hormones and other components of pituitary function. The specific tests ordered will depend on symptoms, medical history and the suspected type of adenoma.
• Specialized hormone testing – Some endocrine disorders require more than a single blood measurement. Suppression tests, stimulation tests, saliva testing or a 24-hour urine collection may be used in selected circumstances to evaluate hormone production more precisely.
• Magnetic resonance imaging (MRI) – Detailed MRI of the pituitary, typically performed with contrast, can identify small abnormalities within the gland and establish their dimensions and location. Specialized imaging protocols may be particularly useful because some microadenomas measure only a few millimeters.
• Additional testing – Depending on the suspected endocrine disorder, further evaluation may be needed to determine how abnormal hormone levels are affecting other organs and body systems.
• Pathology evaluation – Tissue analysis is generally performed only when a microadenoma is surgically removed. Examination of the tumor can establish its pituitary cell lineage and hormone expression and provide information relevant to subsequent care.

How pituitary microadenoma is treated

Finding a pituitary microadenoma does not necessarily mean treatment is needed. Management depends primarily on whether the adenoma is functioning, what type of hormone it produces, whether it is causing clinically significant effects and whether its characteristics change over time. As a result, two patients with similarly sized microadenomas may have very different treatment plans.

Treatment approaches for pituitary microadenoma may include:

• Observation – A nonfunctioning microadenoma that is not causing clinically significant abnormalities may be followed rather than treated immediately. Follow-up can include repeat imaging, laboratory testing and clinical assessment to determine whether the adenoma changes or pituitary abnormalities emerge.
• Medication – Prolactinomas are particularly responsive to medical treatment. Dopamine agonists can reduce prolactin production and often decrease tumor size, making medication the usual initial treatment for many prolactin-secreting adenomas. Medications may also help control hormone excess associated with certain other types of pituitary adenomas.
• Transsphenoidal surgery – Surgical removal is an important treatment for certain functioning microadenomas, including many ACTH-secreting tumors responsible for Cushing disease and growth hormone-secreting tumors associated with acromegaly. Pituitary surgery is generally performed through the nasal passages and sphenoid sinus, allowing the surgeon to reach the gland without a traditional open craniotomy.
• Radiation therapy – Radiation therapy is not commonly needed for pituitary microadenoma but may be considered if hormonally active tumor tissue persists or recurs and cannot be adequately controlled with other treatments.
• Clinical trials – Research studies may provide an option for eligible patients with persistent, recurrent or difficult-to-control pituitary tumors. Clinical trials can evaluate investigational medications and other approaches intended to improve tumor or hormone control.

Treatment may also include management of the broader health effects caused by prolonged hormone excess. Endocrinologists and other specialists can address complications and monitor whether hormone levels return to an appropriate range following therapy.

Frequently asked questions (FAQs) about pituitary microadenoma

The discovery of a pituitary microadenoma can be confusing, particularly because the word “tumor” may suggest a more serious diagnosis than the imaging finding actually represents. Understanding the significance of a microadenoma requires looking beyond its size to consider its hormone activity, symptoms and changes over time. The following answers address several common questions about these tumors:

What is the exact difference between a pituitary microadenoma and a macroadenoma?

The distinction is based on the tumor’s maximum diameter. A pituitary adenoma smaller than 1 centimeter (10 mm) is considered a microadenoma. Once an adenoma measures 1 centimeter or larger, it is classified as a macroadenoma.

This size difference has clinical implications. Macroadenomas have more potential to extend beyond the pituitary and place pressure on the optic chiasm, normal pituitary tissue and other nearby structures. Microadenomas are less likely to produce these pressure-related effects, although their small size does not prevent them from causing significant problems if they release excess hormones.

Is a pituitary microadenoma considered brain cancer?

No. A pituitary microadenoma is benign and is not considered brain cancer. Pituitary adenomas do not behave like malignant brain tumors and do not metastasize to distant organs.

A benign diagnosis, however, does not automatically mean that an adenoma can be ignored. Hormones released by a functioning microadenoma can affect many organs and body systems, which is why some of these tumors require treatment despite their noncancerous nature.

Can a microadenoma cause serious symptoms even though it is very small?

Yes. A functioning pituitary microadenoma can cause significant symptoms by producing excess hormones. Pituitary hormones travel through the bloodstream and regulate processes throughout the body, so abnormal hormone production can have widespread effects.

For example, a tiny ACTH-secreting adenoma can drive excess cortisol production and cause Cushing disease. Other microadenomas may produce excess prolactin or growth hormone and cause their own characteristic endocrine problems. For this reason, physicians consider hormone activity as well as measurements on an MRI when assessing the significance of a microadenoma.

Does a pituitary microadenoma always require surgical removal, or can it be monitored?

Surgical removal is not necessary for every pituitary microadenoma. A nonfunctioning adenoma without concerning symptoms or hormone abnormalities may be managed with observation and appropriate follow-up.

Even when treatment is required, surgery is not always the first choice. Prolactinomas are commonly treated with dopamine agonist medication, while surgery has a more prominent role for certain ACTH- and growth hormone-secreting adenomas. The appropriate approach depends on the type of tumor and its effects rather than its size alone.

What happens if a pituitary microadenoma is found accidentally on an MRI?

An unexpected pituitary finding usually requires further assessment to determine whether it is affecting hormone function and whether its imaging characteristics warrant follow-up. The evaluation may include a review of symptoms, laboratory testing and, when appropriate, dedicated pituitary imaging.

Finding a microadenoma incidentally does not necessarily mean that treatment will be needed. If testing shows that the adenoma is nonfunctioning and there are no other concerning findings, the healthcare team may recommend observation instead.

Can a pituitary microadenoma become a macroadenoma?

A microadenoma can grow enough to reach 1 centimeter and therefore be reclassified as a macroadenoma, but growth is not inevitable. Some microadenomas remain stable over time.

Whether repeat imaging is necessary and how frequently it should be performed depend on the adenoma’s characteristics, hormone activity and the patient’s clinical circumstances. Follow-up can allow the healthcare team to identify meaningful changes if they occur.

Can a pituitary microadenoma affect fertility?

It can, particularly when the adenoma alters hormones involved in reproductive function. Prolactinomas are a notable example because elevated prolactin can interfere with the hormonal signals necessary for normal ovulation, menstruation and testosterone production.

Other pituitary hormone disorders can also affect reproductive health. When infertility or menstrual or sexual changes are related to a functioning adenoma, treating the underlying hormonal problem may help restore normal reproductive function.

The Moffitt Cancer Center difference for pituitary microadenoma

A pituitary microadenoma may require specialized evaluation to determine whether it is affecting hormone function and which management approach is appropriate. Determining whether an imaging finding requires observation or active treatment may involve detailed endocrine testing, specialized pituitary imaging and careful consideration of symptoms that affect multiple body systems.

At Moffitt, patients have access to specialists who collaborate across disciplines to evaluate each adenoma and its hormonal effects. Through Moffitt’s Endocrine Program, diagnostic findings can be integrated with the patient’s symptoms and overall health to develop an individualized plan that may include surveillance, medical therapy, pituitary surgery or other specialized treatment when appropriate. 

To learn more about pituitary microadenoma, you can request an appointment with a specialist in Moffitt’s Endocrine Program by calling 1-888-663-3488 or submitting a new patient registration form online. We do not require referrals.