Across 3 studies, 2 reported beneficial small-sized effects on increasing serum free thyroxine levels, with 1 neutral finding. Effects were predominantly small. Most studies involved clinical populations with autoimmune thyroid disease (Hashimoto thyroiditis, Graves-Basedow disease) and dosing around 200 μg/day. Effects typically observed at 8–12 weeks.
Caveats: Evidence base is small (only 3 studies) — conclusions should be considered preliminary. The neutral study was in hemodialysis patients, suggesting the benefit may not extend to all clinical populations. Most studies had small effect sizes, and the two beneficial findings come from meta-analytic or systematic review designs rather than individual large-scale RCTs.
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.