Well over ninety percent of what the thyroid releases into the blood is thyroxine — a molecule with four iodine atoms that binds only weakly to the receptors in the cell nucleus. The form that acts there only arises afterward: in the liver, kidneys, and nervous tissue, through the removal of a single iodine atom. The enzymes that make this cut carry selenocysteine in their active site. Authorized claim: “Selenium contributes to the normal thyroid function” — In accordance with Regulation (EU) No 432/2012.
Go to the guideThe thyroid is one of the few organs that produce hydrogen peroxide on purpose. Without this compound, no iodine would reach the carrier protein from which the hormones are later cut. The price is a chemistry that the gland cell has to handle in the same breath.
The tissue is built from vesicles about two tenths of a millimeter in diameter, the follicles. A single layer of cells encloses an inner space filled with a viscous mass of thyroglobulin — a protein of roughly 660 kilodaltons carrying more than a hundred tyrosine residues. These residues are the sites where iodine is attached.
Iodide enters the cell on the side facing the blood, pulled in by a transport protein that brings two sodium ions along with it. It travels through the cell and exits on the opposite side into the follicle lumen. Only there, at the membrane boundary, does the actual incorporation take place: an enzyme system supplies hydrogen peroxide, and thyroid peroxidase uses it to oxidize iodide and attach it to the tyrosine residues.
Two iodinated residues that are then joined together form the finished hormone — still as part of the long protein chain. The cell takes back the colloid in portions, breaks down the chain, and releases what remains in the end: overwhelmingly thyroxine, with a smaller share of triiodothyronine.
Any hydrogen peroxide that is not used up stays behind in a tissue whose cells rarely divide and therefore remain in service for a long time. That is why several selenium-containing enzymes can be detected in the thyroid: glutathione peroxidases in the cytosol, at the membrane, and in the secretion, plus thioredoxin reductases. All of them carry selenocysteine at the catalytically active position.
Converted to weight, the glandular tissue is among the most selenium-rich tissues measured in humans — even though the organ weighs only about twenty grams and accounts for hardly any of the body’s total store of about fifteen milligrams. Concentration and quantity are two different statements here.
In plasma, thyroxine travels almost entirely bound: to a specialized transport protein, to transthyretin, and to albumin. Only a fraction of about two hundredths of a percent is free, and only this fraction passes into cells. It stays in the blood for about a week, considerably longer than triiodothyronine at about one day.
Most of the triiodothyronine circulating in an adult’s blood does not come from the gland but from conversion in peripheral tissues. This is precisely where the link lies that the European Union lists for selenium.
Since the 1970s, feeding experiments had documented an observation that at first lacked an explanation. Animals given a low-selenium diet showed a shifted ratio in their blood: thyroxine was higher than in the control animals, triiodothyronine lower. Iodine intake was the same in both groups, so it could not be the cause.
In 1990 and 1991, two independent research groups — one led by Dietrich Behne in Berlin, one by John Arthur and Geoffrey Beckett in Scotland — presented the explanation: the enzyme that removes the first iodine atom from thyroxine is itself a selenoprotein. If this building block is missing, the enzyme works more slowly, and the ratio of the two hormone forms shifts measurably.
It is a long way from lab work to an entry in a regulation. The European Food Safety Authority reviewed the dossiers submitted on selenium in 2009 and 2010; the Commission compiled the result into a common list in 2012. Since then, the rule has been: what may be stated is what is written there, exactly as it is written there.
The sentence on thyroid function reads: “Selenium contributes to the normal thyroid function” — In accordance with Regulation (EU) No 432/2012. It describes a contribution to a normal process. It makes no statement about symptoms, test results, or their course, and no wording on this page is meant to be read that way.
Everything stated here about follicles, enzymes, and hormone forms is biochemistry and is labeled as such. Authorized claims appear only as quotations, each with its source reference.
Three entries from the Union list are used on this page. Each goes back to its own assessment and relates to a different tissue. The overview below first gives the biochemical context and then, beneath it, the officially recorded wording.
The liver and kidneys account for the largest share of the conversion that turns thyroxine into the form active at the nuclear receptor. The enzymes involved remove an iodine atom from the outer ring of the molecule. Their active site contains selenocysteine — an amino acid that the body incorporates at exactly the intended position only with considerable effort.
The brain takes its own route: there, a second type of enzyme handles the conversion within the tissue itself, so the nerve cell does not have to draw its supply from the blood.
Per Regulation (EU) No 432/2012
The hair bulb is not a passive recipient. Receptors for thyroid hormones have been detected in its cells, as have the enzymes that can remove an iodine atom locally or deactivate a molecule. The tissue therefore partly sets its own hormone balance rather than taking it entirely from the blood.
There is a second factor: the matrix beneath the bulb is among the most actively dividing tissues in the body, and selenium-containing peroxidases are regularly detectable in such tissues. The register has a separate entry for this link.
Per Regulation (EU) No 432/2012
When an immune cell encounters the fragment its receptor is made for, it begins to multiply on a daily cycle: within a week, one cell becomes thousands. Each division requires new building blocks for genetic material, new membrane surface, and a new stock of enzymes — including members of both selenium-containing families.
That immune cells store a lot of selenium relative to their mass has long been described in the literature. The authority assessed this functional area separately; the third statement quoted here resulted from that.
Per Regulation (EU) No 432/2012
The guide traces the hormone’s path station by station: follicle, bloodstream, target cell. It includes three tables — the hormone forms side by side, the three enzyme types with their tissues, and a timeline of findings from 1817 to today’s Union list.
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