Iodine intake in figures: reference values, foods, sources

Part one

A trace element the soil does not supply

Iodine is a halogen and never occurs in pure form in nature. By far the largest reserve lies dissolved in seawater and in marine sediments. On land, by contrast, the element has been leached out of the upper soil layers over millennia by rain, rivers and — particularly markedly in Alpine regions — by draining glacial meltwater, and carried back to the sea.

This has an immediate consequence for nutrition: plants take up only what is present in the soil. Cereals, potatoes and vegetables grown in Central Europe therefore supply small amounts of iodine — regardless of how carefully they were cultivated. Geology stands at the beginning of this chain, not agriculture.

How Germany came to the iodised chain

The observation that goitre occurred more frequently in inland and mountain regions than on coasts was documented as early as the 19th century. The Swiss physician Eugen Bircher and later the American David Marine are among the names associated with systematic research into this connection. In 1922 Switzerland became the first country to introduce iodised table salt.

In Germany, iodised salt could initially only be sold subject to conditions. Only in 1981 did it become freely tradable, and in 1989 authorisation followed for commercial food production. Since then iodine has reached the household mainly by three routes: the salt in one's own salt shaker, processed products such as bread and sausage, and — indirectly — the iodised mineral feed used in livestock farming.

What appears on the ingredient list

A glance at the packaging quickly shows whether iodised salt was used. Food law requires labelling as “iodised table salt” or “table salt with iodine”. If this addition is missing, non-iodised salt was used — which is the norm for numerous organic products and imported goods.

Part two

The thyroid as a chemical workshop

In adults the organ weighs between 18 and 25 grams and consists of thousands of spherical follicles. Each of these follicles is surrounded by a single layer of cells and filled inside with a viscous mass, the colloid. The storage protein thyroglobulin is deposited in it — and with it the body's iodine reserve.

Thyroglobulin as the storage form of the hormones

Thyroglobulin is a very large glycoprotein with around 2,750 amino acids. About 130 of them are tyrosine residues to which iodine can be bound. Thyroid peroxidase oxidises the absorbed iodide and transfers it to these residues. Two iodinated tyrosines then form one hormone molecule: two diiodotyrosines give thyroxine, one mono- and one diiodotyrosine give triiodothyronine.

The reserve in the colloid is, on paper, enough for several weeks to months. That explains why short-term fluctuations in intake are not immediately reflected in blood levels.

Feedback via TSH from the pituitary

Release follows a control loop with three levels. The hypothalamus releases TRH, the pituitary responds with TSH, and TSH binds to receptors on the follicular cells. If the hormone level in the blood rises, TSH secretion falls; if it drops, secretion rises. In laboratory diagnostics the TSH value is therefore regarded as the first point of reference — but its interpretation belongs exclusively in medical hands.

Under Regulation (EU) No 432/2012

Part three

How much iodine ends up on the plate

The values below come from food analyses and give orders of magnitude, not point values. The iodine content of a specific product depends on the catch area, feeding, processing and preparation, and can deviate considerably.

Figures are reference values from published food composition tables. Variation between batches is the rule, not the exception.

What one gram of iodised table salt contributes

Iodised table salt in Germany contains between 15 and 25 milligrams of iodine per kilogram of salt. One gram of salt — roughly a knife tip — thus supplies about 15 to 25 µg. For comparison, the second table shows how this relates to the EU reference intake.

The reference intake of 150 µg is a labelling value for packaging and not an individual intake recommendation.

Losses during cooking and storage

Iodine is volatile. When cooking in open water, part of it passes into the cooking liquid, and at high heat a further share escapes. Iodised salt stored open also loses content over months, especially with humidity and exposure to light. In practice this means: add salt only towards the end of cooking and keep the container closed.

Part four

T3 in the cell nucleus: from gene reading to basal metabolic rate

Once triiodothyronine has reached a cell, it binds to receptors in the cell nucleus. These receptors already sit on particular sections of the DNA and act as switches: without bound hormone they slow the reading, with hormone they release it. In this way a single molecule influences the production rate of numerous proteins at once.

The gene products affected include subunits of the sodium-potassium ATPase, which moves ions against a gradient at every cell membrane and consumes considerable amounts of ATP in doing so. Also among them are enzymes of the respiratory chain in the mitochondria and proteins involved in building up and breaking down fats and carbohydrates. The sum of these processes is measurable as basal metabolic rate.

In nervous tissue the pathway is similar, but the outcome concerns different gene products: proteins of the myelin sheaths, growth factors and building blocks of synaptic connections. That is why iodine supply in pregnancy and early childhood has been in the focus of nutrition research for decades.

Where the limit of the authorised statement runs

An authorised claim describes a contribution to a normal bodily function. It says nothing about what an additional intake achieves in a person who is already adequately supplied, and it makes no statement about diseases. For this reason Lazurstein quotes the three sentences exactly as they stand in the Union register — without rephrasing and without additions.

Part five

Four follow-up questions we receive often