Selenium functions as an essential component of antioxidant systems such as glutathione peroxidases and thioredoxin reductases, and it is also incorporated into iodothyronine deiodinases
, which are involved in thyroid hormone metabolism (Roman et al.
Although both T4 and T3 are secreted by the thyroid following TSH stimulation, 80 percent of circulating T3 is derived from the conversion of T4 by enzymes called deiodinases
in the liver.
Thyroid gland has a high concentration of selenium, as constitutive part of type 1 and 2 deiodinases
(Dio 1 and 2), which convert thyroxine (T4) to triiodothyronine (T3); Gpx 1 and 4, which protect thyreocytes against H2O2 released during thyroglobulin iodination; Gpx3, regulating H2O2 concentration in the lumen of thyroid follicle; selenoprotein P and Txnrd 1, having an antioxidant effect .
This showed a definite relationship between these two hormones (T3 and T4) as T4 is transformed to T3 within the target cells with the help of deiodinases
(types of deiodinases
D1 D2 D3).
Stapleton, "Halogenated phenolic contaminants inhibit the in vitro activity of the thyroid-regulating deiodinases
in human liver," Toxicological Sciences, vol.
Presence of thyroxine deiodinases
in mammary gland: Possible modulation of the enzyme-deiodinating activity by somatotropin.
Early expression of thyroid hormone deiodinases
and receptors in human fetal cerebral cortex.
There are three homologous iodothyronine deiodinases
which catalyses these reactions.
At least three different molecular alterations may cause reduced sensitivity to TH in humans: (a) mutations in the gene encoding TR[beta] isoform causing RTH, (b) mutations of the specific TH transporter, monocarboxylate transporter 8 (MCT8), and (c) mutations in selenocysteine insertion sequence binding protein 2 (SECISBP2) which reduces the synthesis of selenoproteins, including the TH deiodinases
Consequently, at cold stress which body needs more deiodinases
and active thyroid hormones, more selenium is needed.
Selenium deficiency, thyroid hormone metabolism, and thyroid hormone deiodinases
The selenium-dependent iodothyronine deiodinases
have a specific role in the assimilation of iodine, which explains why selenium deficiency can exacerbate the effects of coexisting iodine deficiency