The main action of the thyroid is to secrete hormones, thyroxine and tri-iodothyronine which speed up body metabolism. They do this by increasing the usage of oxygen necessary to metabolic processes. It is as if the body fires are stoked up and burn more brightly and rapidly. The hormones are synthesised by the gland from iodine circulating in the blood, this iodine is derived from food. The thyroid acts as an iodine trap. The synthesised hormone is bound to a protein and stored as colloid in the vesicles of the gland.
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