Thyroid hormone deficiency in humans has a variety of causes. For example, iodide deficiency may result in a reduction in thyroid hormone production. Autoimmune diseases, such as Hashimoto's disease, impair thyroid hormone synthesis (see Clinical Focus Box 33.1). Other causes of thyroid hormone deficiency include heritable diseases that affect certain steps in the biosynthesis of thyroid hormones and hypothalamic or pituitary diseases that interfere with TRH or TSH secretion. Obviously, radioiodine ablation or surgical removal of the thyroid gland also causes thyroid hormone deficiency. Hypothyroidism is the disease state that results from thyroid hormone deficiency.
Thyroid hormone deficiency impairs the functioning of most tissues in the body. As described earlier, a deficiency of thyroid hormones at birth that is not treated during the first few months of postnatal life causes irreversible mental retardation. Thyroid hormone deficiency later in life also influences the function of the nervous system. For example, all cognitive functions, including speech and memory, are slowed and body movements may be clumsy. These changes can usually be reversed with thyroid hormone therapy.
Metabolism is also reduced in thyroid hormone-deficient individuals. Basal metabolic rate is reduced, resulting in impaired body heat production. Vasoconstriction occurs in the skin as a compensatory mechanism to conserve body heat. Heart rate and cardiac output are reduced. Food intake is reduced, and the synthetic and degradative processes of intermediary metabolism are slowed. In severe hypothyroidism, a substance consisting of hyaluronic acid and chondroitin sulfate complexed with protein is deposited in the extracellular spaces of the skin, causing water to accumulate osmotically. This effect gives a puffy appearance to the face, hands, and feet called myxedema. AH of the above disorders can be normalized with thyroid hormone therapy.
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