6983 Pembrolizimab Induced Hypophysitis: Uncommon Side Effect Leading To Late Diagnosis

Abstract Disclosure: T. Chaudhary: None. O. Syed: None. A. Shridar: None. R. Kaur: None. O. Chaudhary: None. M.S. Zubair: None. Background: Pembrolizumab, an immune checkpoint inhibitor, has shown efficacy inducing remission in various malignancies. Associated immune-related adverse effects (irAEs) have been recorded but cases of concomitant hypophysitis and thyroiditis are rare. Clinical case: The patient is a 41-year-old male with a past medical history significant for metastatic melanoma of the left calf, status post sentinel lymph node biopsy and left inguinal lymph node dissection, treated with pembrolizumab for one year. He initially presented with a severe headache that was preceded by two months of fatigue and impaired concentration. CT and MRI head were both unremarkable while lab work was significant for cortisol 0.2 ug/dL and adrenocorticotropic hormone (ACTH) 1.1 pmol/L. A full hormone panel was ordered which showed normal insulin-like growth factor-1 (IGF-1), luteinizing hormone (LH), and follicle-stimulating hormone (FSH). Free testosterone was at the lower end of normal. Prolactin was elevated at 27.3 ng/mL. The thyroid function tests showed primary hypothyroidism with elevated thyroid stimulating hormone (TSH) at 109 uIU/mL, low T4 at 1.5 ug/dL, and low free T4 at 0.4 ng/dL. There were no signs or symptoms of posterior pituitary dysfunction. The patient was diagnosed with hypophysitis and started on hydrocortisone 20 mg once daily along with levothyroxine 175 mcg once daily. DEXA scan showed normal bone mineral density. Subsequently, the patient presented to our care and reported symptom improvement after taking the prescribed medication however he continued to develop fatigue by the end of the day. Therefore, hydrocortisone prescription was switched to 15 mg in the morning and 10 mg at noon. A repeat thyroid function panel, sex hormone binding globulin, and LH were ordered, to be followed up at the patient's next visit. The patient was also advised to wear a medical alert bracelet at all times moving forward. Conclusion: Due to non-specific symptoms at onset, hypophysitis can be undiagnosed or misdiagnosed therefore it is important to have a high clinical suspicion with patients on PD-1 inhibitors presenting with such symptoms. It is also imperative to order a full set of hormones as this patient had concomitant primary hypothyroidism. Presentation: 6/1/2024