The rationale for selenium supplementation in patients with autoimmune thyroiditis, according to the current state of knowledge.

Selenium (Se) supplements are commonly prescribed to autoimmune thyroiditis (AIT) patients by European endocrinologists, despite the lack of official guidelines. The majority of Europe is depleted of natural Se sources, and the daily population intake does not comply with recommended values. Optimal individual plasma Se concentration is reached when the selenoproteins (selenoprotein P, glutathione peroxidase) are fully saturated. However, Se intake has to be regulated because both Se shortage and overdose negatively impact health. In the case of AIT, Se may alleviate symptoms or prevent progression to hypothyroidism and postpartum hypothyroidism. Se supplementation in euthyroid, subclinical, or overt hypothyroid AIT patients decreased thyroid autoantibodies, lowered or maintained the TSH level, decreased the fT4/fT3 ratio, reduced the body's oxidative stress and inflammatory status, and amended quality of life and thyroid ultrasound structure and volume. In pregnant females, adequate Se intake protected them against miscarriages, preeclampsia/hypertension, preterm birth, small-for-gestational-age infants' birth, and improved child's neuropsychological development. In the elderly population, adequate Se supplementation decreased cardiovascular diseases and hypertension risk, but prolonged intake of excessive doses increased the all-cause mortality rate. Routine Se supplementation implementation requires from researchers and clinicians consideration of specific populational differences in natural Se and iodine supply, the patient's clinical situation (supplementation simultaneously or before levothyroxine treatment, AIT/non-AIT hypothyroidism), individual response to supplementation (Se and selenoprotein P assessment), predisposition (genetic testing), the status of other trace elements, and the interplay between those micronutrients. Moreover, the safety of commercially available Se formulations, doses, and duration of treatment should be determined. Proper guidelines are warranted to standardise the medical approach to Se supplementation. This article presents a comprehensive review of recent randomised-controlled trials, meta-analyses, and clinical trials concerning the risks and benefits of Se supplementation in different clinical settings and specific populations with particular emphasis on AIT in a practical manner.