The goal of this observational study is to determine the role of TSI, as well as clinical signs and thyroid function tests in predicting Graves' disease (GD) relapse after withdrawing anti thyroid drug (ATD). The main questions it aims to answer are:

1. To investigate the serum TSI concentration in patients with GD undergoing maintenance-dose ATD.
2. To determine an optimal cut-off of TSI level for predicting GD relapse.
3. To determine the role of TSI in predicting Graves' disease relapse after withdrawing ATD.

Retrospective follow-up study of patients with hyperthyroidism due to Graves' disease, treated at the endocrine outpatient clinic of Medic Medical Center, Ho Chi Minh City from January 2000 to April 2021. TSI was measured several times during the course of treatment when planning to stop medication (at the discretion of the attending physician). ATD withdrawal would be planned when patients achieved euthyroid status clinically with normal FT4 tests for at least 3 months with minimal dose of ATD. The decision was also based on TSI concentration, goiter's characteristics and parenchymal vascularity on Doppler ultrasound.

Inclusion Criteria:

• Patients diagnosed with hyperthyroidism due to Graves' disease according to the criteria of the Japan Thyroid Association.
• TSI: measured during the course of treatment when planning to stop medication (normal FT4 at maintenance-dose of ATD) Methimazole (MMI): at doses of ≤10 mg. Propylthiouracil (PTU): at doses of ≤200 mg.

Exclusion Criteria:

• Hyperthyroidism due to other causes
• Patients who were intolerant to ATD or had serious side effects with ATD.
• TSIs were measured in pregnancy.
• Patients treated with surgery or radioactive iodine.

patients with hyperthyroidism due to Graves' disease, treated at the endocrine outpatient clinic of Medic Medical Center, Ho Chi Minh City, Vietnam from January 2000 to April 2021.