Background: Chronic autoimmune thyroiditis (AIT) is a common autoimmune disease that often leads to impaired function of the thyroid gland, increases in incidence with age, and has an 8-9 time female preponderance. Quality of life is often impaired and complaints persist in a considerable number of patients, even after restoration of euthyroidism. The autoimmune component of the disease has been suggested as an explanation for this. Selenium is a micro nutritive essential for human health and the thyroid gland has the highest selenium concentration of all human tissues. Selenoproteins catalyse thyroid hormone metabolism and anti-oxidative processes in thyrocytes. In addition they are important to immune function. In Denmark, patients with AIT have lower blood selenium concentration than the background population. The majority of 13 randomised trials have shown that selenium supplementation decreases serum thyroid peroxidase antibody levels (TPO-Ab) in patients with AIT, when compared with placebo. We hypothesise that adjuvant selenium may be beneficial in the treatment of AIT.