Medical brain notes 44

  Thyroid Metabolic Hormones The thyroid gland, located immediately below the larynx on each side of and anterior to the trachea, is one of the largest of the endocrine glands, normally weighing 15 to 20 grams in adults. The thyroid secretes two major hormones,  thyroxine  and triiodothyronine , commonly called T 4 and T 3 , respec-tively. Both of these hormones profoundly increase the metabolic rate of the body. Complete lack of thyroid secretion usually causes the basal metabolic rate to fall 40 to 50 per cent below normal, and extreme excesses of thyroid secretion can increase the basal metabolic rate to 60 to 100 per cent above normal. Thyroid secretion is con-trolled primarily by  thyroid-stimulating hormone (TSH)  secreted by the ante-rior pituitary gland.

  Synthesis and Secretion of the Thyroid Metabolic Hormones About 93 per cent of the metabolically active hormones secreted by the thyroid gland is  thyroxine , and 7 per cent  triiodothyronine . However, almost all the thy-roxine is eventually converted to triiodothyronine in the tissues, so that both are functionally important. The functions of these two hormones are qualita-tively the same, but they differ in rapidity and intensity of action. Triiodothy-ronine is about four times as potent as thyroxine, but it is present in the blood in much smaller quantities and persists for a much shorter time than does thyroxine. Physiologic Anatomy of the Thyroid Gland.  The thyroid gland is composed, as shownin Figure 76–1, of large numbers of closed  follicles  (100 to 300 micrometers in diameter) filled with a secretory substance called  colloid  and lined with  cuboidalepithelial cells  that secrete into the interior of the follicles. The maj...

  Physiologic Functions of the Thyroid Hormones Thyroid Hormones Increase the Transcription of Large Numbers of Genes The general effect of thyroid hormone is to activate nuclear transcription of large numbers of genes (Figure 76–5). Therefore, in virtually all cells of the body, great numbers of protein enzymes, structural proteins, transport proteins, and other substances are synthesized. The net result is generalized increase in functional activity throughout the body. Most of the Thyroxine Secreted by the Thyroid Is Converted to Triiodothyronine.  Before acting on the genes to increasegenetic transcription, one iodide is removed from almost all the thyroxine, thus forming triiodothyro-nine. Intracellular thyroid hormone receptors have a very high affinity for triiodothyronine. Consequently, more than 90 per cent of the thyroid hormone mole-cules that bind with the receptors is triiodothyronine. Thyroid Hormones Activate Nuclear Receptors.  The thyroidhormone receptors...

  Thyroid Hormones Increase the Transcription of Large Numbers of Genes The general effect of thyroid hormone is to activate nuclear transcription of large numbers of genes (Figure 76–5). Therefore, in virtually all cells of the body, great numbers of protein enzymes, structural proteins, transport proteins, and other substances are synthesized. The net result is generalized increase in functional activity throughout the body. Most of the Thyroxine Secreted by the Thyroid Is Converted to Triiodothyronine.  Before acting on the genes to increasegenetic transcription, one iodide is removed from almost all the thyroxine, thus forming triiodothyro-nine. Intracellular thyroid hormone receptors have a very high affinity for triiodothyronine. Consequently, more than 90 per cent of the thyroid hormone mole-cules that bind with the receptors is triiodothyronine. Thyroid Hormones Activate Nuclear Receptors.  The thyroidhormone receptors are either attached to the DNA genetic strand...

  Thyroid Hormones Increase Cellular Metabolic Activity The thyroid hormones increase the metabolic activities of almost all the tissues of the body. The basal  metabolic rate can increase to 60 to 100 per cent above normal when large quantities of the hormones are secreted. The rate of utilization of foods for energy is greatly accelerated. Although the rate of protein syn-thesis is increased, at the same time the rate of protein catabolism is also increased. The growth rate of young people is greatly accelerated. The mental processes are excited, and the activities of most of the other endocrine glands are increased. Thyroid Hormones Increase the Number and Activity of Mito-chondria.  When thyroxine or triiodothyronine is givento an animal, the mitochondria in most cells of the animal’s body increase in size as well as number. Furthermore, the total membrane surface area of the mitochondria increases almost directly in proportion to the increased metabolic rate of the w...

  Effect of Thyroid Hormone on Growth Thyroid hormone has both general and specific effects on growth. For instance, it has long been known that thyroid hormone is essential for the metamorphic change of the tadpole into the frog. In humans, the effect of thyroid hormone on growth is manifest mainly in growing children. In those who are hypothyroid, the rate of growth is greatly retarded. In those who are hyperthyroid, excessive skeletal growth often occurs, causing the child to become con-siderably taller at an earlier age. However, the bones also mature more rapidly and the epiphyses close at an early age, so that the duration of growth and the eventual height of the adult may actually be shortened. An important effect of thyroid hormone is to promote growth and development of the brain during fetal life and for the first few years of postnatal life. If the fetus does not secrete sufficient quantities of thyroid hormone, growth and maturation of the brain both before birth and af...

  Effects of Thyroid Hormone on Specific Bodily Mechanisms Stimulation of Carbohydrate Metabolism.  Thyroid hormonestimulates almost all aspects of carbohydrate metabolism, including rapid uptake of glucose by the cells, enhanced glycolysis, enhanced gluconeogenesis, increased rate of absorption from the gastrointestinal tract, and even increased insulin secretion with its resultant secondary effects on carbohydrate metabolism. All these effects probably result from the overall increase in cellular metabolic enzymes caused by thyroid hormone. Stimulation of Fat Metabolism.  Essentially all aspects offat metabolism are also enhanced under the influence of thyroid hormone. In particular, lipids are mobilized rapidly from the fat tissue, which decreases the fat stores of the body to a greater extent than almost any other tissue element. This also increases the free fatty acid concentration in the plasma and greatly acceler-ates the oxidation of free fatty acids by the cells....