Selenocysteine is known as the 21st amino acid and is an essential component of various selenoproteins, many of which play critical roles in the human body, especially in the glandular (endocrine) system. The thyroid gland, in particular, relies heavily on selenoproteins such as glutathione peroxidases and iodothyronine deiodinases. These enzymes are crucial for the synthesis, metabolism, and regulation of thyroid hormones, which are central to the function of the glandular system. Scientific studies have established that adequate selenium intake, often supplied in the form of selenocysteine within proteins, is necessary for optimal thyroid hormone production and protection of the gland from oxidative damage. Deficiency in selenium can lead to thyroid dysfunction, such as hypothyroidism, and increased risk of thyroiditis.

Several clinical and epidemiological studies have demonstrated that selenium supplementation can improve thyroid hormone profiles in individuals with marginal selenium status. Furthermore, selenoproteins are involved in the function of other endocrine tissues by acting as antioxidants and supporting redox balance, which is essential for hormone synthesis and secretion. Thus, the use of selenocysteine as a nutritional support for the glandular system is based on substantial scientific evidence, particularly regarding thyroid health.