Diagnosing a prolactinoma involves a structured clinical pathway designed to confirm the overproduction of the hormone prolactin and identify the presence of a pituitary growth. Because elevated prolactin can be caused by various factors, including stress, medication, and pregnancy, the diagnostic process must be meticulous to differentiate a true prolactinoma from other causes of hyperprolactinaemia. In the United Kingdom, this process typically begins in primary care and moves to a specialist endocrine clinic for detailed biochemical analysis and high resolution imaging. The goal is not only to identify the tumour but also to assess its impact on the rest of the pituitary gland and the surrounding brain structures.
What We’ll Discuss in This Article
- The role of initial and repeat prolactin blood tests.
- Identifying laboratory artifacts such as macroprolactin.
- The importance of a baseline pituitary hormone profile.
- Why MRI is the preferred imaging tool for the pituitary gland.
- The diagnostic criteria for microprolactinomas and macroadenomas.
- The value of visual field testing in large tumours.
- Answers to common questions about the diagnostic experience.
The role of prolactin blood tests in diagnosis
The first step in diagnosing a prolactinoma is a simple blood test to measure the level of prolactin in the circulation. Prolactin levels are measured in milliunits per litre (mIU/L) in the UK. A normal result is typically below 700 mIU/L, though this can vary slightly between laboratories. If a single test shows a raised result, it is standard practice to repeat the test at least once. This is because prolactin is a stress sensitive hormone; even the anxiety of having a needle inserted or a poor night’s sleep can cause a temporary, non pathological rise.
In some cases, a specialist may recommend a “cannulated prolactin” or “rested prolactin” test. This involves the insertion of a small plastic tube (cannula) into a vein, followed by a period of rest usually 30 to 120 minutes before blood is drawn. This ensures that any elevation seen is a true reflection of the pituitary’s output rather than a reaction to the procedure itself. If the rested levels remain consistently high, particularly above 2,000 to 5,000 mIU/L, the likelihood of a prolactinoma increases significantly.
Source: https://www.esht.nhs.uk/wp-content/uploads/2022/03/0961.pdf
Identifying macroprolactin and laboratory artifacts
When a blood test reveals high prolactin, the laboratory will often perform an additional check for “macroprolactin.” Prolactin can sometimes bind to antibodies in the blood, creating large molecules that are detected by the test but are biologically inactive. This condition, known as macroprolactinaemia, does not cause symptoms and does not require treatment. Identifying macroprolactin prevents patients from undergoing unnecessary scans or starting medication for a problem that does not exist.
Another rare but important artifact is the “hook effect,” which can occur in patients with extremely large tumours. If prolactin levels are exceptionally high, they can sometimes overwhelm the laboratory test, resulting in a falsely low reading. If a clinician suspects a large tumour based on symptoms like vision loss, but the blood test shows only a mild elevation, they may ask the laboratory to dilute the sample and retest it. This ensures that a massive prolactinoma is not missed due to a technical limitation of the assay.
The baseline pituitary hormone profile
Because a prolactinoma is a tumour of the “master gland,” it is essential to check the function of the entire pituitary system. A baseline pituitary profile is usually conducted alongside prolactin testing. This involves measuring several different hormones in a single blood sample to ensure that the rest of the gland is functioning correctly and is not being compressed by the growth.
| Hormone Checked | Purpose of the Test |
| TSH and Free T4 | To exclude an underactive thyroid, which can raise prolactin. |
| LH and FSH | To assess the signals to the ovaries or testes. |
| Oestradiol or Testosterone | To check the levels of sex hormones, which are often low. |
| Cortisol (9 am) | To ensure the adrenal glands are functioning properly. |
| IGF-1 | To screen for excess growth hormone. |
These tests help the endocrinologist determine if the patient has hypopituitarism—a deficiency in other hormones caused by the tumour’s pressure. Identifying an underactive thyroid is particularly important, as hypothyroidism can cause prolactin levels to rise. If the thyroid is the cause, treating it with simple hormone tablets will often return the prolactin to normal without the need for further intervention.
Pituitary imaging: The role of MRI
Once biochemical evidence confirms a pathological rise in prolactin, imaging is required to visualize the pituitary gland. Magnetic Resonance Imaging (MRI) is the gold standard for this purpose in the UK. An MRI uses powerful magnets and radio waves to create detailed slices of the brain and pituitary gland. It is far superior to a CT scan for this area because it provides much clearer contrast between the tumour and the surrounding healthy tissue.
During the scan, a specialized “contrast” dye called gadolinium is often injected into a vein. This dye highlights the pituitary gland and helps the radiologist distinguish between a small tumour and normal gland tissue. The scan typically takes 20 to 30 minutes. It is a painless procedure, though the machine is noisy. For patients who feel claustrophobic, the Pituitary Foundation suggests discussing the use of a mild sedative with your GP or specialist beforehand to ensure the best possible images are obtained.
Differentiating microprolactinomas and macroadenomas
The results of the MRI allow clinicians to classify the tumour based on its size. This classification is vital for determining the frequency of monitoring and the potential for complications.
- Microprolactinoma: A tumour that is less than 10 millimetres in diameter. These are the most common and are often entirely contained within the pituitary gland itself.
- Macroprolactinoma: A tumour that is 10 millimetres or larger. These are more likely to cause symptoms through their size, such as pressing on the nerves to the eyes.
While the size is important, it is the combination of the scan and the blood test that provides the most information. For example, if an MRI shows a large mass but the prolactin level is only mildly elevated, the doctor may suspect a “non functioning” tumour that is causing the “stalk effect” rather than a true prolactinoma. This detailed differentiation ensures that the patient receives the correct treatment—usually medication for a prolactinoma and potentially surgery for a non functioning mass.
Visual field testing for larger tumours
If an MRI reveals a macroadenoma, particularly one that is close to the optic nerves, the patient will be referred for a visual field test. This is an essential diagnostic step to see if the tumour is physically pressing against the optic chiasm. The test involves clicking a button when lights flash in the peripheral field of vision. It is a painless, non invasive way to “map” the patient’s sight and identify any areas where the tumour might be causing silent vision loss.
Visual field testing is a critical part of the initial diagnosis and is often repeated periodically once treatment begins. As the medication shrinks the tumour, the visual field should improve or return to normal. According to the Royal Berkshire NHS Foundation Trust, monitoring the relationship between the tumour and the eyesight is one of the most important aspects of long term care for those with larger growths.
Diagnostic summary and the multidisciplinary team
The final diagnosis of a prolactinoma is made by an endocrinologist who reviews all the evidence: the persistent high prolactin, the results of the pituitary profile, the MRI images, and the visual field tests. In the UK, complex cases are often discussed in a Multidisciplinary Team (MDT) meeting. This team includes endocrinologists, neurosurgeons, radiologists, and specialist nurses who work together to ensure the diagnosis is accurate and the treatment plan is safe.
Once the diagnosis is confirmed, the focus shifts to treatment, which for the vast majority of patients involves dopamine agonist medications like cabergoline. These drugs are highly effective at lowering prolactin and shrinking the tumour. The diagnostic phase is often the most anxiety inducing part of the journey, but once the cause is identified, the path to recovery becomes clear and manageable within the specialized framework of the NHS.
Conclusion
Diagnosing a prolactinoma requires a careful combination of repeated blood tests to confirm true hormone elevation and high resolution MRI imaging to visualize the pituitary growth. By excluding artifacts like macroprolactin and assessing the entire pituitary profile, UK specialists can create a precise clinical picture of the patient’s health. For those with larger tumours, visual field testing provides essential information on the safety of the optic nerves. If you experience severe, sudden, or worsening symptoms, call 999 immediately.
Why do I need more than one blood test?
Prolactin is a very sensitive hormone that can rise due to the stress of a needle or poor sleep; a second test ensures the high level is a permanent clinical issue.
Is an MRI scan safe?
Yes, an MRI does not use radiation (unlike X-rays) and is the safest, most detailed way to look at the pituitary gland.
What happens if I have high prolactin but a clear MRI?
This is known as “idiopathic hyperprolactinaemia.” It means the levels are high without a visible tumour, and it is managed similarly with monitoring or medication.
Will the contrast dye during the MRI make me feel unwell?
Most people do not feel any different, but you should tell the radiologist if you have asthma or any known allergies before the injection.
Can a GP diagnose a prolactinoma?
A GP can perform the initial blood tests and identify high prolactin, but the final diagnosis and specialized scans must be arranged by an endocrinologist.
What is a “DEXA” scan and do I need one?
A DEXA scan measures bone density. If your prolactin has been high and your sex hormones low for a long time, it may be recommended to check for bone thinning.
How long does it take to get a final diagnosis?
From the first blood test to the final specialist review and MRI, the process usually takes several weeks to ensure every detail is checked thoroughly.
Authority Snapshot
This article provides a clinical overview of the diagnostic pathway for prolactinomas to assist in patient education in the United Kingdom. The content has been authored by a specialized medical content team and reviewed by Dr. Rebecca Fernandez to ensure the highest standards of clinical accuracy. All information presented is strictly aligned with the latest NHS guidelines and UK endocrine society standards to provide reliable and safe information for patients.
