A Rare Phenomenon of Hashimotos Thyroiditis Converting to Graves Disease

Introduction: Hashimotos thyroiditis and Graves disease are two distinct autoimmune disorders of the thyroid. Conversion of hypothyroidism to hyperthyroidism is even more rare. We report a case of an established Hashimotos thyoiditis patient who converted into Graves disease. Case Description: 67-year old female with a past medical history of iron-deficiency anemia, dyslipidemia, and depression presented with a six-month history of fatigue, cold-intolerance, hair loss, and weight gain in September of 2015. Laboratory tests confirmed diagnosis of Hashimotos thyroiditis with an elevated TSH 80.7 (0.40-4.50 mIU/L), FT4 0.2 (0.8-1.8 ng/dL), and positive thyroid antibodies TPO 24 (0.0-8.9 IU/mL). She was started on Levothyroxine 88 mcg daily. Gradually she had a decreased requirement of Levothyroxine; from February 2016 to March 2017 she maintained a normal TSH range while on 50 mcg/day with resolution of her symptoms. The patient was then lost to follow-up until she presented in the clinic in September 2018 with complaints of several weeks of easy fatigability, 10lb-weight loss, and periorbital edema. She was found to have a suppressed TSH 0.01, and elevated FT4 2.3, and FT3 8.4 (2.3-4.2 pg/mL). Her Levothyroxine 50 mcg/day was discontinued for four days and labs were repeated which still showed suppressed TSH and elevated FT4 and FT3. She was found to have a positive TRAB and a positive TSI which are consistent with hyperthyroidism. Thyroid ultrasound was performed which showed a heterogeneous thyroid gland with increased vascularity, confirming the diagnosis of Graves disease. She was started on Methimazole 10 mg daily. Her Methimazole dose was adjusted according to her thyroid function test until she had a total thyroidectomy in October 2019. She was started on levothyroxine post-operatively and as of March 2020 is on Levothyroxine 50 mcg/daily. Conclusion: Despite the rarity of Hashimotos thyroiditis converting to Graves disease, it is possible that those affected can be encountered by primary care providers and hospitalists and could easily be mistaken for over-replacement of levothyroxine. Close monitoring of the patient along with regular thyroid function tests will be required for ongoing follow-up.