Iodine is an essential trace element for horses, primarily affecting the thyroid and thus the entire metabolism. The daily requirement is low—but the line between deficiency and oversupply is narrow. Many horses in Central Europe receive too little iodine through their basic feed, which can lead to metabolic problems in the long term. In this article, you will learn how much iodine your horse needs, how to recognize iodine deficiency, what happens in case of an excess, and which feedstuffs are particularly critical.

What is iodine and why is it important for horses?

Iodine is an essential trace element. The importance of iodine for horses was already recognized in the 18th century, when researchers discovered a connection between goiter formation and iodine-deficient regions. Goiter is the enlargement of thyroid tissue, caused by a lack of feedback from thyroid hormones due to iodine deficiency.

The role of iodine in the formation of thyroid hormones

Iodine is an essential component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones control vital bodily functions:

• Metabolism and basal metabolic rate: Regulation of energy production from food

• Nervous system: Development and function of nerve cells

• Musculature: Building and maintaining muscle mass

• Body temperature: Regulation of body heat

• Cardiovascular system: Control of heart rate and blood pressure

Iodine is absorbed in the small intestine and actively transported via the bloodstream to the thyroid gland, where it is synthesized into these essential thyroid hormones.

Importance for basal metabolic rate and body temperature

The thyroid hormones T4 and triiodothyronine (T3) are primarily responsible for regulating the basal metabolic rate. They determine how efficiently the organism converts nutrients into energy and thus directly influence:

• Vitality and performance

• Regulation of body temperature

• Growth in foals and young horses

• Pregnancy and fetal development

Iodine requirements in horses

The iodine requirement in horses is quite precisely defined, but it varies depending on the stage of life. The recommended values are based on body weight and special circumstances, such as pregnancy or growth.

Basic maintenance requirement

The maintenance requirement is 0.3 mg of iodine per 100 kg of body weight. In practical terms, this means:

An average 600 kg horse therefore requires between 1 and 3 mg of iodine per day—a very small amount that should, however, be met as precisely as possible.

Increased requirement during special life stages

Certain situations require an increased iodine supply:

• Pregnant mares: The requirement increases by about 50%, as the trace element iodine is essential for the development of the fetus, especially the nervous system.

• Lactating mares: Milk production significantly increases the iodine requirement, as iodine is passed to the foal through the mother’s milk.

• Foals and young horses: Intensive growth demands an optimal supply for the formation of thyroid hormones, which regulate growth.

Covering iodine requirements with mineral feed

High-quality mineral feed should contain about 15–40 mg of iodine per kilogram, supplementing roughly half of the daily iodine requirement for your horse. Premium mineral feeds require only small daily amounts. It’s worth reading the declaration carefully and calculating the feed ration accordingly. Some seemingly inexpensive mineral feeds require 100 g per day, while a high-quality one is effective with just 50 g.

Natural sources of iodine in horse feed

Natural iodine supply varies greatly by region and depends on the iodine content of the soil. In many areas of Germany, the natural supply is not sufficient.

Seaweed as the richest source of iodine

Seaweeds such as Ascophyllum nodosum or Laminaria japonica contain extremely high amounts of iodine:

• Ascophyllum nodosum: 1,500–2,500 mg iodine per kg

• Laminaria japonica: up to 3,000 mg iodine per kg

These high concentrations make seaweed the richest natural source of iodine, but they also carry a significant risk of overdose.

Good to know: The red algae used in Equine 74 is not a classic iodine source. Unlike other seaweeds, it contains only 3.6 mg/kg of iodine. With a recommended daily dose of 50 mg Equine 74 Gastric, this adds up to just 0.18 mg of iodine—such a small amount is insignificant.

Our Equine 74 Stomach Calm Relax paste also contains the seaweed Ascophyllum nodosum. The iodine content per portion is 0.815 mg. Sport horses under heavy load (such as on a competition weekend) should receive a maximum of 3 portions per day, which would total 3 mg of iodine. This value must be included when calculating the total ration.

Other natural sources

Sea fish and seafood: Contain moderate amounts of iodine, but are normally not included in horse feed.

Sea salt: Does contain iodine, but only in comparatively small quantities (about 0.1–0.2 mg per kg).

Agricultural products: Grass, hay, and grains from iodine-poor soils usually have very low iodine content (often below 0.1 mg per kg).

Regional differences

Large parts of Europe are considered iodine-deficient regions. Areas particularly affected are those far from the coast, regions in the south, and places with soils that have little maritime influence. Near the coast, the natural iodine supply is somewhat better due to sea aerosols, but even there it is usually not sufficient to ensure an optimal supply.

Recognizing and treating iodine deficiency in horses

Iodine deficiency leads to an underactive thyroid (hypothyroidism) with far-reaching effects on the entire organism. The symptoms often develop gradually and are therefore frequently overlooked.

Symptoms of Iodine Deficiency

The signs of iodine deficiency are diverse and affect various body systems:

Visible changes:

• Goiter (enlargement of the thyroid gland)

• Weight gain despite loss of appetite

• Coat problems and poor coat shedding

• Edema (fluid retention)

Performance-related symptoms:

• Noticeable weakness in performance

• Fatigue and lack of drive

• Reduced vitality

• Low blood pressure

Reproductive problems:

• Prolonged gestation periods in mares

• Developmental disorders in the fetus

• Miscarriages

• Weak or non-viable foals

Causes of Iodine Deficiency

The most common causes of deficiency are:
Feeding-related factors:

• Feed from iodine-poor soils

• Lack of supplementation with mineral feed

• High nitrate levels in drinking water

Feeds with goitrogenic factors:

• Alfalfa and sainfoin (contain thiocyanates)

• Legumes such as white clover, beans, and peas

• Rapeseed and field mustard (glucosinolates)

These substances inhibit iodine absorption or significantly increase the requirement.

Treatment of Iodine Deficiency

Therapy requires a sustained, needs-based supplementation of iodine for at least 6 to 8 weeks. However, iodine should generally be supplemented since the body cannot produce or store it for long periods. Essential treatment principles:

• Use of high-quality mineral feed with a clearly declared iodine content

• Precise dosing according to body weight

• Consideration of interactions with other trace elements

• Regular monitoring of treatment success

Improvement of symptoms usually occurs only after 6–8 weeks, as the thyroid gland regenerates slowly.

Iodine Excess and Its Risks

The margin between meeting requirements and over-supplying iodine is extremely narrow. Toxic symptoms can appear in a large horse from just 20 mg per day—the equivalent of ten times the usual requirement.

Symptoms of Iodine Poisoning

An excess of iodine causes characteristic symptoms:
Acute symptoms:

• Emaciation despite adequate feeding

• Restlessness and nervousness

• Tremors and convulsions

• Diarrhea

Chronic effects:

• Inflammatory changes to skin and coat

• Paradoxical goiter (iodine goiter)

• Disturbances of thyroid function

• Impaired cardiovascular function.

Critical thresholds for iodine supplementation

The toxicological threshold is:

• Acute overdose: from 20 mg iodine per day

• Chronic oversupply: even persistent doses above 5–10 mg daily

• Reference values in blood count: 10–80 µg/l (values above this indicate oversupply)

Caution with seaweed meal

Seaweed meal poses the greatest risk of overdose. Even 20 g of seaweed meal can provide up to 60 mg of iodine, corresponding to 20 times the daily requirement. Even small amounts can cause serious poisoning.

Rule of thumb: Supplement a maximum of 50% of the daily requirement in addition, never more than 3-4 mg iodine per day for a 600 kg horse.

Influence of selenium and manganese on iodine metabolism

Iodine metabolism does not function in isolation but is closely linked to other trace elements. Selenium and manganese, in particular, play crucial roles.

Selenium as a key element

Selenium is central to the conversion of thyroxine (T4) into the biologically active triiodothyronine (T3) via selenium-dependent enzyme systems (deiodinases).
Simply put: Even with optimal iodine supply, hypothyroidism can occur if there is insufficient selenium.

Interactions between iodine and selenium:

• Selenium deficiency worsens thyroid function despite sufficient iodine

• Optimal ratio: About 1:1 between iodine and selenium

• Both trace elements should be supplemented together

The role of manganese

Manganese is rarely tested, and many laboratory tests use very low reference values, so manganese often appears “normal” in bloodwork, even though the actual level may already be too low. At many labs, manganese must be specifically requested and is not routinely included in a standard “comprehensive blood count.”

Manganese is involved, among other things, in the synthesis and conversion of thyroid hormones. A deficiency can impair the effectiveness of iodine therapy. Manganese also has numerous other important functions in the equine body.

Other important trace elements

Copper and zinc also influence the overall protein and hormone metabolism. An adequate supply of trace elements is therefore essential for:

• Optimal iodine utilization

• Normal thyroid function

• Effective production of thyroid hormones

• Prevention of metabolic disorders

Feedstuffs that influence iodine requirements

Some feeds contain substances that either increase the iodine requirement or inhibit iodine absorption. These so-called goitrogenic factors (which stimulate thyroid gland growth) must be taken into account when calculating rations.

Feeds with goitrogenic factors

Thiocyanate-containing feeds:

• Alfalfa and sainfoin

• Increase iodine requirement by 20–30%

• Inhibit iodine uptake in the thyroid gland

Interfering substances in water and feed

Nitrate and fluorides:

• Inhibit iodine absorption in the intestine

• Especially problematic at high concentrations in drinking water

• Can increase iodine requirement by up to 50%

Feed additives:

• Aluminum hydroxide (in some mineral feeds)

• Iron sulfate

• Impair iodine absorption

More on the topic

On our topic page Feeding Horses with Stomach Problems you will find much more information about horse feeding.

Horses with stomach issues require additional support. It is important to know which feeds are suitable for your horse and which should rather be avoided.