This guide follows thyroid hormone from the follicle cell to the nucleus of the target tissue and notes at each station which enzyme is at work there and where the figures come from. Three tables summarize what reads better as a list than as prose: the hormone forms side by side, the three enzyme types with their tissues, and a timeline of findings since 1817.
Before a single iodine atom is incorporated in the gland, the signal for it has already passed through two stations. The process is set up as a closed loop: what ultimately arrives in the blood helps determine how strong the initial signal is.
In the hypothalamus, an area of the diencephalon, a short peptide made of three amino acids is released. Through its own vascular system, it reaches the anterior pituitary, where it triggers the release of thyroid-stimulating hormone. This in turn binds to the follicle cell and sets several processes in motion at once: the uptake of iodide, the production of the carrier protein, and the retrieval of the colloid.
The hormones themselves handle the opposite direction. Both the anterior pituitary and the diencephalon carry receptors for them and scale back their output when enough arrives in the blood. Remarkably, the pituitary does not use the ready-made triiodothyronine delivered to it, but makes its own locally from thyroxine — with a selenium-containing enzyme that is particularly abundant there.
There is no linear relationship between the amount of free thyroxine and the amount of control hormone sent out. It is described as logarithmic: if free thyroxine falls by a few percent, the value of the control hormone can multiply. Conversely, it drops sharply when the supply rises.
For the specialist literature, this is why the control value is considered more sensitive than the hormone values themselves. For this page, it is mainly a reminder that individual numbers from such a feedback loop say little without the context in which they were measured. No interpretation of test results takes place here, nor can this page replace one.
All the molecules named here share the same basic scaffold: two rings joined by an oxygen atom, plus an amino acid side chain. They differ only in how many iodine atoms are attached to which ring. This one question gives rise to four forms with very different roles.
| Abbreviation | Name | Iodine atoms | Origin of the share in the blood |
|---|---|---|---|
| T4 | Thyroxine | four | almost entirely from the gland |
| T3 | Triiodothyronine | three | mostly from conversion in the liver, kidneys, and other tissues |
| rT3 | reverse triiodothyronine | three | almost exclusively from removal at the inner ring |
| 3,3'-T2 | Diiodothyronine | two | from further breakdown of the two triple-loaded forms |
The classification follows standard endocrinology textbooks. Figures on amounts and ratios vary depending on the study series.
The revealing part of this overview is in the last column. Only the first row describes a product of the gland in the strict sense. The other three arise mostly elsewhere — through conversion, that is, through enzymes that carry selenocysteine. Anyone who looks at the gland alone therefore sees only the first part of the process.
The third row is also worth noting. If the iodine atom is removed not from the outer but from the inner ring, the result is a molecule that triggers nothing at the receptor. The same starting substance thus leads to an active or a silent form depending on the point of attack.
Three enzymes carry out the conversion. They differ in where they occur, in their point of attack, and therefore in their product. What they share is the building block: all three have selenocysteine at the catalytically active position.
| Enzyme type | Mainly detected in | Point of attack on the molecule | What is produced |
|---|---|---|---|
| Type I | Liver, kidneys, thyroid | outer ring, and to a lesser extent the inner ring | Triiodothyronine for the bloodstream, plus breakdown forms |
| Type II | Brain, pituitary, brown adipose tissue, skeletal muscle | outer ring only | Triiodothyronine for the cell’s own use |
| Type III | Placenta, nervous tissue, skin | inner ring only | reverse triiodothyronine, forms that are silent at the receptor |
The classification by type goes back to studies from the 1980s and 1990s and has been standard in textbooks ever since.
The second row explains a feature that is surprising at first glance: nervous tissue largely covers its own needs instead of drawing finished hormone from the blood. The Type II enzyme sits inside the cell there and delivers directly to the nucleus. The third row describes the counterpart: a way to scale back the local supply without anything having to change in the gland.
The link that the Union list records for selenium is rooted precisely in this enzyme family. The wording of the entry, however, names no enzyme but describes a contribution in general terms: “Selenium contributes to the normal thyroid function”.
Per Regulation (EU) No 432/2012
Today’s register is the result of a long series of individual findings, the earliest of which had nothing to do with nutrition. The timeline lists the milestones that textbooks usually refer to.
| Year | Finding | What it is used for today |
|---|---|---|
| 1817 | Jöns Jacob Berzelius describes a new element in the residue of a sulfuric acid chamber and names it after the moon | Starting point of the element’s history |
| 1930s | On pastures of the North American plains, damage to hooves and coats is traced back to selenium-rich plants | Earliest described effects of excess intake |
| 1957 | Klaus Schwarz and Calvin Foltz show that a selenium-containing factor protects test animals from a specific liver change | Shift from toxin to nutrient |
| 1973 | Two research groups independently detect selenium in the enzyme glutathione peroxidase | First evidence of a selenoenzyme |
| 1986 | Selenocysteine is described as a distinct amino acid that enters the chain via a reinterpreted stop codon | Explains the unusual incorporation pathway |
| 1990/1991 | Two research groups identify Type I deiodinase as a selenoprotein | Basis of the link to thyroid function |
| 2003 | A screening of the human genome counts twenty-five genes for selenoproteins | Size of the protein family |
| 2012 | The European Commission publishes the common list of permitted health claims | Source of the statements quoted here |
Years are based on the publication dates of the respective papers. Where two groups published simultaneously, the order is not consistent in the literature.
The term comes up constantly in specialist texts but means different things depending on context. It can refer to the amount in blood plasma, the loading of the transport form, or the activity of a selenium-containing enzyme. These values are related but do not move in parallel.
The amount in plasma covers everything currently in transit — transport form, enzyme portions, and selenium incorporated nonspecifically into protein, all taken together. It responds to intake over the past few weeks. The transport form, a protein made in the liver with up to ten selenocysteine residues, reaches its upper range at a higher intake than enzyme activity in the blood. And the activity of an enzyme in red blood cells reflects a period of about four months, because these cells live that long.
This sequence explains an observation regularly mentioned in the specialist literature: above a certain supply, enzyme values barely rise any further, while the plasma amount still increases. A rising number therefore does not always mean that something is also changing in the enzymes.
For food labeling in the European Union, a reference value of 55 micrograms per day applies; it is set out in Annex XIII of Regulation (EU) No 1169/2011 and is the figure that percentages on packaging refer to. The German-speaking nutrition societies give an estimated value for adults of 70 micrograms for men and 60 for women. In 2014, the European Food Safety Authority specified an adequate intake of 70 micrograms.
The upper end is also quantified: a 2023 assessment gives 255 micrograms per day as the tolerable total intake for adults; an older assessment by the Scientific Committee on Food put it at 300. Both values describe an upper limit for long-term intake from all sources and are not targets.
Where selenium occurs in significant amounts depends less on the food group than on the growing region. Grain from regions with selenium-rich soil contains many times what the same grain grown in Central Europe contains. For fish, organ meats, and eggs the variation is smaller; for Brazil nuts it is by far the largest.
Besides the entry on thyroid function, two further claims are quoted on this page. Both go back to their own assessments and relate to tissues connected to hormone metabolism in different ways.
The hair follicle is one of the few tissues outside the classic target organs in which both receptors for thyroid hormones and two of the three deiodinase types have been detected. The cell there can therefore help determine how much active hormone is present in its immediate surroundings. Added to this is the high division rate of the matrix cells, which comes with a corresponding demand for enzymes.
What is visible on the head is already finished material: keratinized cells that were completed deep below and pushed upward. Changes in supply therefore reach the visible part only with a delay. The register entry reads:
Per Regulation (EU) No 432/2012
An immune cell that encounters its matching fragment begins to multiply in a way hardly any other tissue shows: divisions hours apart, over several days. Each one requires new genetic material, new membrane surface, and a new stock of enzymes. Immune cells store a lot of selenium relative to their mass; this has been described since the 1980s.
This functional area was also assessed separately and received its own statement:
Per Regulation (EU) No 432/2012
The following three statements are the reason selenium may be mentioned on this page at all. They appear here unabridged and unchanged, each with its source reference. Each claim belongs to its own scientific opinion; they are not three versions of the same fact.
Everything else on this page — follicles, enzyme types, timeline, numerical values — is descriptive biochemistry and nutrition science. It has the status of an explanation, not of an authorized claim, and is not presented as one here.
Five questions we have received most often about this text, with the answer we can back up from the literature. We do not comment on individual people’s test results.
The answer that can be derived from physiology has to do with shelf life. Thyroxine stays in the blood for about a week, triiodothyronine for only about one day. The longer-lasting form is therefore better suited as a reserve from which tissues can draw at their own pace. Where and when it becomes the active form is then decided by each tissue through its enzyme makeup.
It is not lost. The released iodide becomes available to the body again and can be taken up by the gland once more. A considerable part of daily iodine turnover comes from this internal cycle rather than from food. Whatever is not reused leaves the body via the kidneys.
Only partly. The amount measured in plasma increases because selenium is also incorporated nonspecifically into protein. The activity of the selenium-containing enzymes, on the other hand, reaches a range at moderate intake above which it hardly rises any further. Two numbers both called “selenium” can therefore mean different things.
No. Each claim in the register goes back to its own scientific opinion, with its own question and its own body of literature. That is why they appear in the list as separate entries and are quoted separately here as well. A statement cannot be transferred to another tissue just because the same element is involved.
Yes, it belongs there. A statement from the Union list is a reviewed wording that may only be used unchanged and with reference to its legal basis. The repetition may feel clunky when reading, but it draws a clean line: what appears in quotation marks with a source reference is reviewed text; everything else on this page is specialist information without that status.
If you would rather have the tables on this page printed out next to you, you will find them in the guide — along with the sources the figures come from and a list of the cited works from 1817 to today.
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