12452 Unraveling An Intriguing Clinical Sequence: Thyrotoxicosis Antecedent To Stiff Person Syndrome

Abstract Disclosure: A. Jain: None. M.N. Rayan: None. Background: Stiff Person Syndrome (SPS) is an autoimmune phenomenon resulting in the progressive rigidity of limbs and trunk muscles. It is associated with GAD-positive antibodies in the CSF and serum and is linked to Type 1 Diabetes, Celiac disease, and other autoimmune diseases. Clinical Case: A 34-year-old female with no past medical history was brought into the ED by family for paranoia, suicidal ideations, and visual and auditory hallucinations. She was found to be tachycardic and altered in the ED. Endocrinology was initially consulted for concerns of hyperthyroid pathology due to lab work showing a TSH of <0.008 uIU/mL and free T4 of 4 pg/mL. Thyroid ultrasound revealed heterogeneous, hypervascular thyroid tissue. Further investigation revealed TRAB 4.05 IU/L (n 0-0.9 IU/L) and TSI 3 IU/L (n < 0.577 IU/L). A diagnosis of Graves’ disease was made, and she was started on methimazole 20mg daily and metoprolol 25mg daily. Due to a lack of response to methimazole, other organic causes were investigated including psychiatric and neurological etiology. The patient's symptoms continued to worsen, with new tremors of the upper and lower extremities. Thyroid labs showed improvement, and hence thyrotoxicosis was not thought to be the underlying etiology of her encephalopathy. Neurology was consulted, and EEG and MRI Brain were performed which were unrevealing. Lumbar puncture revealed elevated oligoclonal bands and GAD-65 antibodies, suggesting autoinflammatory encephalitis. High-dose steroids were started, with no improvement. Due to the failure of steroids to treat her symptoms, she was given 5 days of plasmapheresis and a dose of rituximab. Slowly, the patient began to improve and could follow commands and communicate with staff. A formal diagnosis of SPS was made at this time. Upon seeing her in follow-up 1 month after discharge, her TRAB and TSI antibodies were negative, and she was able to be titrated off of methimazole. Conclusion: SPS is strongly associated with autoimmune diseases, with up to 30-40% having T1DM. The association between hyperthyroidism and SPS, however, is exceedingly rare and the etiology remains unclear. Presentation: 6/1/2024