Selenomethionine is a common supplemental form of selenium, an essential trace element involved in thyroid hormone metabolism. Scientific evidence supports a role for selenium in overall thyroid health, as the thyroid gland contains higher concentrations of selenium than any other organ. Selenium is a component of the enzymes (selenoproteins) that catalyze the conversion of thyroxine (T4) to the active hormone triiodothyronine (T3) and protect the thyroid from oxidative damage.

Several studies have explored selenium supplementation, primarily in the form of selenomethionine, in patients with autoimmune thyroiditis (Hashimoto’s disease), which is a common cause of hypothyroidism. Some randomized controlled trials and meta-analyses have shown that selenium supplementation may reduce thyroid peroxidase antibody (TPOAb) levels, potentially reflecting reduced autoimmune activity. However, the effect on actual thyroid hormone levels and clinical symptoms of hypothyroidism is less clear. Most studies show little or no improvement in thyroid function (TSH, T4, T3) or hypothyroid symptoms in people who are not selenium deficient. Guidelines generally do not recommend universal selenium supplementation for hypothyroidism but suggest it may be considered in cases of proven deficiency or in regions with low selenium intake.

In summary, there is moderate scientific evidence supporting a mechanistic role for selenium (and thus selenomethionine) in thyroid health, particularly in autoimmune thyroiditis, but evidence for a direct therapeutic effect on hypothyroidism itself is limited and not definitive.