Iodine in horses: needs, sources and safe supplementation

Like selenium, iodine is a trace element with a narrow window – but with one crucial difference: with iodine, both ends are dangerous. Too little and too much lead to the same visible sign, and that is exactly what makes supplementation tricky.

Functionally, iodine is an essential building block of the thyroid hormones thyroxine (T4) and triiodothyronine (T3); these hormones control basal metabolism as well as growth and development ¹. If iodine is lacking, the thyroid cannot produce enough hormone – the body responds with more TSH, and the gland enlarges ¹.

How much does a horse need?

Here, for once, the references disagree – unlike selenium, where the GfE and NRC meet. Based on GfE 2014 (German Society of Nutrition Physiology), the Gruber Table gives 1.8 mg iodine per day for a 600 kg warmblood at maintenance ²³. The NRC calculates with 0.35 mg per kg feed dry matter, which gives around 4.2 mg a day for the same horse ⁴⁵ – more than double.

The U-curve: too little and too much end the same way

This is the core of the article. Iodine deficiency reduces hormone production, drives TSH up and makes the thyroid swell – goitre with signs of hypothyroidism ¹. But too much iodine also leads to goitre and hypothyroidism: high iodine concentrations inhibit hormone production in the gland itself, and above a critical amount production collapses ¹.

An enlarged goitre is therefore precisely not a clear sign of “too little iodine” – it occurs at both ends of the curve. Blind topping up can therefore make the problem worse instead of solving it.

The kelp trap and the pregnant mare

The most common source of oversupply is mineral supplements made from seaweed (kelp): their iodine content is very high and varies greatly by species, harvest time and drying, so their effect is hard to predict. Several documented cases of goitre in mares and foals were associated with large amounts of dried seaweed in the feed ¹⁵.

The unborn and newborn foal is particularly sensitive: the foetal thyroid reacts more strongly than the adult one, and the mare passes iodine on via the placenta and milk – disproportionately so on an iodine-rich ration ¹. As a result, the foal gets too much iodine both in the womb and through the milk. Iodine poisoning has been observed in pregnant mares from around 40 mg per day ⁵ – measured against a requirement of a few milligrams, that is only a factor of about 10.

How much is too much – and why the EU limit is not what counts

For equines, there is a legal maximum content of 4 mg iodine per kg complete feed (at 12% moisture), with 3 mg/kg recommended ⁶. Unlike selenium, however, this legal limit is not the first one to take effect for iodine: there is plenty of room between the requirement (around 0.15–0.35 mg/kg DM) and the EU limit. The real guide rail is biological tolerance – and for pregnant mares and foals this is considerably tighter than the law ⁵.

Merck draws a clear conclusion from this: except in regions proven to be severely iodine-deficient, iodine supplementation is usually not necessary in horses ⁵.

Sources – and why only with a lab value

Forage grown on soils that are not iodine-poor generally covers the iodine requirement; iodised salt and common mineral feeds or concentrates cover it easily ⁵. The reverse conclusion matters for your own ration: pure, non-iodised table salt provides no iodine – our salt module deliberately consists of pure sodium chloride and therefore contributes nothing to iodine supply. If you cover iodine, you do so via iodised salt or a mineral feed containing iodine – and then need to make sure that several sources (mineral feed, kelp, iodised salt) do not add up.

Because goitre does not reveal the direction (see above) and the window is narrow in both directions, the rule is: no blanket topping up. Whether the ration provides iodine is shown by a feed or ration analysis, not by the horse's appearance ⁷⁸. We therefore only release an automatic iodine dosage recommendation with a documented lab value – without an analysis, the iodine module is dropped from the recommendation altogether, even if the calculation shows a gap. This is the same logic as for selenium.

Requirement peaks and limits of the statement

In breeding and growth, the iodine requirement according to the GfE rises only moderately – the Gruber Table gives around 1.8 mg each for pregnant and lactating mares and up to 2 mg per day for growing horses ². The real danger in these groups is not a tight requirement but sensitivity to excess.

Foals, young horses during rearing and pregnant and lactating mares therefore belong in veterinary and breeding advice and sit behind the safety and advice gate. Goitre and thyroid disorders are not a feeding project to solve yourself, but a case for the vet ¹.