The thyroid gland is situated in the neck, just in front of the windpipe or trachea (see diagram 16.5). It produces the hormone thyroxine, which influences the rate of growth and development of young animals. In mature animals it increases the rate of chemical reactions in the body.
Thyroxine consists of 60% iodine and too little in the diet can cause goitre, an enlargement of the thyroid gland. Many inland soils in New Zealand contain almost no iodine so goitre can be common in stock when iodine supplements are not given. To add to the problem, chemicals called goitrogens that occur naturally in plants like kale that belong to the cabbage family, can also cause goitre even when there is adequate iodine available.
Diagram 16.5 - The thyroid and parathyroid glands