Stuck at the Checkpoint: Pembrolizumab Induced Hypophysitis With ‘Normal’ Cosyntropin Response

Introduction: Pembrolizumab is an immune checkpoint inhibitor (ICI) that blocks programmed cell death receptor 1 (PD-1) and is indicated in treatment of malignancies including lung adenocarcinoma and melanoma. Anti-PD-1 therapies are responsible for immune related adverse events including endocrine dysfunction. Here we present a case of Pembrolizumab induced hypophysitis causing central adrenal insufficiency apparently missed by a 250 mcg cosyntropin test. Clinical Case: A 68 year old female with history of COPD and stage 4 lung adenocarcinoma with bone metastases presented to the ED with hypotension during her 14th cycle of treatment with Pembrolizumab. Other symptoms included fatigue and anorexia for about 4 weeks. She denied vomiting, diarrhea, bleeding episodes or chronic steroid use. On exam, patient was obese and pale appearing, with decreased bilateral breath sounds without edema. Random cortisol on admission at 4:44 pm was 6.9 ug/dl, ACTH - was in process, Na was 135 (137 - 145 mEq/L) and K was 3.7 (3.5 - 5.1 mEq/L). A 250 mcg cosyntropin stimulation test resulted in 30 min cortisol level of 22.5 µg/dL and 60 min cortisol level of 34 µg/dL. Patient was treated with fluids, salt tabs and eventually midodrine, however BP remained borderline low. On day 5, the patient was started on IV methylprednisolone 40 mg Q8 hrs for COPD exacerbation. A few days later pre-steroid ACTH was reported as <5 pg/ml (6-50 pg/ml) which prompted further workup for hypopituitarism. LH and FSH levels were inappropriately low for a postmenopausal female at 0.2 and 2.6 mIU/ml respectively, Free T3 was 2.02 pg/ ml (2.77-5.27 pg/ml), free T4 was 1.07 ng/dl (0.80-2.20 ng/dl), and TSH was 0.67 uIU/ml (0.47-4.70 uIU/ml). MRI brain showed partially empty sella. Patient was diagnosed clinically with Pembrolizumab induced hypophysitis causing central adrenal insufficiency. Eventually, steroids were tapered to prednisone 5 mg daily maintenance dose and patient was discharged with stress dosing instructions. Conclusion: Diagnosis of adrenal insufficiency is challenging as advanced malignancy and adrenal insufficiency can cause similar symptoms. The finding of an empty sella here is a confounding factor. Checkpoint inhibitors are known to cause hypophysitis, thyroiditis and primary adrenal insufficiency, however incidence is <1%. We report a case of missed adrenal insufficiency by a falsely assuring cosyntropin test. Based on our experience, we conclude that when suspecting a diagnosis of checkpoint inhibitor induced adrenal insufficiency, we should start by checking random cortisol and ACTH value. A standard 250 mcg cosyntropin test should not be used solely to completely rule out this diagnosis.