SNUH Immune Checkpoint Inhibitor-induced Hypothyroidism Recovery Trial

The goal of this study is to find out whether hypothyroidism caused by immune checkpoint inhibitors (ICIs) can recover after stopping the ICIs. The study also aims to identify factors that can help predict which patients will be able to stop taking thyroid hormone replacement. The main questions the study will answer are: * What percentage of patients recover normal thyroid function and can stop levothyroxine after stopping ICIs? * What clinical or laboratory factors can predict successful withdrawal of levothyroxine? Participants will: * Be adult cancer patients who developed hypothyroidism during ICI treatment and are currently taking levothyroxine * Have already stopped ICI therapy * Gradually reduce their levothyroxine dose every 3 months if their thyroid function remains normal * Stop levothyroxine if thyroid function remains normal at low doses (≤0.025 mg) * Be monitored with thyroid function tests and clinical symptoms at each visit