Collateral thyroid damage: A case report of suspected TKI-induced hypothyroidism.

Tyrosine kinase inhibitors (TKIs) are widely used in cancer therapy and are known to induce hypothyroidism in some patients, though the mechanisms remain unclear. We describe a 58-year-old man with metastatic gastrointestinal stromal tumor who developed clinical hypothyroidism during TKI treatment. Imaging revealed thyroid enlargement, and fine-needle aspiration yielded indeterminate cytology. The patient subsequently underwent thyroid lobectomy, which revealed a unique pattern of thyroiditis with multiple hyaline nodules containing entrapped follicular cells and scattered immune cell infiltration. Postoperatively, the patient was in a good clinical condition, but persistently elevated thyroid-stimulating hormone concentrations despite normal T4 concentrations were likely due to pre-existing TKI-induced thyroid dysfunction limiting compensatory function after hemithyroidectomy. The above-mentioned histological presentation is atypical for conventional thyroiditis and may represent a distinct pathological response associated with TKI therapy. The clinical, cytological, and histological findings in this case suggest a potential link between TKIs and thyroid dysfunction, highlighting the need for increased awareness of thyroid complications during targeted therapy. This case underscores the importance of considering iatrogenic causes in new-onset hypothyroidism in oncology patients and suggests that distinct histological features may serve as clues to the underlying pathogenesis.