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Hormonal Control of Metabolism • the Thyroid Gland is located directly in front of the trachea below the larynx. • embedded in the thyroid are four “pea-shaped” parathyroid glands. • the primary function of the Thyroid gland is to release the hormone thyroxine (T4), a peptide hormone made from the amino acid tyrosine. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. • receptors for thyroxine are found on most cells in the body. • it increases the basal metabolic rate and oxygen consumption, especially in the heart, liver and kidney. • thus, individuals who have higher levels of T4 will oxidize sugars and other nutrients at a faster rate. • the release of T4 is controlled by a negative feedback pathway: • if the metabolic rate decreases, receptors in the hypothalamus are activated. • the hypothalamus secretes Thyroid releasing hormone (TRH), which stimulates the release of Thyroic stimulating hormone (TSH), • TSH travels in the bloodstream to the thyroid gland and initiates the release of thyroxine. • thyroxine increases metabolism by stimulating increased oxidation of glucose by somatic cells. • the higher levels of of thyroxine feed back to inhibit the release of TRH which stops the release of TSH from the pituitary. • the Parathyroid Glands are unique because they aren’t regulated by nerves or hormones. • they respond directly to changes in their environment and their job is to monitor blood calcium levels. • low calcium levels will stimulate the release of Parathyroid Hormone (PTH). • PTH acts on the kidneys, intestines and bones to increase the levels of calcium and decrease the levels of phosphate in the blood. • most of the calcium in the body is stored in bones (as caclium phosphate) and when released, calcium goes directly into the bloodstream, while phosphate is excreted in urine. • when calcium levels are high enough, they feed back and inhibit the release of PTH. • calcitonin (produced by the thyroid gland) is released when Ca+2 concentration is too high – this promotes its incorporation into bone.