A small lesion inside your pituitary gland is one of the more common incidental findings on a brain MRI, and one of the most anxiety-inducing to receive by letter. The good news, for most people reading this, is that the plan almost never involves surgery. It involves careful, regular imaging - and the imaging itself has to be done properly to be worth doing at all.
This piece walks through what annual pituitary surveillance actually looks like in the UK in 2026, why a routine brain MRI is not sufficient, what the radiologist is comparing between scans, and what changes the plan.
The one-line answer
A pituitary microadenoma is monitored with a dedicated pituitary MRI at 6 months, at 12 months, then annually if stable, then usually every 2 years - not with a repeat routine brain MRI. The scan and the schedule are different from what most people assume.
Microadenoma vs macroadenoma - the 10 mm line
Pituitary adenomas are divided by size. Anything under 10 mm across is a microadenoma. Anything 10 mm or larger is a macroadenoma. That single millimetre threshold changes almost everything about how the lesion is managed.
A macroadenoma is more likely to press on nearby structures - the optic chiasm above the gland, the cavernous sinus on either side - and is more likely to need earlier neurosurgical or endocrine intervention. A microadenoma, by contrast, is usually watched. Most microadenomas do not grow, and a substantial minority shrink or disappear over the years. The purpose of surveillance is not to treat, it is to catch the small percentage that behave differently.
The three microadenomas most often surveilled
In UK endocrinology practice, three specific patterns account for most of the annual surveillance workload:
- Non-functioning microadenoma. No hormonal excess, no symptoms. Often found incidentally on a brain MRI done for headaches or dizziness. The rationale for follow-up is size stability, not hormones.
- Prolactinoma on medication. A prolactin-secreting microadenoma controlled with a dopamine agonist such as cabergoline. Follow-up MRI checks that the lesion is stable or shrinking alongside the falling prolactin level.
- Pituitary incidentaloma. Found on a scan done for an unrelated reason, no hormonal abnormality, no visual issue. The surveillance is short-term reassurance that nothing is quietly changing.
For all three, the imaging protocol is the same - and it is not the protocol used for a standard brain MRI.
The pituitary MRI protocol - why it is different
A routine brain MRI takes slices around 3 to 5 mm thick across the whole head. That resolution is fine for cortex, ventricles and the major white matter tracts. It is not fine enough to measure a 6 mm lesion in a 10 mm gland with any real confidence.
A dedicated pituitary MRI is targeted at the sella turcica - the small bony hollow that holds the gland - and uses thin slices (typically 2 to 3 mm), coronal and sagittal T1 and T2 sequences, and in most surveillance protocols a dynamic post-contrast series where images are taken every few seconds as gadolinium contrast flows through the gland. The healthy pituitary enhances quickly. Most microadenomas enhance more slowly, which is what makes them visible against the normal tissue.
The scan that first found your microadenoma may not have been the ideal scan to find it. But for tracking it over the years, the protocol matters more than the scanner. Get a proper pituitary MRI, or the surveillance is not really surveillance.
The typical surveillance schedule
There is no single national protocol, but the pattern most UK endocrinologists follow for a stable, non-functioning microadenoma looks like this:
| Point in the pathway | Scan interval | What changes if stable |
|---|---|---|
| After the incidental finding | MRI at 6 months | First direct comparison against the baseline scan |
| Year one | MRI at 12 months | Moves to an annual rhythm if unchanged |
| Years 2 to 3 | MRI annually | Confirms stability across multiple time points |
| Years 4 and beyond | MRI every 2 years | Interval lengthens for lesions that have been reliably stable |
| Prolactinoma on cabergoline | MRI at 12 months, then annually | Interval driven by prolactin levels alongside imaging |
Two things are worth noting. First, this schedule is a starting point, not a rule - an endocrinologist will lengthen or shorten it based on your specific hormonal picture, symptoms, and how confident they are in the baseline imaging. Second, the interval should never be extended purely for logistical reasons. If a scan is due, it is due.
