The goal of this clinical trial is to compare two methods of guiding methimazole therapy in patients with Graves' disease: methimazole dose adjustments based on a new semi-automated computer-guided treatment (based on a mathematical model) or based on the treating physician's decision (i.e. usual care). The main question it aims to answer is whether semi-automated computer-guided treatment is not inferior to usual care in terms of safety, the time it takes to achieve euthyroidism and the methimazole dose required.

Inclusion Criteria:

• Active Graves' hyperthyroidism [Thyroid-stimulating hormone <0.1 mU/L, elevated free thyroxine levels (fT4) above the upper limit of normal and positive Thyrotropin receptor antibody (TRAb) according to local laboratory results], measured within the last month prior to the inclusion date
• Patients not yet receiving antithyroid treatment, or having received antithyroid treatment continuously for less than 4 weeks, or relapse or recurrence of Graves' hyperthyroidism defined as patients previously having received and discontinued treatment with antithyroid drugs for at least 4 weeks)
• Age 18 years or older
• Provision of written informed consent

Exclusion Criteria:

• Previous treatment with radioactive iodine
• Ongoing antithyroid treatment for more than one month
• Pregnancy
• Treatment with propylthiouracil