SUN-331 - An Unusual Case of Grave'S Disease Presenting in A Patient with Stiff Person Syndrome

Introduction
Stiff Person Syndrome (SPS) is a rare condition associated with hyperreflexia, muscle spasms, seizures, and cerebellar ataxia. The pathophysiology of this condition has not been fully established, but is theorized to be driven via antibodies directed at glutamic acid decarboxylase (GAD). Some patients with SPS have other autoimmune conditions, such as pernicious anemia. This is known as autoimmune clustering.

Graves’ disease (GD) is an autoimmune disorder that may be also associated with autoimmune clustering. Lab findings indicate low TSH and elevated T4. The presence of thyroid stimulating immunoglobulins (TSI) drives the production of excess thyroid hormone. There is limited literature that establishes an association between SPS and thyroid disease. However, given the subsequent development of GD in a patient diagnosed with SPS, we aim to explore the potential role of IVIG in the emergence of GD in a 60 year old female patient with SPS.

Case Presentation
60 year old female with pertinent history of stiff person syndrome, Type 2 DM, presenting for hair loss, chronic diarrhea, and weight loss. The patient sees neurology for SPS with a positive GAD level of 417. She was initially treated with Diazepam and Baclofen, however, her ataxia worsened in 2025. IVIG therapy was initiated with resulting improvement of her symptoms. A few months later, she came in with weight loss, tachycardia and fatigue, TSH was < 0.01 with a T4 of 7.9 and T3 of 7.8. TSI was 192. Thyroid screening from 1 year prior was unremarkable. Ultrasound demonstrated a heterogenous thyroid without distinct nodules, suggesting thyroiditis. She was diagnosed with Grave’s Disease and was initiated on methimazole. Repeat thyroid levels were improved.

Discussion
Autoimmune clustering may explain the development of the patient’s symptoms. Information about SPS with GD is extremely rare. Prior data suggests treatment, such as plasma exchange or immunotherapy, for the underlying SPS may improve symptoms of GD(2). However, while our patient's neurological symptoms improved with IVIG therapy, she only developed her symptoms of GD and altered thyroid levels after initiating this treatment.

Conclusion:
It may be that SPS, or its treatment, contributed to the development of GD in our 60-year-old female patient through autoimmune clustering. Despite the safety of IVIG therapy, given the spontaneous emergence of GD in our patient, a paradoxical immune activation may have led to the emergence of GD. Future research would be beneficial as SPS is a rare condition, and their is limited data to correlate its treatment with the development of GD.

1.Bose S, Jacob S. Stiff-person syndrome. Pract Neurol. 2025;25(1):6-17. Published 2025 Jan 16. doi:10.1136/pn-2023-003974
2.Rayan MN, Alshawabkeh Y, O’Donnell B. Stiff person syndrome and Graves’ disease: a rare autoimmune overlap with therapeutic insights. Open J Clin Med Case Rep. 2024;2371.

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