MON-356 When Thyroiditis Meets Grave's: Untangling the Mystery of Atypical Hyperthyroidism

Abstract Disclosure: A. Mohammad: None. N.N. Gamarra: None. J. Doad: None. F. Abdo: None. L. Trancito: None. A. Sood: None. Introduction: Subacute thyroiditis (SAT) is a self-limited inflammatory thyroid disorder, often triggered by viral infections. It typically presents with neck pain, tenderness, and transient thyrotoxicosis. Graves’ disease, an autoimmune hyperthyroidism caused by thyroid-stimulating immunoglobulins, is characterized by persistent hyperthyroidism with hallmark features such as diffuse goiter and orbitopathy. The coexistence of SAT and Graves’ disease is rare, complicating diagnosis and management. This case highlights the challenges of hyperthyroidism in a patient lacking classic features of either condition.Case Presentation A 50-year-old woman with cutaneous T-cell non-Hodgkin lymphoma (in remission, treated with mogamulizumab since 2020, currently on weekly therapy), untreated subclinical hyperthyroidism, and hypertension presented with fatigue, weakness, and a viral illness 8 weeks prior.Initial evaluation revealed undetectable TSH (<0.01 mIU/L), elevated free T4 (FT4) >6.0 ng/dL, and elevated C-reactive protein (CRP) at 6.80 mg/dL, consistent with thyrotoxicosis. Repeat thyroid function tests (TFTs) over several days showed spontaneous improvement without intervention. FT4 levels decreased from 3.01 ng/dL to 2.51 ng/dL, while TSH rose slightly from <0.01 to 0.01 mIU/L.Further workup revealed mildly elevated thyroid peroxidase (TPO) antibodies (41 IU/mL) and markedly elevated TSH receptor antibodies (TRAbs) >40 IU/mL. Thyroid ultrasound showed a heterogeneous gland without increased vascularity or nodules. The absence of diffuse hypervascularity on Doppler ultrasound and rapid biochemical improvement supported SAT. However, the elevated TRAbs suggested underlying Graves’ disease, prompting methimazole initiation. Discussion Distinguishing SAT from Graves’ disease requires careful integration of clinical, laboratory, and imaging findings: SAT: Low iodine uptake, elevated inflammatory markers (e.g., CRP), preceding viral illness, and normal or decreased vascularity on Doppler ultrasound.Graves’ disease: High iodine uptake, positive TRAbs, and diffusely increased vascularity on Doppler ultrasound. In this patient, elevated CRP, recent viral illness, normal thyroid vascularity, and rapid TFT improvement supported SAT. The absence of thyroid tenderness, however, complicated the diagnosis. Elevated TRAbs confirmed Graves’ disease, necessitating methimazole. This coexistence of SAT-like inflammation and Graves’ disease posed a unique diagnostic and therapeutic challenge. Conclusion This case underscores the diagnostic complexity of hyperthyroidism when typical features of SAT and Graves’ disease are absent or atypical. Individualized evaluation is crucial to refine management strategies in cases with overlapping features. Presentation: Monday, July 14, 2025