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PubMed This is a summary of 21 peer-reviewed journal articles Updated
Endocrinology · Immunotherapy-Induced Hypophysitis

Diagnosis and the Normal MRI Trap

At a Glance

Immunotherapy-induced hypophysitis can cause low pituitary hormones even when the MRI looks normal. Diagnosis depends on symptoms and blood tests such as 8 AM cortisol, ACTH, and Free T4; an early normal cosyntropin test may not exclude it.

When doctors suspect immunotherapy-induced hypophysitis (IIH), they are essentially looking for a failure of your master gland. Unlike many other conditions where a scan provides the answer, IIH is primarily diagnosed through specialized blood work and a careful review of your symptoms [1][2]. Understanding the diagnostic process can help you navigate why your doctor might be ordering specific tests at precise times.

The Preferred Initial Assessment: 8 AM Blood Tests

The most important step in evaluating your pituitary function is checking your hormone levels. Because your body’s cortisol levels naturally peak around 8 AM, doctors strongly prefer to draw your blood first thing in the morning [3][4].

  • Serum Cortisol: While cutoffs vary depending on the specific laboratory assay and guidelines, an 8 AM cortisol level below 5 mcg/dL (140 nmol/L) is often a strong indicator of adrenal insufficiency [3].
  • ACTH (Adrenocorticotropic Hormone): This is the signal your pituitary gland sends to your adrenal glands. In IIH, your ACTH will usually be low or “inappropriately normal” (meaning it should be high to compensate for low cortisol, but it isn’t) [5][6].

A Critical Exception: While 8 AM is preferred for diagnosis in a stable patient, if you are acutely ill with an adrenal crisis, a random cortisol test is still highly clinically important. Emergency steroid treatment must NEVER be delayed to wait for a perfect 8 AM blood draw [7].

The “Normal MRI Trap”

One of the most common pitfalls in diagnosing IIH is relying too heavily on imaging. While an MRI is an important tool to rule out other issues—such as cancer spreading to the brain (pituitary metastasis) or severe swelling pressing on the optic nerves—it frequently fails to show any inflammation in patients with IIH [8][9].

Why the MRI Often Looks Normal

  • PD-1/PD-L1 Inhibitors: If you are taking drugs like Nivolumab (Opdivo) or Pembrolizumab (Keytruda), MRI abnormalities are less frequent. In these cases, the immune system often causes damage to specific cells without causing the entire gland to visibly swell [10][11].
  • Timing: Even if swelling does occur (more common with CTLA-4 inhibitors like Ipilimumab), the inflammation may have already subsided by the time the scan is performed, leaving behind a gland that looks normal or even slightly shrunken [11][8].

The Bottom Line: A normal MRI does not rule out hypophysitis. If your blood tests show low hormones and you have symptoms, you still need treatment [10][1].

The Cosyntropin Stimulation Test

If your morning cortisol test is borderline, your doctor may order a cosyntropin (or Synacthen) stimulation test. During this test, you are given a synthetic version of the ACTH hormone to see if your adrenal glands respond by producing cortisol [4].

However, this test has a significant weakness early in IIH:

  • Residual Adrenal Reserve: Your adrenal glands are like a battery. Even if the “charger” (the pituitary gland) breaks, the battery can stay charged and responsive for several weeks [12][13].
  • False Reassurance: If your hypophysitis is very new, your adrenal glands might still be able to respond to the test, giving you a “normal” result even though your pituitary gland is failing [14][15].

Because of this, a normal stimulation test in the first few weeks of symptoms cannot be used to definitively rule out IIH. An endocrinologist will individualize the timing of any repeat testing. Do not stop taking prescribed steroids on your own to wait for a repeat test [12][16].

Evaluating Other Hormones

While cortisol is the most critical hormone to replace, the pituitary gland controls several other systems that your doctor should check:

  • Thyroid Axis: To evaluate central hypothyroidism, doctors monitor Free T4 levels rather than relying solely on TSH (Thyroid Stimulating Hormone). Because the pituitary is malfunctioning, TSH levels are unreliable and cannot be used alone to adjust your medication [17][18].
  • Sex Hormones: The immune attack can also lower testosterone in men or estrogen in women, which contributes to fatigue and low mood [19][20].

If an adrenal crisis is suspected, your medical team should not wait for these test results or an MRI. Life-saving steroid treatment should be started immediately based on your symptoms alone [7][21].

Common questions in this guide

Can immunotherapy-induced hypophysitis be present if my MRI is normal?
Yes. A normal pituitary MRI does not rule out immunotherapy-induced hypophysitis because the condition may damage hormone-producing cells without causing visible swelling, or the inflammation may have settled before imaging. Diagnosis relies heavily on symptoms and hormone tests.
Why are cortisol and ACTH checked early in the morning?
Cortisol normally reaches its daily peak around 8 AM, so an early-morning sample helps clinicians assess adrenal function. In hypophysitis, cortisol may be low and ACTH may be low or not appropriately increased.
Can a normal cosyntropin test miss hypophysitis?
Yes, especially early in the illness. The adrenal glands may still respond to synthetic ACTH for several weeks after the pituitary has stopped sending enough ACTH, so a normal early result does not definitively exclude hypophysitis. An endocrinologist should decide whether and when to repeat testing, and prescribed steroids should not be stopped without medical guidance.
Which thyroid test is most useful when hypophysitis affects the pituitary?
Free T4 is important for assessing the thyroid hormone system because pituitary dysfunction can make TSH unreliable. TSH should not be used by itself to adjust thyroid medication in this situation.
What should happen if an adrenal crisis is suspected?
An adrenal crisis is an emergency that requires immediate steroid treatment based on symptoms and clinical judgment. A random cortisol sample may be useful if it can be obtained promptly, but treatment should never be delayed for an 8 AM blood draw, MRI, or other test.
Could recent prednisone or dexamethasone change my cortisol or ACTH results?
Yes. Steroid medicines can affect the interpretation of cortisol and ACTH tests, so tell your healthcare team which medicine you took, along with the dose and timing. Do not stop prescribed steroids without medical instructions because untreated adrenal insufficiency can be dangerous.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What were my exact 8 AM cortisol and ACTH levels, and how do they compare to the normal range?
  2. 2.Since my MRI was normal, what other evidence are you using to confirm my diagnosis?
  3. 3.If my cosyntropin stimulation test was normal, how are we managing my symptoms while we wait to see if we need a repeat test?
  4. 4.Are we checking my Free T4 to monitor my thyroid axis?
  5. 5.Do my imaging results show any swelling or pressure on the optic nerves?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

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This page is for informational purposes only and does not constitute medical advice. If you may have an adrenal crisis, seek emergency care; your endocrinology and oncology teams should interpret your tests and MRI.

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