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ICI Thyroiditis Biphasic — SCE Medical Oncology MCQ

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ModerateGynaecological CancersICI Thyroiditis BiphasicSCE Medical Oncology

A 55-year-old woman with advanced endometrial cancer has pembrolizumab added to carboplatin-paclitaxel (per NRG-GY018 or RUBY protocols). She also takes metformin for type 2 diabetes. She develops immune-related thyroiditis, progressing from thyrotoxicosis to hypothyroidism over 8 weeks. At what point should levothyroxine be started?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: E — Start levothyroxine only when TSH is elevated and fT4 is low (hypothyroid phase) — the thyrotoxic phase is self-limiting and treated symptomatically with beta-blockers; premature levothyroxine during thyrotoxicosis would worsen hyperthyroid symptoms

ICI-induced thyroiditis follows a predictable biphasic course: (1) transient thyrotoxicosis (2-6 weeks, caused by destructive thyroiditis releasing stored hormone — treat symptomatically with beta-blockers, NOT anti-thyroid drugs), followed by (2) permanent hypothyroidism (thyroid gland destroyed — requires lifelong levothyroxine). Starting levothyroxine during the thyrotoxic phase is inappropriate and worsens symptoms. TSH and fT4 monitoring every 2-4 weeks during the transition guides the timing of levothyroxine initiation.

Reference: https://www.nice.org.uk/guidance/conditions-and-diseases/cancer