Iodine supply · Thyroid hormones
Four atoms in thyroxine, three in triiodothyronine. Lazurstein collects what nutritional science has compiled about this trace element — and which claims the EU Commission has authorised for it.
To the analysisFor iodine, the EU Commission has authorised three health claims on the basis of opinions from the European Food Safety Authority. Each of them describes a distinct physiological relationship. Below, each appears in its authorised wording alongside the biochemical explanation.
The thyroid sits below the larynx and actively draws iodide out of the blood — via a transport protein called the sodium-iodide symporter. In the follicular cells the iodide is bound to the storage protein thyroglobulin. From this reserve the two hormones thyroxine (T4) and triiodothyronine (T3) are formed, whose molecules carry four and three iodine atoms respectively. Without iodine from the diet, the building block for both is missing.
Under Regulation (EU) No 432/2012
Thyroid hormones cross the blood-brain barrier via specialised transporters such as MCT8 and OATP1C1. In nervous tissue they bind to receptors in the cell nucleus and there influence the reading of genes whose products are involved in the myelination of nerve fibres and in the formation of synapses. This relationship has been described in the specialist literature since the studies on iodine supply in mountain regions of Europe.
Under Regulation (EU) No 432/2012
Triiodothyronine acts as a ligand of nuclear receptors that control the activity of numerous genes. Those affected include the sodium-potassium ATPase of the cell membrane, enzymes of the mitochondrial respiratory chain and proteins of fat and carbohydrate turnover. Physiologists have measured this relationship for over a hundred years as basal metabolic rate — the amount of energy a resting body needs per day.
Under Regulation (EU) No 432/2012
An adult body contains a total of about 10 to 20 milligrams of iodine. Around three quarters of it is in the thyroid, the rest distributed across blood plasma, muscle, salivary glands and gastric mucosa. The amount is tiny — and yet the entire hormone production of the organ rests on it.
Iodine is taken in with food almost exclusively as iodide and is absorbed almost completely in the small intestine. Via the blood it reaches the thyroid, where a transport protein sits at the basal membrane of the follicular cells: the sodium-iodide symporter, NIS for short. It carries two sodium ions and one iodide ion together into the cell — against a marked concentration gradient. The thyroid can thereby enrich iodide to many times the blood concentration.
What does not end up in the gland is excreted via the kidneys. That is precisely why the iodine concentration in urine serves in epidemiological surveys as an indicator of a population's supply situation.
At the apical membrane of the follicular cell waits an enzyme called thyroid peroxidase. It oxidises the iodide and attaches it to tyrosine residues of the storage protein thyroglobulin. This first produces monoiodotyrosine and diiodotyrosine. When two of these building blocks are coupled, the result is thyroxine with four iodine atoms or triiodothyronine with three. The finished thyroglobulin is stored in the colloid of the follicle until the body calls for hormone.
Retrieval is controlled by a feedback loop: the pituitary releases the hormone TSH, which binds to receptors on the follicular cells and triggers release. If the hormone level in the blood falls, TSH secretion rises — and vice versa.
The thyroid releases mostly thyroxine, even though T3, not T4, is the biologically more active form. The conversion takes place only in the target tissue: enzymes of the deiodinase family split a single iodine atom off the T4 molecule. Liver, kidneys and skeletal muscle account for most of this conversion, and the brain additionally has a deiodinase system of its own.
One point is remarkable: the iodine that is split off is not lost. It enters what is known as the intrathyroidal cycle and remains available for renewed hormone synthesis. The body therefore handles this trace element exceptionally sparingly.
The authorised claim in this field reads, in its exact wording: “Iodine contributes to normal thyroid function” — Under Regulation (EU) No 432/2012.
Iodine is an element of marine origin. In the oceans it is present in dissolved form, whereas on land it has been washed out over geological timescales by precipitation and meltwater. Central Europe is therefore geologically one of the iodine-poor regions — this applies to arable soils just as much as to the vegetables and cereals growing on them.
Dietary practice has responded to this in three ways. First, with iodised table salt, which has been commercially available since the 1980s and may also be used in commercial food production. Second, through animal husbandry: mineral feeds contain iodine, which is why milk and dairy products are among the most significant sources by volume. And third, through sea fish and seafood, which take up the element directly from seawater.
The reference value for daily intake that the EU has laid down for nutrition labelling is 150 µg of iodine. You will find this figure on packaging as a percentage of the reference intake.
What should not count as a source of iodine: non-iodised sea salt. During the drying and cleaning of sea salt, most of the iodine originally present escapes, so that in the end only traces in the range of a few micrograms per hundred grams of salt remain. Anyone wanting to obtain iodine via salt has to use expressly iodised salt — recognisable from the ingredient list.
At the other end of the scale are dried seaweeds. Brown algae such as kombu can reach iodine levels in the range of several thousand micrograms per gram, and the variation between individual batches is considerable. For that reason, seaweed products sold in Europe carry a content declaration and a recommended intake that should be observed.
Lazurstein brings these figures together, orders them by food group and states the source for every number. Assessing which of them applies to your own diet can, in the end, only be done by someone who knows your personal situation — if in doubt, together with a medical practice.
Around forty pages of running text, table values and source references on iodine, the thyroid and the three claims that the EU Commission has approved for this trace element.
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