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Thyroid Hormones

Thyroid Hormones are a collection of three hormones: thyroid stimulating hormone (TSH) which signals production of T4 (a longer lasting and 'active storage' hormone) and T3 (quick usage and more potent). The latter two hormones are active, and increase metabolic rate.

Our evidence-based analysis on thyroid hormones features 9 unique references to scientific papers.

Research analysis led by and reviewed by the Examine team.
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Research Breakdown on Thyroid Hormones

1Structure and Origin In vivo

1.1Molecules 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).[1] 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.[2]


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.[1]

2Systemic Transport and Transformation

2.1Transport Proteins

T3 and T4 are hydrophobic molecules, and as such require some manner of vehicle to travel in the blood.[3][4] They tend to be more soluble in alkaline solutions.[1]

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.[1]

2.2Free versus bound Thyroid Hormones

The circulating levels of free thyroid hormones are generally seen as causative of the effects of thyroid hormone.[5][6][7] 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[8] 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).[1] 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.[9]



  1. ^ a b c d e Hulbert AJ. Thyroid hormones and their effects: a new perspective. Biol Rev Camb Philos Soc. (2000)
  2. ^ Mansourian AR. Metabolic pathways of tetraidothyronine and triidothyronine production by thyroid gland: a review of articles. Pak J Biol Sci. (2011)
  3. ^ Davis PJ, Davis FB. Nongenomic actions of thyroid hormone. Thyroid. (1996)
  4. ^ Davis PJ, Leonard JL, Davis FB. Mechanisms of nongenomic actions of thyroid hormone. Front Neuroendocrinol. (2008)
  5. ^ ROBBINS J, RALL JE. The interaction of thyroid hormones and protein in biological fluids. Recent Prog Horm Res. (1957)
  6. ^ Ekins R, Jackson T, Edwards P. Thyroid hormone assay. Br J Nutr. (1986)
  7. ^ Mendel CM. The free hormone hypothesis: a physiologically based mathematical model. Endocr Rev. (1989)
  8. ^ Franklyn JA, Ramsden DB, Sheppard MC. The influence of age and sex on tests of thyroid function. Ann Clin Biochem. (1985)
  9. ^ Stockigt JR. Guidelines for diagnosis and monitoring of thyroid disease: nonthyroidal illness. Clin Chem. (1996)