Abstract
Iodine deficiency depletes thyroid iodine stores, inducing hypothyroidism and goiter formation. Development is retarded, particularly of the fetus, resulting in a high rate of stillbirths. In later life, hypothyroidism results in general depression of metabolism and growth. The thyroid gland weight is found to be increased after slaughter or at autopsy. The majority of farm animal feed is plant matter which usually contains little iodine (< 20-50 mu g/kg dry matter) and the iodine content of water is generally low (2-7 mu g/l). Animal feeds vary greatly in composition and water content. For comparison, feeds are either standardized to dry matter or to grain dry matter equivalents, i.e., approximately 880 g grain dry matter equivalent/kg feed. Animal nutrition societies recommend iodine supplements of 120-250 mu g/kg dry matter for growing pigs and beef cattle. This prevents iodine deficiency, facilitates high performance, maintains adequate iodine stores and sustains thyroid function. The recommendations for lactating animals are higher (500-600 mu g/kg dry matter) resulting in iodine concentrations above 100 mu g/l milk. Piglets, calves and lambs require iodine to maintain their rapid rates of growth. German feed manufacturers usually add three to seven times the recommended amounts to pig and cattle diets. Thus, the iodine status of farm animals in Germany is generally good. Deficiency has been virtually eliminated and toxicity is rare. Hypothyroidism is generally due to a combination of insufficient dietary iodine and consumption of iodine antagonists. Iodine antagonists include nitrates, nitrites, thiocyanates and the products of cyanogenic glycoside and glucosinolate degradation. Protein feeds based on rapeseed contain glucosinolates. Iodine supplements are best administered orally. Premixed vitamin and mineral supplements are added to feeds in intensive animal husbandry. Mineral blocks or iodized salt licks are used on pastureland with large numbers of roaming livestock. Administration of large doses of iodine (> 4-10 mg .iodine/kg feed) to dairy cows and laying hens greatly increases the iodine concentrations of milk and eggs. However, in growing pigs the effects on the iodine content of meat are small. Excessive iodine intake in humans can induce hyperthyroidism, and so the iodine content of milk and eggs must be controlled. In 2005 European Union legislation limited the maximum iodine content of cow and hen diets to 5 mg/kg grain dry matter equivalent (previously 10 mg/kg). This should protect consumers and presumably farm animals, but could perhaps be reduced further for pigs. This chapter reviews the effect of iodine intake on the health of animals and the food obtained from animals (e.g., milk, eggs and meat).