A Case of Thyrotropin-Secreting Pituitary Carcinoma With Bone Metastases

A 65-year-old man with a history of thyrotropin (TSH)-secreting pituitary adenoma presented with progressive spinal pain 7 years after diagnosis. Laboratory examinations revealed hyperthyroidism with markedly elevated TSH 28.5 mIU/mL (reference range, 0.3-5.0 mIU/mL) and a large pituitary tumor. Despite 3 transsphenoidal surgeries and long-term octreotide therapy, the tumor gradually enlarged. Radioiodine (RAI) therapy using iodine-131 (I-131; hereafter "I-131 therapy") was administered for refractory thyrotoxicosis, after which thyroid hormone levels normalized but TSH rose dramatically (>500 mIU/mL). Subsequently, magnetic resonance imaging (MRI) revealed multiple spinal metastases. Biopsy of a vertebral lesion demonstrated chromophobic adenocarcinoma with positive TSH immunostaining, confirming metastatic TSH-secreting pituitary carcinoma. Palliative spinal irradiation and pamidronate temporarily relieved symptoms, but the patient died of pneumonia 8 years after onset. Autopsy revealed widespread bone metastases without any other primary malignancy. This case illustrates the malignant transformation of a TSH-secreting pituitary adenoma into carcinoma with distant bone metastases and suggests that I-131 therapy may have accelerated tumor progression. Early recognition of central hyperthyroidism and avoidance of I-131 therapy before pituitary-directed therapy are crucial to prevent treatment-related tumor exacerbation.