Thyroid related
Thyroid physiology

Topic Completed: 1 March 2011

Revised: 31 December 2018, last major update March 2011

Copyright: (c) 2002-2016,, Inc.

PubMed Search: Thyroid physiology [title]

Nat Pernick, M.D.
Page views in 2019: 123
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Cite this page: Pernick N. Thyroid physiology. website. Accessed January 21st, 2020.
Definition / general
  • Theodor Kocher (Swiss surgeon) won Nobel Prize in 1909 for his work on thyroid physiology (Award Ceremony Speech)
  • Hypothalamus secretes thyrotropin releasing hormone (TRH), which acts on anterior pituitary to secrete thyroid stimulating hormone (TSH, thyrotropin), which increases thyroid follicular cell update of iodide and stimulates synthesis of T3 and T4
  • Free T3 and free T4 act as negative feedback on hypothalamus and pituitary
  • Thyroid cells actively take up iodide, then iodinate tyrosine residues on thyroglobulin, then form mono- and di-iodotyrosine, which are coupled to form T3 and T4
  • Thyroglobulin undergoes proteolysis to release free iodotyrosine, T4 and T3
  • Iodotyrosines are deiodinated to reuse the iodide
  • T4 is deiodinated to T3
  • Thyroid follicles contain colloid, composed primarily of thyroglobulin, the storage site for T3 and T4
  • T3 formation:
    • 80% from deiodination of T4 in nonthyroid tissue, 20% produced in thyroid gland from coupling of mono-iodotyrosine plus di-iodotyrosine (i.e. tyrosine plus 3 iodides) and deiodination of T4
    • T4 is formed from two di-iodotyrosine molecules (i.e. tyrosine plus 4 iodides)
    • 70% of T4 is bound to thyroxine binding globulin (TBG), 20% to transthyretin (formerly called prealbumin), 10% to albumin
    • T3 primarily binds to TBG, but affinity is only 10% that of T4 to TBG
    • Free components of T4 and T3 are metabolically active - 0.03% of T4 and 0.3% of T3
  • Interpretation of thyroid function tests may be difficult if:
    • Abnormal protein binding proteins
    • Nonthyroidal illness
    • Acute psychiatric illness or mediations that affect thyroid hormone binding or the hypothalamic-pituitary-thyroid axis
Diagrams / tables

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Thyroid hormones chemistry

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