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.