Diagram of steps in thyroid hormone synthesis. This diagram depicts two follicle cells: one in the process of thyroglobulin synthesis (on the left) and the other in the process of thyroglobulin resorption (on the right). The numbers, which are described more fully in the text, indicate the sequential steps that occur: 7, synthesis and secretion of thyroglobulin; 2, uptake and concentration of iodide from the blood, oxidation to iodine, and release into the colloid; 3, iodination of thyroglobulin in the colloid; 4, formation of T3 and T„ hormones in the colloid by oxidative coupling reactions; 5, resorption of colloid by receptor-mediated endocytosis; and 6, release of T, and T3 from the cell into the circulation.
An extensive network of fenestrated capillaries derived from the superior and inferior thyroid arteries surrounds the follicles. Blind-ended lymphatic capillaries are present in the interfollicular connective tissue and may also provide a second route for conveying the hormones from the gland.
Thyroid gland function is essential to normal growth and development
The thyroid gland produces three hormones, each of which is essential to normal metabolism and homeostasis (Table 20.7):
• Thyroxine (tetraiodothyronine, TJ and triiodothyronine (T3) are synthesized and secreted by the follicular
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