The pituitary gland is roughly the size of a pea, sat in a bony recess called the sella turcica at the base of the skull. It runs your thyroid, your adrenals, your reproductive hormones, your growth, and your body’s response to stress. When something goes wrong here, the endocrinologist needs to see the gland itself, not the brain around it. And that is where most imaging goes wrong.
The one-line answer
If your endocrinologist suspects a pituitary problem, ask specifically for a dedicated pituitary MRI with dynamic contrast, ideally on a 3T scanner and reported by a neuroradiologist. A standard brain MRI is not enough.
Why a standard brain MRI often misses a microadenoma
A standard brain MRI is optimised to cover the whole brain. It uses slice thicknesses of around 4 to 5 mm, standard T1 and T2 sequences, and typically no dynamic contrast run. That is perfect for looking at strokes, tumours, multiple sclerosis or the aftermath of a bleed. It is not the right tool for a 3 mm lesion sitting inside a 10 mm gland.
Microadenomas, the small pituitary tumours that most commonly cause prolactinoma and Cushing’s disease, can measure between 2 and 8 mm. On a 5 mm slice thickness they can fall entirely between two slices, or be averaged out with normal tissue and become invisible. Even when they are captured, without dynamic contrast timing they enhance at the same rate as the healthy gland and vanish into the background.
The result is a report that reads "no intracranial abnormality" while a lesion causing real, measurable hormone disruption sits there unseen. It is one of the most common preventable misses in outpatient neuroimaging.
The pituitary MRI protocol
A proper pituitary MRI is not one sequence, it is a specific stack of them. A well-run UK centre will include:
- Thin-slice coronal T1 through the sella, typically 2 to 3 mm with no gap. This is the workhorse view of the gland itself.
- Sagittal T1 to profile the gland, the stalk (infundibulum), the optic chiasm above and the sphenoid sinus below.
- Sagittal or coronal T2 to characterise cystic components, fluid signals, and Rathke cleft cysts that mimic adenomas.
- Dynamic post-contrast coronal T1, the single most important sequence for finding a microadenoma. Gadolinium is injected while imaging is happening, and images are captured every 15 to 30 seconds for several minutes. A microadenoma enhances more slowly than the normal pituitary, and appears as a relative dark spot in the first minute after injection before catching up.
- Delayed post-contrast T1 to confirm the finding and assess any extension outside the sella.
- Sometimes dedicated stalk imaging if a stalk lesion, thickening or ectopic posterior pituitary is suspected.
The whole study runs 25 to 35 minutes in the scanner, longer than a straightforward brain MRI. On a 3T scanner the resolution is meaningfully better than on 1.5T, which matters for the small structures involved.
Symptoms that trigger a pituitary MRI
You do not scan the pituitary for a hunch. The scan is triggered by a specific cluster of symptoms plus blood results that point at a hormone excess or deficiency. The common triggers:
- Suspected prolactinoma. Galactorrhoea (milky nipple discharge outside of pregnancy or breastfeeding), amenorrhoea (loss of periods), infertility, low libido or erectile dysfunction in men, combined with a raised serum prolactin. Prolactinoma is the most common secreting pituitary tumour.
- Bitemporal hemianopia. A loss of the outer half of the visual field in both eyes, caused by a larger tumour pressing up on the optic chiasm. Any adult reporting this needs imaging urgently.
- Acromegaly features. Enlarging hands, feet or jaw in adulthood, increased ring or shoe size, coarsening facial features, sweating, joint pain, sleep apnoea, and raised IGF-1 on bloods. Suggests growth-hormone-secreting adenoma.
- Cushingoid features. Central weight gain, thin skin, easy bruising, purple abdominal striae, proximal muscle weakness, and hypertension, with biochemistry showing high cortisol not suppressing on dexamethasone. Suggests ACTH-secreting adenoma.
- Central hypothyroidism or hypogonadism. Low thyroid or sex hormones with inappropriately low TSH, FSH or LH suggest the pituitary is under-signalling rather than the target gland failing.
- Sudden severe headache with visual change or collapse. Possible pituitary apoplexy, a haemorrhage or infarction within an adenoma. This is an emergency and needs same-day imaging.
Microadenoma (<10mm) vs macroadenoma (>10mm)
Pituitary tumours are classified by size, and the size changes the entire management pathway.
A microadenoma is under 10 mm. Most are functional, meaning they secrete a hormone, and the diagnosis is usually made because of the endocrine picture rather than because the lesion itself is causing mass effect. Prolactinomas are typically microadenomas at presentation. Treatment is usually medical, with dopamine agonists such as cabergoline, and the MRI is repeated periodically to confirm the lesion is shrinking or stable.
A macroadenoma is 10 mm or larger. These can cause symptoms through mass effect on adjacent structures, most notably the optic chiasm above (causing visual field loss) and the cavernous sinuses either side (where the carotid arteries and cranial nerves run). Non-functioning macroadenomas often present precisely this way, with headaches, visual disturbance and hypopituitarism from compression of the normal gland. Treatment is more likely to involve transsphenoidal surgery, sometimes followed by radiotherapy, alongside hormone replacement.
Reports should state maximum diameter, extension into the cavernous sinus, the position of the optic chiasm relative to the tumour, and whether the tumour is cystic, solid or haemorrhagic. Anything less is not a useful pituitary report.
