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CASE FILES: BIOCHEMISTRY
B IO C H E M IS T R Y PE A R L S
The major circulating forms of thyroid hormone are thyroxine (T4),
containing four iodine atoms per molecule, and triiodothyronine
(T3), with three iodine atoms per molecule.
TSH binds a G protein-coupled receptor to activate adenylate
cyclase and trigger a signaling cascade leading to thyroid hor-
mone biosynthesis.
In developed countries, where iodine deficiency is rare because of
the use of iodized salt, autoimmune disorders are a leading
cause of thyroid disease.
Thyroid hormone receptors bind their sites on the promoter regions
of DNA in the absence of bound hormone, usually resulting in
transcriptional repression.
Binding of thyroid hormone triggers a conformational change in
the receptor converting it to a transcriptional activator.
REFERENCES
Barrett EJ. The thyroid gland. In: Boron WF, Boulpaep EL. Medical Physiology:
A Cellular and Molecular Approach. Philadelphia, PA: W.B. Saunders, 2003.
Litwack G, Schmidt TJ. Biochemistry of hormones I: polypeptide hormones. In:
Devlin TM, ed. Textbook of Biochemistry with Clinical Correlations, 5th ed.
New York: Wiley-Liss, 2002.
Pathophysiology of the endocrine system. An online textbook from Colorado State
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