Across 3 studies on selenium supplementation for increased serum free thyroxine levels, 2 reported beneficial small-sized effects and 1 reported a neutral effect. The evidence primarily comes from clinical populations with Hashimoto thyroiditis or Graves-Basedow disease, with a median study duration of 84 days (12 weeks), indicating effects may be observed at this timeframe. Selenium shows moderate evidence for a small beneficial effect on thyroid function in these clinical populations.
Caveats: Evidence base is small (only 3 studies) — conclusions should be considered preliminary. The neutral finding came from a small RCT in hemodialysis patients, a distinct population from the two beneficial studies, suggesting benefits may not generalize to all clinical groups. The beneficial meta-analysis and systematic review both involved patients with autoimmune thyroid conditions, so the effect may be specific to that population.
Significant effects on declining the TSH titers were found after 6 months (SMD = -0.18, 95%CI = -0.35 to -0.01; P = .03).
selenium supplementation was associated with significant improvements in TSH, FT4, FT3, TPOAb, TgAb, and TRAb levels
Although there were no significant differences in ... FT4 (p = 0.76) ... between the groups at the end of the trial.