Synopsis

Source of validity: Scientific
Rating (out of 5): 3

Selenocysteine, known as the "21st amino acid," is the active form of selenium in various selenoproteins, several of which are critical for thyroid hormone metabolism and antioxidant defense within the thyroid gland. Scientific evidence supports the relevance of selenium (and by extension, selenocysteine) in thyroid health, particularly because two key selenoproteins—iodothyronine deiodinases—are directly involved in the activation (T4 to T3 conversion) and inactivation of thyroid hormones. Additionally, glutathione peroxidases, also dependent on selenocysteine, protect thyroid tissue from oxidative damage during hormone synthesis. Multiple clinical studies and meta-analyses have investigated selenium supplementation in patients with hypothyroidism, especially in those with autoimmune thyroiditis (Hashimoto's). Some research indicates that selenium supplementation can reduce thyroid peroxidase antibody levels and may modestly improve well-being and thyroid function in these patients. However, results are inconsistent, and not all studies show clear clinical benefit in terms of thyroid hormone levels or symptoms. The evidence is stronger for selenium’s role in autoimmune thyroid disease than in non-autoimmune hypothyroidism. Guidelines from several endocrine societies recommend considering selenium supplementation in regions with documented selenium deficiency but do not broadly endorse its use for all hypothyroid patients. In summary, there is a scientific rationale and some clinical evidence for selenium/selenocysteine in thyroid support, but the benefit is moderate and context-dependent.

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