Table 1 Eligibility criteria

1. Age 19–75 years of either gender

2. Patients with mild-to-moderate GO with one of the following:

- Confirmation of mild-to-moderate GO by a professional ophthalmologist

- NO SPECS class 2a or 2b (mild or moderate involvement of soft tissue) [61]

- Proptosis of 22 mm or less

- No treatment for GO other than symptom control since diagnosis

3. Patients with Graves’ disease with normal thyroid function as indicated:

- Patients maintaining euthyroid status for at least 2 months with ATDs

- Patients with maintenance of euthyroid status from 6 months after RAI treatment

- If hypothyroidism occurred after Grave’ disease treatment including with ATDs, RAI, or surgery, patients who have euthyroid status under conditions of thyroid hormone replacement

- Patients who have been diagnosed with GO by an ophthalmologist but have not yet developed thyrotoxicosis and who are positive for anti-thyroid receptor antibodies

4. Patients who have voluntarily signed the written consent form

1. Patients with severe GO who meet at least one of the following criteria:

- NO SPECS class 2C (marked involvement of soft tissue)

- Proptosis greater than 22 mm

- Diplopia or ocular torticollis in a resting position

- Optic neuropathy

- Confirmation of severe GO by an ophthalmologist

2. Patients who have been treated with a high dose of steroids or any dose of selenium within 3 months:

- High-dose steroid treatment includes oral methylprednisolone more than 1 g per day or equivalent and intravenous steroids

3. Patients with contraindications for selenium supplementation meeting at least one of the following criteria:

- Hypersensitivity to selenium or a component of the pill

- Selenium toxicity

- Hereditary fructose intolerance, glucose-galactose malabsorption, or deficiency of sucrase or isomaltase

4. Patients with comorbidities or active disease under treatment

5. Pregnant or lactating women