SAT-351 From Silence To Storm: Seroconversion From Hashimoto’S Thyroiditis To Graves’ Disease

Abstract Disclosure: A.J. Zaidi: None. T. Mushtaq: None. S. Zaidi: None. M.F. Siddiqui: None. Background: Hashimoto’s thyroiditis (HT) and Graves’ disease (GD) are opposing autoimmune thyroid disorders: HT causes hypothyroidism via gland destruction in ∼7.5% globally (predominantly women aged 30-50), while GD induces hyperthyroidism via stimulating antibodies in 2-3%of women and 0.5% of men . Although 15-20% of GD cases progress to HT, HT to GD conversions are extremely rare (∼50 cases, median 7 years post-HT in levothyroxine-treated women who develop TSH-receptor antibodies). We present two cases of transition from HT to GD highlighting their clinical course, immunologic shifts and the importance of vigilant monitoring.Case Report:Patient 1, A 32-year-old woman with MAFLD presented with fatigue, weight gain, and cold intolerance; labs confirmed Hypothyroidism due to Hashimoto’s thyroiditis [TSH 195.6µIU/mL(0.45-4.5), FT4 0.63 ng/dL(0.82-1.77), anti-Thyroid peroxidase antibodies (anti TPO Ab) 600IU/ml (<35), anti-Thyroglobulin antibody (anti TG) 653.6 IU/mL (<4) , requiring treatment with levothyroxine with good biochemical response. 1.5 yrs later, she developed clinical and biochemical hyperthyroidism (TSH 0.005, FT4 4.34) requiring gradual discontinuation of levothyroxine and subsequent initial of Methimazole due to elevated TSI. Thyroid ultrasound showed hyper vascularity, confirming diagnosis of Graves’ disease. She showed good response to Methimazole. Patient 2, A 36-year-old man with PMHx of Smoking presented with fatigue, constipation, weight gain and labs suggestive of Hypothyroid due to HT [TSH 170.8 mIU/L(0.45- 4.5), FT4 11ng/dL(0.82-1.77)] and positive TPO Ab 145 IU/mL<35), treated with levothyroxine. 5 years later, he developed symptoms of hyperthyroidism palpitations, heat intolerance, tremors and weight loss, with labs showing TSH 0.53 mIU/L, elevated FT4/T3 and rising TSI titers [peaking at 325IU/mL(0.5) in 2023], indicating transition to GD. Levothyroxine was discontinued and methimazole was initiated and continued till now with gradual titration to lower dose based on biochemical and clinical improvement.Conclusion: These cases highlight a very rare phenomenon of autoimmune seroconversion from thyroid blocking antibodies (TBAb) to thyroid stimulating antibodies (TSAb).One of the Possible mechanisms proposed is Dual presence of autoantibodies with shift in dominance due to multiple risk factors including environmental (intake of excessive iodine, smoking) genetic factors, presence of other autoimmune conditions.Another mechanism emphasizes the recovery of damaged thyroid tissue while simultaneously being stimulated by TSAb. There are atleast 50 previous reported cases of such conversion, with variable conversion time averaging 7 years. Clinicians must be aware of this phenomenon especially in patients with HT who develop symptoms of hyperthyroidism or have difficulty in controlling Thyroid levels. Presentation: Saturday, July 12, 2025