Thyroid gland fulfills two functions. On one hand, it synthesizes and builds up stocks of thyroid hormones in thyroglobulin molecules of the colloid in its follicles, such as they can maintain the hormonal secretion during several days and even weeks. To do this, it captures and concentrates plasma iodide through a specific membrane transporter and it oxidizes iodide through the action of thyroperoxidase and H2O2. This makes it able to bind to tyrosine residus of thyroglobulin. Then, the iodotyrosines can form the thyroid hormones (T4 and T3) by a coupling reaction. On the other hand, thyroid secretes the hormones after internalization and proteolysis of thyroglobulin. All the steps of synthesis and secretion are regulated by pituitary TSH, through a negative feed-back action of T4 and T3. Thus, any increase or decrease of circulating thyroid hormones induces the opposite modification of TSH. In addition, an important fraction of plasma and tissue T3 is produced through the extrathyroidal monodeiodination of T4 by enzymes (5' deiodases) which are regulated by the nutritional status and by thyroid hormones.