Objective: To report on 2 patients with moderate to severe Graves ophthalmopathy (GO) who developed dysthyroid optic neuropathy following levothyroxine (LT4) withdrawal in preparation for 131I treatment for thyroid carcinoma.Methods: Two patients referred to a center for the treatment of thyroid diseases were evaluated.Results: Patient 1, a 55-year-old woman, had active (clinical activity score CAS, 5 out of 7 moderate to severe GO. After LT4 withdrawal, her left eye visual acuity decreased from 10/10 to 1/10, and her omolateral visual field was impaired. Euthyroidism was rapidly restored and GO was treated with intravenous glucocorticoids. Nevertheless, as the patient's visual acuity was still impaired, orbital decompression was performed. Patient 2, a 50-year-old man, had active (CAS, 3 out of 7) moderate to severe GO. After LT4 withdrawal, the patient developed a right dysthyroid optic neuropathy. His visual acuity decreased from 10/10 to 4/10, and his omolateral visual field was impaired. After prompt restoration of euthyroidism and treatment with intravenous glucocorticoids, normalization of his visual acuity was achieved.Conclusion: In moderate to severe GO, dysthyroid optic neuropathy may be precipitated in thyroidectomized patients following LT4 withdrawal, even if triiodothyronine at doses that do not prevent elevated serum thyroid-stimulating hormone concentrations are administered, suggesting that hypothyroidism should be carefully avoided in patients with such grades of GO.Abbreviations: CAS clinical activity score DON dysthyroid optic neuropathy GD Graves disease GO Graves ophthalmopathy LE left eye LT4 levothyroxine RE right eye rhTSH recombinant human thyroid-stimulating hormone T3 triiodothyronine TRAb thyroid-stimulating hormone receptor autoantibody TSH thyroid-stimulating hormone TTA total thyroid ablation