PMON109 A Case of Non-functioning Pituitary Adenoma With Unexpectedly High Prolactin Level

INTRODUCTION: Lactotroph or prolactin (PRL)-secreting adenomas account for approximately 45% of all pituitary adenomas. They are classified by size as microprolactinomas (<10 mm) and macroprolactinomas (≥10 mm). A giant prolactinoma is arbitrarily defined as an adenoma > 4 cm. PRL levels are often proportional to the size of tumor ranging from <200 ng/mL in adenomas smaller than 1 cm, 200 ng/mL to 1000 ng/mL in 1 cm to 2 cm adenomas and more than 1000 ng/ml in tumors larger than 2 cm. Nonfunctioning pituitary adenomas (NFPA) can cause mild increases in PRL levels due to pituitary stalk compression, however, levels more than 6 times the upper limit of normal have not been reported. We present a rare case of NFPA with significantly elevated prolactin levels Case: 42 yo female presented with headache and left eye blurry vision worsening over 2 years. MRI revealed a pituitary macroadenoma measuring 4.1×4.7×3cm with supra-sellar extension, mass effect including severe compression and displacement of the optic apparatus, and potential cavernous sinus invasion. Initial biochemical profile: TSH 1.01 (0.30–4.20 uU/mL), FT4 0.9 (0.6–1.5 ng/dL), AM ACTH of 20 (7.2-20 pg/ml), peak cortisol 18.8 mg/dl after cosyntropin stimulation, FSH 14.6 mIU/mL, LH 2.4 mIU/mL, IGF-1 71 (52-328 ng/mL) and prolactin 452 (<25.0 ng/mL), confirmed by dilution. PRL levels were concerning for prolactinoma but did not correlate with tumor size making giant prolactinoma an unlikely diagnosis. Yet, levels were higher than those expected from stalk compression. An interdisciplinary decision was made to proceed with operative management. The patient underwent trans-sphenoidal pituitary resection (TSPR). Tumor pathology showed sparsely granulated gonadotroph adenoma with weak LH expression and negative PRL staining, confirming NFPA. PRL level one-month post op was 33.2 ng/mL and vision is improved. CONCLUSION: Medical treatment with dopamine agonists is first line treatment for any size prolactinoma with a high remission rate of 90% and prescribed before TSPR even in those with vision disturbance or neurologic symptoms. Giant NFPAs with compressive symptoms are best surgically managed.To our knowledge, this is the first report of significant PRL elevation due to stalk compression. Not only was the PRL higher than that previously associated with stalk compression, but also well above 250 ng/mL, which is typically diagnostic of prolactinoma. This case also highlights the importance of assessing the relationship between PRL level and tumor size to determine the correct course of management. Presentation: Monday, June 13, 2022 12:30 p.m. - 2:30 p.m.