Purpose: We investigated whether selenium supplementation improves quality-of-life (QoL) in patients with autoimmune thyroiditis (ID:NCT02013479). Methods: We included 412 patients ≥18 years with serum thyroid peroxidase antibody (TPOAb) level ≥100 IU/mL in a multicentre double-blinded randomised clinical trial. The patients were allocated 1:1 to daily supplementation with either 200 μg selenium as selenium-enriched yeast or matching placebo tablets for 12 months, as add-on to levothyroxine (LT4) treatment. QoL, assessed by the Thyroid-related Patient-Reported-Outcome questionnaire (ThyPRO-39), was measured at baseline, after 6 weeks, and after 3, 6, 12, and 18 months. Results: In total, 332 patients (81%) completed the intervention period, of whom 82% were women. Although QoL improved during the trial, no difference in any of the ThyPRO-39 scales was found between the selenium group and the placebo group after 12 months of intervention. In addition, employing linear mixed model regression no difference between the two groups was observed in the ThyPRO-39 composite score (28.8 (95% CI: 24.5–33.6) and 28.0 (24.5–33.1), respectively; P = 0.602). Stratifying the patients according to duration of the disease at inclusion, ThyPRO-39 composite score, TPOAb level, or selenium status at baseline did not significantly change the results. TPOAb levels after 12 months of intervention were lower in the selenium group than in the placebo group (1995 (95% CI: 1512–2512) vs 2344 kIU/L (1862–2951); P = 0.016) but did not influence LT4 dosage or free triiodothyronine–free thyroxine ratio. Conclusion: In hypothyroid patients on LT4 therapy due to autoimmune thyroiditis, daily supplementation with 200 μg selenium or placebo for 12 months improved QoL to the same extent.