Thyroid gland

Topic Completed: 1 March 2009

Minor changes: 11 September 2019

Copyright: 2003-2018,, Inc.

PubMed Search: physiology thyroid

See also: Thyroid gland physiology in clinical chemistry chapter

Shahid Islam, M.D., Ph.D.
Page views in 2019: 173
Page views in 2020 to date: 104
Cite this page: Islam S. Physiology. website. Accessed July 10th, 2020.
Definition / general
  • Purpose of thyroid gland is to produce T4 and T3, which regulate metabolism, increase protein synthesis and increase oxygen consumption in all cells in body; T4 and T3 are also important for growth and development, and maturation of peripheral and central nervous system
  • Hypothalamus releases thyroid releasing factor (TRF) into hypothalamic-pituitary portal blood circulation, which travels to pituitary, which releases thyroid stimulating hormone (TSH) into blood
  • Follicular cells normally synthesize thyroglobulin and secrete it into follicular lumen
  • Thyroid peroxidase, found in apical membrane of thyroid follicular cells, catalyzes iodination of tyrosine residues on thyroglobulin molecule and coupling of iodotyrosyl residues to form T4 (thyroxine) and T3, which are still bound to thyroglobulin, making them inactive; they are then stored as colloid
  • In response to TSH, follicular cells pinocytose colloid, release the thyroglobulin, and secrete now active T4 and T3 into bloodstream
  • Body needs 100 mg of iodide per day from diet to synthesize adequate T4; iodide uptake is mediated by human sodium iodide symporter, then oxidized to iodine by iodide peroxidase, which binds to tyrosine
  • Most T4 / T3 is reversibly bound to thyroid binding globulin, which maintains levels within narrow limits
  • Free T4 / T3 enters cells, binds to nuclear receptors, increases protein synthesis and catabolism of carbohydrates and fats (basal metabolic rate)
  • Decreased serum T4 / T3 stimulates release of TRF and TSH via negative feedback regulation; elevated levels have opposite effect
  • Chronically stimulated (hyperplastic) follicular cells are tall and columnar, may be papillary
  • C cells secrete calcitonin, which lowers serum calcium by promoting bone absorption of calcium and inhibiting bone resorption by osteoclasts; in humans, has only a minor role in calcium homeostasis
  • Major role of calcitonin may be to protect skeleton during periods of calcitonin stress, such as growth, pregnancy and lactation
  • Goitrogens: suppress T4 / T3 synthesis by interfering with iodide uptake or other parts of biochemical pathways, causing an increased TSH, which causes goiter (enlargement of thyroid gland); examples include propylthiouracil, which inhibits oxidation of iodide and blocks T4 / T3 production; iodides in large doses, which inhibit thyroglobulin proteolysis; vegetables such as cabbage, turnips and cassava
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