FRI370 Hyperthyroidism And Acromegaly In A Patient With A Mature Plurihormonal PIT1 Lineage PitNET/Adenoma

Disclosure: D. Slack: None. A.C. Levine: None. Background: TSH- and GH-secreting pituitary neuroendocrine tumors (PitNETs) are both rare entities, with an incidence of 1 per million people and 1-4 per million people, respectively. Thyrotroph, somatotroph, and lactotroph cells have the same progenitor, regulated by the PIT1 transcription factor. Thus, it is not unusual for PIT1 positive tumors to exhibit immunhistochemical expression of TSH, GH, and prolactin. It is vanishingly rare, however, for these tumors to secrete multiple biologically active hormones with apparent clinical sequelae. We herein report on a case of a patient with a plurihormonal PIT1 lineage PitNET secreting prolactin with secondary amenorrhea, TSH causing central hyperthyroidism and GH resulting in acromegaly. Clinical Case: A 42-year-old woman with a 1-year history of amenorrhea presented with 3 months of progressive weight loss, fatigue, palpitations, and tremulousness. She was found to have central hyperthyroidism, with an elevated free T4 (>7.77 ng/dL, nl 0.82 – 1.77) and inappropriately normal TSH (3.25 uIU/mL, nl 0.45 – 4.5). Full endocrine workup confirmed central hyperthyroidism with elevated free T4 (5.02 ng/dL) and TSH (4.68 uIU/mL) and also demonstrated hyperprolactinemia (122 ng/mL, nl 4.8 – 23.3 ng/mL), elevated IGF-1 (795 ng/mL, nl 91 – 308 ng/mL), GH (19 ng/mL, nl 0 – 10) and free alpha subunit (28 ng/mL, nl <1.02). Over the ensuing 4 months, she noted enlargement of her lips, nose, feet, and hands. MRI demonstrated a pituitary macroadenoma measuring 2.2 cm x 2.7 cm x 2 cm with asymmetric extension into the left cavernous sinus and mild elevation of the adjacent optic chiasm. She underwent an uncomplicated transsphenoidal resection of the tumor with no residual tumor noted on post-op MRI. Discharge medications were hydrocortisone 10 mg AM and, 5 mg early afternoon and levothyroxine 50 mcg daily. One month postoperatively, labs were notable for a low TSH (0.16 mIU/L), normal free T4 (1.62 ng/dL), down-trending IGF-1 (339 ng/mL), and normal prolactin (15.1 ng/mL). Additionally, AM cortisol and ACTH levels remained normal after holding her morning dose of hydrocortisone. She reported resolution of all presenting symptoms of thyrotoxicosis as well as decreasing size of her hands, feet, and nose. Pathology revealed a mature plurihormonal PIT1-lineage PitNET, with immunoreactivity for PIT1, GH, TSH, and prolactin and MIB-1 index of 3%. Conclusion: This case demonstrates the potential for PitNETs to secrete multiple biologically active pituitary hormones, including prolactin, TSH and GH, highlighting the importance of maintaining an index of suspicion for pituitary causes in the evaluation of thyrotoxicosis and of considering the plurihormonal potential of PitNETs in patients presenting with multiple endocrinopathies. Presentation: Friday, June 16, 2023