New-Onset Graves' Disease in the Background of Hashimoto's Thyroiditis: Spectrums of the Same Disease With Changing Autoantibodies.

Hashimoto's thyroiditis and Graves' disease represent two spectrums of the same autoimmune thyroiditis. Evolution from Graves' disease to Hashimoto's thyroiditis is a common scenario but conversion is a rare occurrence that we observed in the presented case of a 56-year-old Hispanic female with a history of Hashimoto's thyroiditis with positive anti-thyroperoxidase (TPO) antibodies who was euthyroid on levothbefore75 µg for six years prior to her presentation to the emergency department (ED) with complaints of palpitations, exertional dyspnea, and unintentional weight loss of 20 lbs for four months. The patient denied any recent change in levothyroxine dosage. The patient was noted to have tachycardia at rest, hyperdynamic circulation, and fine tremors on outstretched hands-on examination but no exophthalmos, thyromegaly, or pedal swellings. Her initial laboratory workup revealed thyroid-stimulating hormone of <0.01 mIU/L, with elevated free T3 and free T4 levels. Levothyroxine was held and beta-blockade therapy was started for symptom control. Further workup showed elevated thyroid-stimulating immunoglobulin and thyroid receptor antibody levels and normalization of anti-TPO antibody levels. The radioactive iodine uptake scan was initially delayed because the patient underwent a pulmonary angiogram in the ED. A later scan showed thyromegaly with heterogeneous uptake of 82% in both lobes. Hence, the patient was diagnosed with Graves' disease and managed with radioactive iodine ablation therapy. On follow-up, the patient developed post-ablation hypothyroidism; she was started back on levothyroxine therapy and became euthyroid. This case highlights that patients can develop Graves' disease in the background of a hypothyroid state, and this conversion might be postulated secondary to a combination of atypical destructive thyroiditis and a switch of autoantibodies from blocking to stimulating ones. Clinicians should suspect the possibility of changing antibodies when there is a change in the patient's euthyroid state.