Produced by the thyroid gland in the neck, thyroid hormones regulate basal metabolic rate, oxygen uptake by tissues, foetal growth, hair growth, and energy metabolism in many tissues. The thyroid releases two hormones known as T3 Triiodothyronine, and T4 Thyroxine, both of which require iodine for their synthesis, so most of the iodine in the body is associated with the thyroid gland and these hormones.
A deficiency of iodine in the diet can cause hypothyroidism, with a characteristic enlargement of the thyroid gland known as Goiter. Foals born of iodine deficient dams may have goiter at birth and be stillborn, or if alive may be weak, dysmature and suffer flexural limb deformities and a soft silky hair coat. The mares themselves may suffer irregular oestrus cycles. Another feature of hypothyroidism is a dull rough coat and delayed shedding of the winter coat.
Deficiency may occur where soils are iodine deficient and little or no supplementary feeds are offered. Certain plants like kale, cabbage and unprocessed soybean interfere with iodine absorbtion and hence are regarded as goitrogenic. High pasture Nitrate levels may possibly also interfere with normal thyroid function and whilst these have not been shown to cause goiter, they may be implicated in dysmaturity of newborn foals via the thyroid hormones. Most commercial feeds are fortified with iodine compounds, such as iodised salt or potassium iodide, as are most mineral and trace element supplements including blocks in common use, so deficiency is fairly uncommon, and goiter is rare.
Excess dietary iodine can be equally detrimental and also cause goiter when the ration is over supplemented. One cause of this is the use of seaweed as a supplement which can have an iodine content as high as 1,800 mg/kg DM of Iodine against a daily requirement of only 3.5mg-4.0mg for an adult 500kg horse. Toxicity has been noted in mares on an intake of only 40mg/day equivalent to as little as 22 grams of seaweed, so care needs to be taken with these supplements.
Treatment revolves around ration analysis with correction of any iodine deficiency or excess. Tests for blood T3 levels can be performed with oral supplementation of thyroid hormones available if appropriate, under veterinary supervision.