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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.
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• 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.