The Thyroid System

The thyroid gland, one of the largest endocrine glands in the body, synthesizes thyroid hormones (THs), which present a wide variety of functions affecting metabolism, growth and maturation, and other organ-specific effects. From a metabolic perspective, thyroid hormones increase oxygen consumption and heat generation. Moreover, they increase protein catabolism, promote gluconeogenesis, increase utilization of glucose and promote lipid metabolism. With regard to other organ-specific effects, thyroid hormones influence cardiac function by increasing heart rate, myocardial contractility, blood volume, and cardiac output while decreasing peripheral vascular volume. They stimulate the production of cytokines, growth factors and other factors to stimulate bone development and growth. In addition, thyroid hormones also promote increased motility in the gastrointestinal system and increase adrenergic activity and sensitivity in the central nervous system. Because they also promote cell differentiation, growth and maturation, thyroid hormones are essential in early fetal life to promote normal growth and brain development. Deficiencies and elevations in thyroid hormone levels may cause many clinical signs and symptoms. This chapter discusses the anatomy and physiology of the thyroid gland and the thyroid disorders that could be induced, especially, by toxic substances.

Keywords: Hyperthyroidism, hypothyroidism, thyroid functions, thyroid gland, toxicity.