The thyroid gland
- Created by: Former Member
- Created on: 02-04-11 21:08
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The thyroid gland
- largest pure endocrine gland
- well vascularised
- located in the neck below the larynx
- has two loves joined by a thin band (isthmus)
- the only endocrine gland that stores its hormones in an extracellular compartment
- composed of more than one million spherical follicles if varying sizes
- blood vessels and lymph vessels exist between follicles
- follicles separated by basement membrane
- single layer of cells
- the follicular cells synthesise thyroxine (T4) and triiodothyronine (T3)
- these are amine hormones derived from the amino acid tyrosine
- T4 contains four iodine molecules
- T3 contains three iodine molecules
- T3 and T4 are synthesised on a backbone of thyroglobulin, a large protein that is made within the follicular cells
- synthesis of the thyroglobulin polypeptide chain, including addition of tyrosine molecules takes place in the rough endoplasmic reticulum of the cell; carbohydrate is added later in the Golgi body
- thyroglobuin is secreted into the follicular lumen where it is stored in the colloid
- the colloid is essentially a store of thyroglobulin
Iodide uptake into the follicular cells
- iodine cannot be synthesised by the body but must be obtained in the diet
- iodide is co-transported with sodium ions from the extracellular fluid into the follicular cells
- sodium ions are pumped back out of the cell by the Na/K ATPase pump, which maintains the concentration gradient
- iodide diffuses into the colloid along a concentration gradient
Iodination of thyroglobulin
- free iodine in the follicular lumen rapidly attaches itself to the tyrosine residues on the thyroglobulin molecules in a process called iodination
- iodination occurs at the apical membrane (colliod-facing) membrane and involves hydrogen peroxide and oxidation by the enzyme thyroid peroxidase
Coupling to form iodotyrosines to form T4
- thyroid peroxidase adds two iodine molecules to thyroglobulin to form diiodotyronise (DIT)
- thyroid peroxidase then joins two DITs to form T4
Coupling of iodotyrosines to form T3
- thyroid peroxidase adds one iodine molecule to thyroglobulin to form monoiodotyrosine (MIT)
- thyroid peroxidasejoints one DIT and one MIT to form T3
Secretion of thyroid hormone
- colloid and iodinated thyroglobulin is reabsorbed into the thyroid cells by endocytosis
- thyroglobulin is broken down by lysosomes containing hydrolytic enzymes to release T4 and T3
- T4 and T3 are released into the bloodstream
- the thyroid secretes mainly T4, however T4 is a relatively inactive hormone which is converted to T3 (more active form) in the peripheral tissues by iodinase enzymes
- about half of the T4 output is converted to reverse T3 which is inactive
- T4 is more stable than T3, with a longer half-life; T3 is degraded by deiodination in the tissues or by conjugation in the liver
- once secreted, over 90% of thyroid hormones are rapidly bound to plasma proteins
- 70% is bound to thyroxine binding globulin (TBG)
- 30% is bound to thyroxine binding pre-albumin (TPBA) and albumin (a general carrier protein)
- only 0.4% of T3 and 0.04% of T4 are not bound to proteins
- it is only the free fraction of T3 that is responsible for hormonal activity
Action of thyroid hormones
- action is via…
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