D-vitamin And Graves' Disease; Morbidity And Relapse Reduction

Inclusion Criteria: * A first time diagnosis of Graves' hyperthyroidism within the last three months, confirmed by TSH below 0.01 IU/L, and T3 or T4 levels above the reference interval necessitating ATD therapy * Positive TRAb * Speak and read Danish * Written informed consent Exclusion Criteria: * Previously diagnosed hyperthyroidism * ATD treatment initiated more than 3 months prior to inclusion * Planned ablative therapy (radioactive iodine or thyroid surgery) * Intake of more than 10 µg D-vitamin/day that the participant wishes to continue. * Chronic granulomatous illness * Persistent hypercalcemia (plasma calcium \> 1.40 mmol/L) * Reduced kidney function (eGFR \< 45 ml/min) * Treatment with immunomodulatory drugs * Active malignant disease * Alcohol or drug abuse * Pregnancy at inclusion * Major comorbidity, making the participant unlikely to continuously receive trial intervention. * Allergy towards the components in the D-vitamin or the placebo pills. * Unable to read and understand Danish * Lack of informed consent.