Once a thyroid hormone has its job behind it, the iodine is not thrown out; the whole molecule is rebuilt — in several steps, in different places and with a share that finds its way back into circulation. The sections below walk through those steps in order and put the legal text at their end.
At the edge of a thyroid hormone sits a free hydroxyl group, that is, an oxygen atom with a hydrogen on it. Chemically it is the most reachable spot on the whole molecule. That is exactly where the enzymes this page is about go to work, and nowhere else.
The iodine atoms themselves stay untouched in this step. They sit firmly in place and are neither moved nor taken off. What changes is the behavior of the carrier molecule: it becomes friendlier to water and so fit for routes that were closed to it before.
A free hydroxyl group is easy for an enzyme to grab: it juts out of the molecule, it carries a hydrogen that can come off, and it can be swapped for a larger group without the rest of the frame having to be rebuilt. That is why both of the routes described below run over the same point and rule each other out at that position.
The first route hangs on a sulfate group. It is small, carries a negative charge and comes off again comparatively easily. In the literature this state is therefore often described as an interim stage and not as an end point.
The second route hangs on a whole sugar group instead of the small one. The result is bulkier, charged as well, and binds more tightly. A molecule carrying that group is as a rule moved toward the exit. Which of the two routes prevails depends on the tissue and on which enzyme variants are made there.
| Enzyme variant | Attached group | Where the literature places it |
|---|---|---|
| SULT1A1 | Sulfate group | Liver cell |
| SULT1E1 | Sulfate group | Liver cell, gut wall |
| SULT1B1 | Sulfate group | Gut wall |
| UGT1A1 | Glucuronic acid | Liver cell |
| UGT1A3 | Glucuronic acid | Liver cell, bile duct cells |
Selected from review papers on the reworking of the body’s own molecules. The assignment to tissues is simplified and comes out differently from one study to the next. None of these names is part of an authorized claim.
A liver cell has two outer faces with different jobs. One looks toward the blood, the other borders a tiny channel that later becomes bile. On this second face sit transport proteins that move charged molecules outward in an active step that costs the cell ATP; the one described most fully for such molecules goes by the short name MRP2 in the literature.
On the blood face other proteins work in the opposite direction. They pull the hormone out of the plasma into the cell, and only then does it come within reach of the enzymes from the previous section. Uptake, rebuilding and release therefore sit in the same cell, but at three different places.
The transport proteins on the bile side pick out molecules with a negative charge. That explains why attaching the sulfate group or the sugar group comes before the export and not the other way round: without the charged group the molecule would hardly be recognizable to this pump.
The bile carries the tagged molecule into the gut. Further along, in the large intestine, sit bacteria whose enzyme kit takes exactly these groups off again. After that an untagged molecule stands in the gut contents, and it can cross the gut wall a second time. By way of the portal vein it comes back to the liver cell, where the process can start over. This loop is called the enterohepatic circulation.
The part that is not fetched back leaves the body with the stool. How the two shares split depends, among other things, on which bacteria sit in the gut and how long the contents are on the move. Figures on this scatter considerably in the literature, and none of them carries over to any one person.
| Segment | What happens there |
|---|---|
| Bile canal of the liver cell | Release of the tagged molecule to the outside |
| Small intestine | Mixing with the chyme, hardly any removal |
| Large intestine | Bacterial enzymes cut the sulfate group and the sugar group off |
| Gut wall | Part of the freed molecule is taken up again |
| Portal vein | Return to the liver cell |
The order is given schematically and leaves intermediate steps out. It describes a process documented in physiology and no effect of a food.
Turn a package over and next to the iodine content you find a percentage. Its reference point is a single figure from the annex to the European labeling rules: the EU reference intake of 150 µg set by Regulation (EU) No 1169/2011. It serves to compare two products on the shelf and says nothing about what any one person should take in.
| Item | Entry |
|---|---|
| EU reference intake for iodine | 150 µg per day |
| Where it is written down | Annex XIII of Regulation (EU) No 1169/2011 |
| What it is for | Working out the percentage printed on a package |
No personal intake amount follows from this value. It belongs to European labeling and not to the wording quoted further down.
In the register of the European Union this wording carries not one ID number but two: 274 and 1237. The reason is that two separately examined findings were drawn together into a single sentence. Whoever uses it uses both parts, and neither of them may be sharpened on its own.
“Iodine contributes to the normal production of thyroid
hormones and normal thyroid function”
EU-authorized wording · Regulation (EU) No 432/2012
The list is short: no amount, no enzyme, no transport route, no organ other than the one named in the sentence itself, and no age. Neither the sulfotransferases nor the glucuronosyltransferases nor MRP2 turn up in the register. They stand on these pages as the origin of the description, not as part of the authorization.
Just as little does the line say anything about an intake above the ordinary. The word normal names a process that goes on anyway when intake is already covered, and no gain beyond that.
In part. One share returns to the pool through the loop described here, another leaves the body with urine and stool. How that splits in any one person is beyond what a text can say; measured values would be needed for that.
Above all the make-up of the gut population, how long the gut contents stay on the way, and which groups the molecule carries. Published figures on this differ widely, because the conditions of the studies differ widely too.
No. The sentence names one element and two states. Everything on this page about enzymes, pumps and segments comes from the scientific literature and stands beside the official text, not inside it.
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