Abstract Disclosure: J.M. Ramos: None. Female patient, 36 years old, presented to our service complaining of a headache lasting five weeks, mainly in the frontal and left periorbital region, radiating to the occipital region. The pain was intense and intermittent, lasting about 40 minutes. No nausea or vomiting was reported, but the patient experienced dizziness and double vision. During the symptomatic interview, the patient mentioned increased thirst and urinary volume. In her medical history, she denied comorbidities, with only frequent episodes of tension-type headaches. She had no continuous medication use and no known drug allergies. The patient had undergone two cesarean sections, with the last one in December 2021. On physical examination, she had horizontal binocular double vision in all three gazes to the left, accompanied by pain during eye movement. Increased facial sensitivity on the left in branches V1 and V2 was observed. The rest of the physical examination showed no abnormalities. Initial general laboratory tests were normal. Evaluation of pituitary function was mostly normal, except for slightly elevated prolactin. A cranial MRI revealed an expansive lesion with sellar and suprasellar components measuring 1.2x1.1x1.2cm. Additional imaging of the sella turcica showed an intra- and suprasellar expansive lesion, predominantly isointense on T1, hyperintense on T2, with discreetly heterogeneous enhancement with gadolinium. In consultation with the neuroradiology team, a hypointensity on T2 at the left margin of the pituitary was noted, suggesting an inflammatory etiology. Based on this, laboratory tests were conducted, and treatment with methylprednisolone 1g per day for 7 days was initiated. After treatment completion, the IgG4 level was found to be 146.7 mg/dL (upper limit of reference: 86 mg/dL). A follow-up MRI, five days after the steroid pulse therapy, showed a reduction in lesion size to 0.8 cm in the cranio-caudal dimension, with a preserved appearance of the optic chiasm. The patient experienced considerable symptom improvement and was discharged with oral corticosteroids and a scheduled initiation of rituximab. The patient meets criteria confirming the diagnosis of IgG4-related hypophysitis, a rare condition representing 30% of hypophysitis cases. Isolated pituitary involvement is even rarer, and its association with diabetes insipidus has been described. Treatment involves corticosteroids, and rituximab can be considered, especially in recurrent cases. Presentation: 6/3/2024