Iodine is probably the most ignored of all elements essential for your horse’s life. The requirement is very small, only a few mg a day, but deficiency in foods is widespread all over the world.
An iodine requirement is our legacy from the sea.
When life left the iodine-rich oceans for land, reliance on iodine came with it. Iodine is a volatile element and levels in soils decline with exposure to weather, leading to significant deficiencies in many areas on earth. Fish, shellfish and seaweed are good sources of iodine but most land-based foods are low. What is available concentrates in foods essential to new life – eggs, milk/dairy, some beans and grains.
Whether a concentrater food or not, iodine levels are still heavily dependent on soil content. Beans and grains grown in deficient areas will also be very low in iodine. Sufficiency can only be assumed for foods grown close to coastal areas.
The body uses iodine to make thyroid hormones with each molecule of thyroxine (T4) containing 4 iodine and each triiodothyronine (T3) having three. Most of the body’s iodine is concentrated in the thyroid gland. Although details remain unknown, iodine is also required for normal immune responses.
The most obvious external sign of iodine deficiency is goiter – a swelling of the thyroid gland. The horse’s thyroid sits high in the neck, straddling the trachea (windpipe). When there is iodine deficiency, the gland enlarges to allow it to trap more iodine.
Pregnant mares and growing horses are the most sensitive to iodine deficiency and have higher requirements. In addition to goiter, signs include reduced fertility, abortion, prolonged gestation in mares. Foals are born weak with tendon and bone deformities, hernias, poor muscling, altered mental status and may need to be euthanized. Hypothyroid adults show lethargy, low heart rate and poor coat with delayed shedding.
Interestingly enough, excessive iodine intake also leads to goiter and hypothyroidism. This is called the Wolff-Chaikoff effect. The details of the mechanism are unknown but believed to be related to high iodine causing suppression of enzyme systems in the thyroid. In addition to the hypothyroidism signs above, excess iodine causes heavy tear formation and skin rashes.
The average size adult nonpregnant horse needs 4.5 to 5 mg of iodine/day. Pregnant, exercising and heavily lactating animals need at least 1 to 1.5 mg more. Feed and supplement manufacturers use highly concentrated forms of iodine like EDDI to manufacture iodine supplemented products in bulk but owners supplementing iodine for individual animals also have access to seaweed based supplements with a known iodine content. It is NOT advisable to feed seaweed with an unknown iodine level as toxicity can easily occur.
Tiny requirement but huge effects for iodine. Make sure it isn’t overlooked in your horse’s diet assessment.
Eleanor Kellon, VMD