Iodine is necessary for the production of the hormone thyroxin by the thyroid gland. This gland regulates the rate of body metabolism and if there is a shortage of iodine in the diet a condition called goitre arises. This denotes an increase in the size of the thyroid gland. Substances called glucosinolates found in winter sown rape seeds are sometimes present in the diet and can prevent the gland using iodine.

Canola meal, an improved rape seed with low levels of glucosinolates, has enabled them to be used safely in swine diets. An iodine deficiency should be considered where large numbers of litters are born with piglets weak and hairless. With modern diets this would be rare. Iodised salt containing 0.008% of iodine provides sufficient iodine in the diet.