Iodine
Iodine is an important component of thyroid hormones and thereby is involved in regulation of metabolic rate. Feedstuffs grown in low-iodine soil, as in the case of sandy areas, are deficient in iodine. Also, canola or rapeseed may contain increased levels of goitrogenic compounds called glucosinolates that interfere with iodine metabolism. Iodine deficiency is characterized by goiter (thyroid enlargement), lethargy, and low growth rate.