When “hot” means malignant: Bone-metastatic invasive encapsulated follicular variant of papillary thyroid carcinoma masquerading as autonomously functioning thyroid nodule

ABSTRACT The follicular variant of papillary thyroid carcinoma (FVPTC) is rarely associated with bone metastasis, and the connection with autonomously functioning thyroid nodules (AFTNs) is an uncommon finding. We present the case of a 53-year-old female who had painless swelling in the occipital region that gradually progressed over 3 years. The swelling measured 5 cm × 5 cm, was firm, and was nonpulsatile. Computed tomography scan of the brain revealed a lytic lesion in the posterior skull, raising suspicion of metastasis. On physical examination, she had a palpable right thyroid nodule. Thyroid function tests revealed suppressed thyroid-stimulating hormone with elevated T3 and T4 levels. The Tc-99m thyroid scan showed a hyperfunctioning thyroid nodule (AFTN) in the right thyroid lobe. Ultrasound of the neck revealed a right lobe thyroid nodule categorized as TIRADS 4. Fine-needle aspiration cytology suggested a Bethesda category IV lesion, suggestive of a follicular neoplasm. A whole-body Tc-99m-methylene diphosphonate scan revealed a rim of increased uptake with central photopenia in the occipital lesion, suggestive of metastasis. The patient was started on antithyroid drugs and underwent a total thyroidectomy after achieving euthyroidism. A histopathological examination confirmed the diagnosis of the invasive encapsulated FVPTC. Early use of functional imaging, re-evaluation of suspicious cytology, and awareness of atypical metastatic presentations were crucial for timely diagnosis and treatment.