MON-336 - Delayed Immune-Mediated Thyroiditis Presenting with Severe Hypothyroidism after Cemiplimab Therapy

Delayed Immune-Mediated Thyroiditis Presenting With Severe Hypothyroidism After Cemiplimab Therapy

Background
Immune checkpoint inhibitors (ICIs), including programmed cell death protein-1 (PD-1) inhibitors, are associated with immune-related adverse events (irAEs), most commonly affecting the thyroid gland. Thyroid dysfunction typically occurs within the first few months of therapy; however, delayed presentations after treatment discontinuation are increasingly recognized. We describe a case of severe hypothyroidism due to delayed immune-mediated thyroiditis occurring months after cessation of cemiplimab.

Case Presentation
A 71-year-old male with basal cell carcinoma of the right forearm treated with cemiplimab (Libtayo) for 20 months presented with several weeks of progressive shortness of breath, fatigue, and alternating diarrhea and constipation. He was referred to the emergency department after outpatient laboratory testing revealed a markedly elevated thyroid-stimulating hormone (TSH) level of 300 µIU/mL with undetectable free thyroxine (FT4).
During active immunotherapy, thyroid function tests had been within normal limits (TSH 4.8 µIU/mL). Notably, the patient had discontinued cemiplimab eight months prior to presentation. He was admitted for further evaluation and management. Serial testing demonstrated profound hypothyroidism with gradual biochemical improvement following initiation of thyroid hormone replacement (TSH 303 → 274 → 56.4 → 7.4 µIU/mL).
Based on the clinical course, prior immune checkpoint inhibitor exposure, and absence of alternative etiologies, a diagnosis of delayed immune-mediated thyroiditis was made.
Management and Outcome
The patient was treated with thyroid hormone replacement and a prolonged corticosteroid taper over six weeks. His symptoms improved significantly, and follow-up with outpatient endocrinology demonstrated continued biochemical recovery with TSH improving to 7 µIU/mL.

Conclusion
This case highlights delayed immune-mediated thyroiditis as a rare but important complication of PD-1 inhibitor therapy that may occur months after treatment discontinuation. Long-term monitoring of thyroid function is essential in patients exposed to immune checkpoint inhibitors.

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