Effects of block-replace regimen in patients with autoimmune hypothyroidism converted to Graves' disease

Purpose We present two cases of autoimmune hypothyroidism converted to Graves' disease (GD) and their medical management. Methods We tested thyroid function and thyroid antibodies and performed an ophthalmologic examination and neck ultrasound in two patients with autoimmune hypothyroidism converted to GD during a follow-up of several years. Case reports The first case is a 33 year-old woman with hypothyroidism due to Hashimoto's thyroiditis (HT). She developed signs and symptoms of hyperthyroidism after 7 years of treatment with the same dose of levothyroxine (LT4). Even when LT4 therapy was discontinued, she remained thyrotoxic, with mild Graves' ophthalmopathy (GO) and very high thyroid-stimulating hormone receptor antibodies (TRAb > 40 IU/L, reference range: <1.75 IU/L). Antithyroid medication was started on a titration regimen, without achievement of euthyroidism. She was switched to a block and replace regimen, using 20 mg of methimazole (MMI) and 75 mcg of LT4 daily, with normalization of thyroid hormones and improvement of GO without steroids. The second case is a 57 year-old man with a 2-year positive medical history of HT and 6 months of LT4 treatment. He developed hyperthyroidism and moderate-severe GO. Despite stopping LT4 and initiating antithyroid medication in a titration regimen, he did not achieve euthyroidism and had active GO. Pulse glucocorticoid therapy and switching to a block-replace regimen was required to achieve euthyroidism and reduce ocular proptosis and diplopia. Conclusion Spontaneous autoimmune conversion of hypothyroidism to hyperthyroidism can occur at any time: it is important to promptly identify these cases so as to manage them effectively.