MON-594 Alternating Hypothyroidism and Hyperthyroidism Complicated by Thyroid Eye Disease

Abstract A.M. Chen: None. B.G. Barnett: None. Conversion of hypothyroidism to hyperthyroidism, as well as alternating hypothyroidism and hyperthyroidism, is a rarely described phenomenon believed to be largely mediated by alterations in thyrotropin receptor autoantibodies. A 70-year-old Caucasian woman with a history of glaucoma was diagnosed with primary hypothyroidism in 2004 and had remained biochemically euthyroid on thyroid hormone replacement therapy, taking a combination of daily levothyroxine 100 mcg and desiccated thyroid extract (DTE) 15 mg. In January 2018, she was found to have biochemical thyrotoxicosis on routine thyroid testing. Accordingly, her dose of DTE was stopped by her primary care provider. Her dose of levothyroxine was sequentially decreased and eventually stopped in September 2018. She reported having an upper respiratory tract infection a few weeks before this conversion in thyroid function was observed. She was referred to endocrinology that November, and was noted to have fatigue, hair loss, red and dry eyes, and double vision requiring ophthalmology evaluation. Her home medications included estradiol patch, multivitamin, and cetirizine. Mild proptosis of her left eye was observed. Labs showed subclinical hyperthyroidism and positive thyroid antibody assays: TSH <0.01 mIU/L, free T4 1.32 ng/dL, total T3 115 ng/dL, TSI 374 % basal activity, TBII 4.73 IU/L, and TPO antibody 39 IU/mL. Given her symptoms, the patient was initiated on methimazole 5 mg daily. Repeat labs two months later revealed she had become overtly hypothyroid again. Methimazole was discontinued. A period of biochemical euthyroidism ensued, until she became overtly hypothyroid again in October 2019 and levothyroxine was restarted at 50 mcg daily. Her disease course was complicated by severe thyroid eye disease (TED) with left optic neuropathy and restrictive strabismus. Between 2019 and 2020, her TED was treated with a course of IV methylprednisolone, followed by left 3-wall orbital decompression and subsequent left inferior rectus recession. Of note, she was diagnosed with breast cancer in 2021 and is now status post mastectomy and on anastrozole therapy. The patient has remained biochemically euthyroid on levothyroxine 50 mcg daily since October 2019, based on her age-adjusted TSH reference range. Her TED Clinical Activity Score has improved to 0 from initial pre-treatment score of 2. This case illustrates the potential for a patient with autoimmune hypothyroidism to convert to and from a hyperthyroid state, even after more than a decade interval. It is presumed that she had a predominance of TSH receptor blocking antibodies that caused her previous hypothyroidism, and then shifted to a predominance of TSH receptor stimulating antibodies causing thyrotoxicosis. Clinician awareness and further research into this disease entity is crucial to elucidate its pathogenic basis and identify risk factors and potential treatment options. Monday, June 3, 2024