Abnormal Cortisol: Adrenal and Pituitary MRI

Cortisol is your body's primary stress hormone, produced by the adrenal glands (which sit on top of your kidneys) under the direction of the pituitary gland (a pea-sized gland at the base of your brain). When cortisol levels are consistently too high or too low, MRI of the adrenal glands and/or pituitary gland can identify the structural cause.

How Cortisol Regulation Works

Understanding the pathway helps make sense of why MRI is needed:

1. The hypothalamus (brain) releases CRH (corticotropin-releasing hormone)
2. CRH stimulates the pituitary gland to release ACTH (adrenocorticotropic hormone)
3. ACTH travels through the blood to the adrenal glands
4. The adrenal glands produce cortisol in response
5. Rising cortisol feeds back to the hypothalamus and pituitary, reducing CRH and ACTH production (negative feedback)

Problems at any level — hypothalamus, pituitary, or adrenal glands — can disrupt this system. MRI is the imaging tool of choice for both the pituitary gland and the adrenal glands because of its superior soft tissue detail.

High Cortisol: Cushing's Syndrome

Cushing's syndrome is the clinical picture caused by prolonged exposure to high cortisol. Symptoms develop gradually and can include:

• Weight gain, particularly around the face (moon face), upper back (buffalo hump), and abdomen
• Thinning skin that bruises easily
• Purple-red stretch marks (striae) on the abdomen, thighs, or arms
• Muscle weakness, especially in the thighs
• High blood pressure
• High blood sugar / type 2 diabetes
• Mood changes — anxiety, depression, irritability
• Menstrual irregularity in women
• Reduced libido in men
• Osteoporosis

The most common cause: medication

By far the most frequent cause of Cushing's syndrome is long-term use of corticosteroid medications (prednisolone, dexamethasone, hydrocortisone) for conditions like asthma, autoimmune diseases, or after transplants. This is called exogenous Cushing's — the excess cortisol comes from outside the body, not from overproduction. In this case, MRI isn't usually needed because the cause is known.

When the body overproduces cortisol (endogenous Cushing's)

This is where MRI becomes essential. There are three main sources:

Pituitary adenoma (Cushing's disease) — 70% of endogenous cases

A small, usually benign tumour in the pituitary gland produces too much ACTH, which drives the adrenal glands to make too much cortisol. These tumours (microadenomas) are often tiny — less than 10mm — and require a dedicated pituitary MRI with thin slices and dynamic contrast imaging to visualise them.

MRI of the pituitary gland shows:

• The size and location of the adenoma
• Its relationship to surrounding structures (optic chiasm, cavernous sinuses)
• Whether surgery (transsphenoidal adenomectomy — through the nose) is feasible

Adrenal tumour — 15% of endogenous cases

An adrenal adenoma (benign) or rarely an adrenal carcinoma (malignant) produces cortisol autonomously, ignoring the normal feedback system. Adrenal MRI shows:

• The size and characteristics of the tumour
• Whether it has features suggesting benign or malignant behaviour
• Whether one or both adrenal glands are affected

Ectopic ACTH production — 15% of endogenous cases

Rarely, a tumour elsewhere in the body (most commonly in the lung) produces ACTH. This requires broader imaging — CT of the chest and abdomen — with MRI used for specific follow-up.

Low Cortisol: Adrenal Insufficiency

Too little cortisol is also a serious condition. There are two types:

Primary adrenal insufficiency (Addison's disease)

The adrenal glands themselves are damaged and can't produce enough cortisol. The most common cause in the UK is autoimmune destruction — the immune system attacks the adrenal glands. Other causes include tuberculosis (historically the most common cause), adrenal haemorrhage, and metastatic cancer.

Symptoms include:

• Extreme fatigue
• Weight loss
• Low blood pressure, especially on standing (postural hypotension)
• Salt cravings
• Darkening of the skin (hyperpigmentation), especially in skin creases, scars, and gums
• Nausea, vomiting, abdominal pain

Adrenal MRI in Addison's disease may show small, atrophic adrenal glands (in autoimmune disease) or enlarged adrenals (in tuberculosis, haemorrhage, or infiltrative disease). It can also identify adrenal masses if metastatic disease is the cause.

Secondary adrenal insufficiency

The pituitary gland doesn't produce enough ACTH, so the adrenal glands aren't stimulated properly. This can result from:

• Pituitary tumours compressing normal pituitary tissue
• Previous pituitary surgery or radiation
• Pituitary apoplexy (sudden bleeding into a pituitary tumour)
• Sheehan's syndrome (pituitary damage from severe blood loss during childbirth)
• Long-term steroid use that suppresses the pituitary (the gland "forgets" how to produce ACTH)

Pituitary MRI is essential in secondary adrenal insufficiency to identify the underlying cause and guide treatment.

Adrenal Incidentalomas

Adrenal incidentalomas are adrenal masses found by accident during imaging done for other reasons. They're surprisingly common — found in about 5% of abdominal CT scans and a similar proportion of MRI scans. Most are non-functioning adenomas (benign tumours that don't produce hormones), but a small percentage are:

• Functioning adenomas producing cortisol (causing subclinical Cushing's)
• Phaeochromocytomas (producing adrenaline — causing episodic high blood pressure, headaches, and sweating)
• Aldosterone-producing adenomas (causing Conn's syndrome — high blood pressure with low potassium)
• Adrenal carcinomas (rare but serious)
• Metastases from cancers elsewhere (lung, breast, kidney, melanoma)

When an adrenal incidentaloma is found, MRI with specific sequences (chemical shift imaging) can characterise it as most likely benign or potentially concerning, guiding whether further investigation or monitoring is needed.

Blood Tests Before MRI

If abnormal cortisol is suspected, your doctor will typically order:

• Morning cortisol: A blood test taken between 8–9am (when cortisol is normally at its peak). Below 100 nmol/L is concerning for deficiency. Above 500 nmol/L may suggest excess.
• ACTH: Measured alongside cortisol. High ACTH + high cortisol = pituitary or ectopic source. Low ACTH + high cortisol = adrenal source. Low ACTH + low cortisol = secondary adrenal insufficiency.
• 24-hour urinary free cortisol: Measures total cortisol output over a day, avoiding the variability of a single blood test.
• Late-night salivary cortisol: Cortisol should be low at night. If it's elevated at midnight, this supports Cushing's syndrome.
• Dexamethasone suppression test: Taking a dose of dexamethasone (synthetic steroid) at night should suppress cortisol by the next morning. If it doesn't, cortisol production is autonomous.

When to Get Scanned

MRI of the pituitary or adrenal glands is indicated when:

• Blood tests confirm abnormal cortisol levels (either high or low)
• ACTH levels point to a specific source (pituitary or adrenal)
• An adrenal mass has been found incidentally
• Symptoms consistent with Cushing's or Addison's are present alongside abnormal bloods
• Monitoring known pituitary or adrenal conditions

Cortisol problems are relatively uncommon but genuinely serious when they occur. The combination of careful blood testing and targeted MRI identifies the cause — and knowing the cause is the first step to effective treatment.