What iodine and selenium have to do with an alert thyroid

The thyroid as the metabolic control centre

Hardly any organ influences so many bodily functions at once as the thyroid. Through its hormones thyroxine (T4) and triiodothyronine (T3) it influences basal metabolic rate, body temperature, the cardiovascular system and the development of the brain even before birth.

For this control loop to work, two trace elements matter as raw material and as tool respectively: iodine supplies the actual building block of the hormones, while selenium, as part of certain enzymes, takes part in activating them. The two roles are considered separately in the following sections.

The European Food Safety Authority (EFSA) has assessed the state of scientific knowledge on both minerals. The claims derived from it and authorised by the EU Commission form the framework of this guide.

Iodine as the raw material of thyroid hormones

From iodide uptake to hormone formation

Iodide, the form of iodine occurring in the body, is absorbed in the small intestine and transported through the bloodstream to the thyroid. There the sodium-iodide symporter (NIS) provides for active uptake into the thyroid cells — an energy-dependent process in which the iodine concentration in the cell lies well above that in the blood.

Inside the cell, iodide is oxidised and bound to tyrosine residues of the protein thyroglobulin. The coupling of two iodinated tyrosine residues produces the finished hormones T4 and T3, which are then released from thyroglobulin and secreted into the blood.

Where the body gets iodine from

The iodine content of foods depends strongly on the iodine content of the soil or water in question. The following sources supply appreciable amounts:

Because iodine levels in seaweed products vary so widely, regular consumption of larger amounts is often advised against — an oversupply can impair thyroid function just as an undersupply can.

Selenium as a cofactor of the deiodinase enzymes

How selenium converts T4 into the more active T3

The T4 released by the thyroid in larger amounts only develops its full effect in the target tissues after conversion into T3. This conversion takes place via deiodinase enzymes, which split an iodine atom off the T4 molecule. The active centre of these enzymes contains selenocysteine, an amino acid that carries selenium in the molecule in place of sulfur.

Besides hormone activation, selenium is also involved, via enzymes of the glutathione peroxidase family, in reducing the oxidative stress in the thyroid that arises as a by-product of hormone production.

Selenium in the daily diet

The selenium content of foods varies considerably with the selenium content of the soil on which they are grown. The following sources are regarded as particularly rich in selenium:

Cognitive function: an additional authorisation for iodine

Even before birth, thyroid hormones are involved in the maturation of the nervous system — a window in which an adequate maternal iodine supply is particularly relevant. After birth as well, the hormones remain important for neuronal signal transmission.

Because iodine is the prerequisite for the formation of these hormones, the EU Commission has authorised a second, independent claim on cognitive function alongside the claim on thyroid function. This one refers exclusively to iodine — no corresponding authorisation exists for selenium in this context.

Questions that reach us about iodine and selenium

Why is iodine named both for the thyroid and for cognitive function?

The two claims are causally linked: cognitive function relies on thyroid hormones, whose formation in turn requires iodine. Because these are two separately assessed and authorised relationships, Magnetitpfad states both claims individually.

Is there anything against taking iodine and selenium together?

At customary amounts oriented to the reference values, taking iodine and selenium at the same time is usually regarded as unproblematic. Where thyroid disorders already exist, it can make sense to discuss this individually with qualified medical staff, particularly before adding dietary supplements.

Which foods supply appreciable amounts of iodine and selenium?

Iodine is found above all in sea fish, iodised table salt and dairy products, while selenium occurs particularly in Brazil nuts, fish and wholegrain products. A varied diet covers a large part of the requirement for both minerals in most adults.

What does the qualifier “normal” mean in the EU claims?

The qualifier “normal” describes the physiologically proper function where supply of the mineral concerned is adequate. It does not describe any performance enhancement beyond that and is not to be equated with a therapeutic effect.