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ICI-Induced Hypothyroidism — RACP Adult Medicine MCQ

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ModerateMedical OncologyICI-Induced HypothyroidismRACP Adult Medicine

A 72-year-old woman with metastatic NSCLC (PD-L1 TPS 80%) on pembrolizumab develops new-onset fatigue, constipation, and cold intolerance 4 months after starting treatment. Her TSH is 45 mIU/L and free T4 is 4 pmol/L. What is the diagnosis and management?

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Correct answer: A — Hypothyroidism from pembrolizumab – commence levothyroxine; pembrolizumab may continue

ICI-induced hypothyroidism is one of the most common immune-related adverse events (irAE), occurring in 5–10% of patients on anti-PD-1 therapy. It often follows a transient thyrotoxic phase (destructive thyroiditis). Management is levothyroxine replacement. Unlike most other irAEs, ICI-induced hypothyroidism does NOT require discontinuation of immunotherapy – pembrolizumab can be continued with thyroid hormone replacement. This is a grade 1–2 endocrine irAE managed with hormone replacement alone.

Reference: ASCO – 2021 – irAE Management Guidelines; eTG – 2025 – Endocrinology