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The follicles in the thyroid, intake iodine and bind them onto the tyrosine rings (attached to the thyroglobulin). This produces either MIT or DIT (tyrosine rings with 1 or 2 iodines attached respectively).

Then the DIT joins with either another DIT or a MIT to form T3 or T4. This makes sense.

But it seems to me, that there could be a MIT attaching to another MIT, forming T2. Why doesn't T2 get created?

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    $\begingroup$ What makes you think that T2 doesn't exist? I did a search for "T2 thyroid hormone" and got several papers as hits ... $\endgroup$ – tyersome May 27 '20 at 4:06
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    $\begingroup$ I think you've already answered your own question. DIT = 'T2', so yes, it gets produced. $\endgroup$ – pbond May 27 '20 at 12:31
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    $\begingroup$ ok so it appears T0, T1, T2, T3, T4 actually all exist, except T0, T1, T2 are simply byproducts of synthesis and don't do anything. $\endgroup$ – John Hon May 28 '20 at 2:06
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In fact, there are two T2 hormones, depending on the position of iodine atoms on the thyronine backbone molecule [5].

One of them, 3,5-diiodothyronine (3,5-T2) is the most active of all thyroid hormones with a relative T4 potency of 8 to 15. Thereby, it is 2 to 5 times as potent as T3. Its plasma concentration is rather low, however, being 0.2 to 0.37 nmol/l [1–3]. It is weakly bound to plasma proteins and has a short half-life [7, 8]. 3,5-T2 seems to be a risk factor for atrial fibrillation [3], and it is able to suppress pituitary thyrotropin (TSH) secretion [4]. It is upregulated in critical illness (NTIS/TACITUS) [1, 3, 6].

The second isomer, 3’,5’-T2 is an inhibiting thyroid agent that is able to block the activity of deiodinases and receptors for thyroid hormones, similar to reverse-T3 (rT3) [5].

The 27 known molecules derived from the thyronine skeleton (Hoermann et al. 2015)

References

  1. Pinna G, Meinhold H, Hiedra L, Thoma R, Hoell T, Gräf KJ, Stoltenburg-Didinger G, Eravci M, Prengel H, Brödel O, Finke R, Baumgartner A. Elevated 3,5-diiodothyronine concentrations in the sera of patients with nonthyroidal illnesses and brain tumors. J Clin Endocrinol Metab. 1997 May;82(5):1535-42. PMID 9141546. https://pubmed.ncbi.nlm.nih.gov/9141546/
  2. Lehmphul I, Brabant G, Wallaschofski H, Ruchala M, Strasburger CJ, Köhrle J, Wu Z. Detection of 3,5-diiodothyronine in sera of patients with altered thyroid status using a new monoclonal antibody-based chemiluminescence immunoassay. Thyroid. 2014 Sep;24(9):1350-60. doi: 10.1089/thy.2013.0688. Epub 2014 Aug 1. PMID: 24967815. https://pubmed.ncbi.nlm.nih.gov/24967815/
  3. Dietrich JW, Müller P, Schiedat F, Schlömicher M, Strauch J, Chatzitomaris A, Klein HH, Mügge A, Köhrle J, Rijntjes E, Lehmphul I. Nonthyroidal Illness Syndrome in Cardiac Illness Involves Elevated Concentrations of 3,5-Diiodothyronine and Correlates with Atrial Remodeling. Eur Thyroid J. 2015 Jun;4(2):129-37. doi: 10.1159/000381543. PMID 26279999. https://pubmed.ncbi.nlm.nih.gov/26279999/
  4. Pietzner M, Lehmphul I, Friedrich N, Schurmann C, Ittermann T, Dörr M, Nauck M, Laqua R, Völker U, Brabant G, Völzke H, Köhrle J, Homuth G, Wallaschofski H. Translating pharmacological findings from hypothyroid rodents to euthyroid humans: is there a functional role of endogenous 3,5-T2? Thyroid. 2015 Feb;25(2):188-97. doi: 10.1089/thy.2014.0262. PMID 25343227. https://pubmed.ncbi.nlm.nih.gov/25343227/
  5. Hoermann R, Midgley JE, Larisch R, Dietrich JW. Homeostatic Control of the Thyroid-Pituitary Axis: Perspectives for Diagnosis and Treatment. Front Endocrinol (Lausanne). 2015 Nov 20;6:177. doi: 10.3389/fendo.2015.00177. PMID: 26635726; PMCID: PMC4653296. https://pubmed.ncbi.nlm.nih.gov/26635726/
  6. Langouche L, Lehmphul I, Perre SV, Köhrle J, Van den Berghe G. Circulating 3-T1AM and 3,5-T2 in Critically Ill Patients: A Cross-Sectional Observational Study. Thyroid. 2016 Dec;26(12):1674-1680. PMID 27676423. https://pubmed.ncbi.nlm.nih.gov/27676423/
  7. Louzada RA, Carvalho DP. Similarities and Differences in the Peripheral Actions of Thyroid Hormones and Their Metabolites. Front Endocrinol (Lausanne). 2018 Jul 19;9:394. doi: 10.3389/fendo.2018.00394. PMID: 30072951. https://pubmed.ncbi.nlm.nih.gov/30072951/
  8. Köhrle J, Lehmphul I, Pietzner M, Renko K, Rijntjes E, Richards K, Anselmo J, Danielsen M, Jonklaas J. 3,5-T2-A Janus-Faced Thyroid Hormone Metabolite Exerts Both Canonical T3-Mimetic Endocrine and Intracrine Hepatic Action. Front Endocrinol (Lausanne). 2020 Jan 8;10:787. doi: 10.3389/fendo.2019.00787. PMID: 31969860; PMCID: PMC6960127. https://pubmed.ncbi.nlm.nih.gov/31969860/
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