Probable Hypophysitis Recognized More Than Two Years After Pembrolizumab Discontinuation

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Immune checkpoint inhibitor-associated hypophysitis is a rare endocrine complication characterized by inflammation of the pituitary gland during cancer immunotherapy. A recent case published in Cureus details a 70-year-old male who developed probable hypophysitis over two years after discontinuing pembrolizumab treatment for non-small cell lung cancer, highlighting the need for long-term clinical vigilance.

This case pushes the boundaries of standard clinical timelines, proving that immune-related adverse events can manifest long after a patient has stopped receiving their cancer therapy. For treating physicians, oncologists, and endocrinologists worldwide, recognizing delayed-onset toxicities is critical to preventing misdiagnosis and ensuring timely hormone replacement therapy.

In Plain English: The Clinical Takeaway

• Delayed Autoimmune Risks: Severe inflammation of the pituitary gland—known as hypophysitis—can emerge more than two years after a patient finishes pembrolizumab treatment.
• Vital Hormonal Testing: Persistent fatigue, confusion, or low blood pressure following immunotherapy demands a comprehensive panel evaluating adrenocorticotropic hormone (ACTH), cortisol, and thyroid markers.
• Interdisciplinary Management: Affected patients typically require lifelong hormone replacement therapy to manage secondary adrenal insufficiency and related endocrine deficiencies.

Deciphering Late-Onset Pembrolizumab-Induced Hypophysitis

The patient, a 70-year-old man with a documented medical history of non-small cell lung cancer, had successfully completed roughly two years of pembrolizumab therapy. His final dose was administered approximately 28 months prior to his eventual hospital presentation. He initially presented with progressive fatigue, anorexia, hypotension, functional decline, and confusion, leading to an initial hospitalization for multifocal pneumonia and septic shock.

Although the acute infection and hemodynamic instability resolved with standard interventions, severe neurological and physical symptoms persisted. Encephalopathy, profound weakness, poor oral intake, and dysphagia remained uncorrected. Subsequent neuroimaging revealed significant clinical findings. Brain magnetic resonance imaging demonstrated new pituitary enlargement compared to scans obtained roughly three months earlier. Dedicated pituitary imaging verified an enlargement measuring approximately 1.5 by 2.0 cm, with no definitive differential enhancement pattern that would indicate a classic pituitary adenoma.

Endocrine evaluations confirmed severe physiological deficits. Laboratory testing revealed a serum cortisol level of 3.3 micrograms per deciliter, paired with an inappropriately low adrenocorticotropic hormone level of 6.5 picograms per milliliter. Free thyroxine registered at 0.36 nanograms per deciliter alongside a thyroid-stimulating hormone level of 2.0 microunits per milliliter, accompanied by suppressed gonadotropins and testosterone.

Understanding the Mechanism of Action and Immune-Related Adverse Events

Pembrolizumab is a monoclonal antibody functioning as an immune checkpoint inhibitor. Its primary mechanism of action involves blocking the programmed cell death protein 1 (PD-1) pathway. By inhibiting this receptor, pembrolizumab prevents cancer cells from evading T-cell-mediated destruction, thereby restoring antitumor immunity.

Clinical and Laboratory Profile of Late-Onset Hypophysitis

Parameter	Patient Finding	Clinical Significance
Time Since Last Dose	~28 Months	Illustrates exceptionally delayed toxicities
Pituitary Dimensions	1.5 x 2.0 cm	Indicates pronounced gland enlargement on MRI
Serum Cortisol	3.3 µg/dL	Reflects secondary adrenal insufficiency
ACTH Level	6.5 pg/mL	Inappropriately low relative to low cortisol

Future Trajectory in Oncological Endocrinology

As immunotherapy continues to dominate cancer treatment paradigms, the medical community faces an expanding spectrum of delayed autoimmune complications. Documenting outliers where adverse events surface over two years post-discontinuation reshapes follow-up guidelines. Future longitudinal studies will need to address whether subclinical immune memory or persistent tissue-resident T-cells drive these exceptionally late manifestations, ensuring survivorship care keeps pace with pharmacological innovation.