The pituitary gland is a small, pea sized organ located at the base of the brain, tucked safely within a bony structure called the sella turcica. Because of its protected location and the way it functions, many tumours that develop here can remain undetected for years. While some tumours make their presence known early by producing excess hormones, a significant number of these growths are “non-functioning,” meaning they do not produce any hormones at all. These silent tumours often only cause symptoms once they have grown large enough to press against nearby brain structures or the gland itself. Understanding the difference between hormonal and structural symptoms is key to knowing why some pituitary tumours are discovered only after reaching a significant size.
What We’ll Discuss in This Article
- The distinction between functioning and non-functioning pituitary tumours.
- How the anatomy of the sella turcica hides small growths.
- The concept of “mass effect” and its impact on surrounding structures.
- Why hormonal symptoms can sometimes be subtle or overlooked.
- The role of incidental discovery during scans for unrelated issues.
- When a growing tumour becomes a clinical priority.
Functioning versus Non-Functioning Adenomas
The primary reason a pituitary tumour might not cause early symptoms is related to its hormonal activity. Medical professionals categorise these tumours into two main groups: functioning and non-functioning adenomas. A functioning adenoma is one that produces an excess of a specific hormone, such as prolactin, growth hormone, or ACTH. Because even a tiny increase in these powerful chemicals can cause noticeable changes in the body, such as irregular periods or changes in facial features, these tumours are often caught while they are still very small, often referred to as microadenomas.
In contrast, a non-functioning adenoma does not produce any hormones. Because there is no chemical signal alerting the body to its presence, the tumour can grow slowly over many years without causing any metabolic or physical changes. These growths are typically only identified when they become macroadenomas, which are tumours larger than one centimetre in diameter. By the time they reach this size, they often begin to push against the normal parts of the pituitary gland or the surrounding nerves, which is usually the point at which symptoms finally appear. According to the NHS, non-functioning tumours are frequently discovered later in life for this exact reason.
The Protective Anatomy of the Sella Turcica
The physical location of the pituitary gland plays a significant role in masking the growth of a tumour. The gland sits in a small, saddle shaped depression in the sphenoid bone known as the sella turcica. This bony cradle provides excellent protection for the gland, but it also means there is very little “extra” room. However, because the bone is rigid, a small growth can often expand slightly within this space without putting immediate pressure on the brain itself.
Until the tumour reaches the top of the bony cradle, it may not interfere with any vital functions. The brain and its surrounding nerves are located just above this bony cavity. It is only when the tumour grows “upward” out of the sella turcica that it begins to interact with the optic nerves or the hypothalamus. This anatomical shielding explains why a tumour can exist for a long time without causing headaches or vision problems, as the initial expansion happens in a confined space that does not contain pain sensitive structures or major nerve pathways.
Subtle Symptoms and Misdiagnosis
In some cases, even functioning tumours that produce hormones might not be diagnosed early because their symptoms are so subtle or common. For example, a tumour that produces a small amount of excess prolactin might cause a slightly reduced sex drive or mild fertility issues, which a patient might not immediately report to their doctor. Similarly, the early signs of acromegaly, caused by excess growth hormone, involve very slow changes to the hands, feet, and face that may be attributed to “just getting older” by the patient and their family.
Because these changes happen over years rather than weeks, the brain and body often adapt to them. It is only when the symptoms become undeniable, or when the “mass effect” symptoms like persistent headaches begin, that the patient seeks medical advice. This slow progression is a hallmark of pituitary disorders and is why clinicians must maintain a high index of suspicion when patients present with a combination of vague hormonal and neurological complaints.
Incidentalomas and Accidental Discovery
Interestingly, many pituitary tumours that cause no symptoms are discovered by accident. These are often referred to as “incidentalomas.” With the increasing use of high-resolution MRI and CT scans for other reasons, such as investigating a head injury or a standard headache, doctors sometimes find a small growth on the pituitary gland that was not causing any problems.
Studies suggest that a small percentage of the general population may have a tiny, harmless pituitary tumour that they will never know about during their lifetime. If these incidentalomas are small and not producing hormones, doctors often choose to monitor them with periodic scans rather than treating them immediately. This highlights the fact that not all pituitary tumours are aggressive or symptomatic, and many can exist quietly without ever impacting a person’s health. The NICE guidelines on pituitary adenoma suggest that observation is a valid pathway for small, non-functioning incidentalomas.
When Growth Becomes an Emergency
While most pituitary tumours grow very slowly, there is a rare but serious complication called pituitary apoplexy. This occurs when a large tumour suddenly bleeds or loses its blood supply, causing it to swell rapidly. Because the tumour is already large, this sudden increase in pressure can cause an immediate and severe headache, sudden vision loss, and a dangerous drop in hormone levels.
This is a medical emergency because the rapid expansion can cause permanent damage to the optic nerves and lead to acute adrenal failure. This situation underscores why even “silent” tumours need to be monitored once they are discovered. While they may have caused no symptoms for years, their presence means the balance of pressure at the base of the brain is delicate, and any sudden change can have significant consequences for the patient’s health.
Conclusion
Pituitary tumours often cause no symptoms until they are large because many do not produce hormones and the gland’s location allows for some growth before vital structures are compressed. Functioning tumours are typically caught early due to chemical changes, whereas non-functioning tumours rely on “mass effect” to become noticeable. Regular monitoring and awareness of subtle hormonal or visual changes are essential for early detection. If you experience severe, sudden, or worsening symptoms such as a sudden severe headache or vision loss, call 999 immediately.
What is the difference between a microadenoma and a macroadenoma?
A microadenoma is a tumour smaller than 1 centimetre, while a macroadenoma is 1 centimetre or larger and is more likely to cause symptoms by pressing on nearby structures.
Can a small tumour cause a headache?
Usually, small tumours do not cause headaches unless they are producing specific hormones; most headaches related to pituitary tumours are caused by larger growths pressing on the brain’s lining.
Why does a pituitary tumour affect my peripheral vision?
The tumour can grow upwards and press on the optic chiasm, which is the part of the brain where the nerves responsible for your side vision cross over.
Is it common for pituitary tumours to be cancerous?
No, most pituitary tumours are benign, meaning they are non-cancerous and do not spread to other parts of the body, though they can still cause problems by growing locally.
How do doctors monitor a tumour that isn’t causing symptoms?
Doctors typically use regular MRI scans and blood tests to check hormone levels to ensure the tumour is not growing or becoming hormonally active.
Can a tumour cause me to feel constantly tired?
Yes, if a large tumour presses on the healthy part of the pituitary gland, it can lower the production of hormones that regulate your energy and metabolism.
Will a non-functioning tumour ever need treatment?
Yes, if the tumour grows large enough to threaten your vision or significantly interfere with normal hormone production, surgery or medication may be required.
Authority Snapshot (E-E-A-T Block)
This article explains the clinical reasons why some pituitary tumours remain asymptomatic until they reach a significant size. It has been authored by the medical content team and reviewed by Dr. Rebecca Fernandez to ensure the information is accurate and reflects standard UK endocrinology practices. The content is strictly aligned with NHS and NICE guidance regarding the diagnosis and management of pituitary adenomas.
