SAT-355 - When the Pituitary Mass Isn't an Adenoma: A Rare Case of Papillary Thyroid Carcinoma Metastasis

Pituitary metastases are rare, comprising 0.5-1% of pituitary masses. Breast and lung cancers are the most common sources, followed by renal cell carcinoma. Thyroid carcinoma metastasis to the pituitary is exceedingly rare, with follicular histology more commonly reported than papillary. Pituitary metastases frequently mimic adenomas clinically and radiographically, with up to 18% initially misdiagnosed. We present a rare case of metastatic papillary thyroid carcinoma to the pituitary.

A 65-year-old woman with stage IVb metastatic papillary thyroid carcinoma to lungs and bones, diagnosed in 2021, underwent total thyroidectomy, radioactive iodine therapy (272 mCi), and palliative radiotherapy (3,000 cGy in 10 fractions) for spinal metastases. She had a thyroglobulin of 16,068.9 ng/mL on initial presentation. She presented in 2024 with progressive blurry vision. MRI revealed a 1.0×1.2×1.8 cm enhancing sellar and suprasellar mass, not seen on prior MRI in 2022, compressing the optic chiasm and displacing the normal pituitary. Pituitary function testing revealed inappropriately low LH (4.6 mIU/mL) and FSH (14 mIU/mL), suppressed TSH (0.008 mcIU/mL) and elevated free T4 (2.03 ng/dL) from levothyroxine, normal ACTH (17.6 pg/mL) and cortisol (12.6 mcg/dL), and slightly elevated prolactin (36 ng/mL) from stalk effect. She underwent transsphenoidal resection; histopathology revealed papillary architecture. Immunohistochemistry was negative for all pituitary markers but strongly positive for thyroid transcription factor-1 and thyroglobulin, confirming metastatic papillary thyroid carcinoma. Molecular profiling revealed EIF1AX, KRAS Q61R, and TERT promoter mutations, a high-risk combination associated with aggressive behavior and radioiodine refractoriness. Following resection, LH, FSH, and prolactin normalized (FSH 5.6 mIU/mL, LH 1.4 mIU/mL, prolactin 13 ng/mL). She received pituitary radiation (3,250 cGy in 13 fractions), with MRI showing interval mass reduction. Cortisol remained normal following treatment. She was initiated on lenvatinib 20 mg daily for progressive radioiodine-refractory disease and developed fatigue, oral ulcers, and hand-foot syndrome; the dose was reduced to 10 mg daily. She maintains stable disease on imaging and thyroglobulin levels. Thyroglobulin antibodies remained negative throughout.

In patients with known malignancy presenting with sellar masses, metastatic disease should be considered, even when the primary tumor is uncommon. Distinguishing metastases from adenomas can be challenging due to overlapping features. Imaging features favoring metastasis include rapid tumor growth on serial imaging, pituitary stalk thickening, cavernous sinus invasion, and isointensity to brain on T1- and T2-weighted sequences. Histopathologic evaluation with immunohistochemistry remains the most reliable diagnostic modality.

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