previous diagnosis of toxic nodular goitre , Graves ' hyperthyroidism , postpartum thyroiditis , Graves ' orbitopathy ; previous antithyroid drug treatment , radioiodine therapy or thyroid surgery ; immune-modulatory or other medication affecting thyroid function ; pregnancy , planned pregnancy or breastfeeding ; allergy towards any intervention or placebo component ; intake of selenium supplementation > 55 g/day ; inability to read or understand Danish or lack of informed consent .