Summary of Thyroid Hormones
Primary Information, Benefits, Effects, and Important Facts
Thyroid hormones are a class of hormones secreted from the thyroid gland which regulate a wide variety of mechanisms, usually divided between genomic effects (those mediated through the genome and DNA transcription) and those that are not.
It is most well known for, in higher than normal levels, increasing the basal metabolic rate and inducing fat loss.
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Structure and Origin In vivo
Molecules known as Thyronines
There are three main Thyroid hormones that exert biological activity; T3 (3',5,3-L-triiodothyronine), T4 or Thyroxine (3',5',3,5-L-tetraiodothyronine) and T2 (3,5',-L-diiothyronine). The thyronine molecules known as reverse T3 (rT3, or 3',5',3-L-triiodothyronine) and 3,5,-diiodothyronine also exert some thyroid-like effects.
Synthesis of thyroid hormones is the only known bodily function of dietary iodine.
Thyroid hormones are synthesized in the thyroid gland of mammals. The cells of the thyroid gland take up the essential mineral iodine and form a protein known as thyroglobulin with the amino acid L-Tyrosine.
Systemic Transport and Transformation
T3 and T4 are hydrophobic molecules, and as such require some manner of vehicle to travel in the blood. They tend to be more soluble in alkaline solutions.
Thyroid hormones are typically regulated via free thyroid hormones and protein bound thyroid hormones, with the former regulated in a picomolar concentration and the latter in a greater (nanomolar) concentration.
Free versus bound Thyroid Hormones
The circulating levels of free thyroid hormones are generally seen as causative of the effects of thyroid hormone. The average level of overall (protein bound + free) thyroid hormones in adult vertabraes is about 91.6-115 nanomoles for T4 and 1.9-2.2 nanomoles for T3 while the average levels of free thyroid hormones is 40-50 picomoles (with ranges of 14.3-16.4pM and 4.1-6.4pM for T4 and T3 respectively). This gives a free thyroid hormone content of 0.01-0.1% on average.
Somebody classified as euthyroidic would have a T4 range of 60-140nM, a T3 range of 1-3nM, free T4 of 10-25pM and free T3 of 3-8pM.
Scientific Support & Reference Citations
Thyroid hormones and their effects: a new perspective.
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Metabolic pathways of tetraidothyronine and triidothyronine production by thyroid gland: a review of articles.
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Nongenomic actions of thyroid hormone.
Davis PJ, Leonard JL, Davis FB.
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Front Neuroendocrinol. (2008)
ROBBINS J, RALL JE.
The interaction of thyroid hormones and protein in biological fluids.
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Endocr Rev. (1989)
Franklyn JA, Ramsden DB, Sheppard MC.
The influence of age and sex on tests of thyroid function.
Ann Clin Biochem. (1985)
Guidelines for diagnosis and monitoring of thyroid disease: nonthyroidal illness.
Clin Chem. (1996)
Cite this page
"Thyroid Hormones," Examine.com, published on 2 July 2013, last updated on
29 April 2017,