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'''Thyroid's secretory capacity''' (GT, also referred to as thyroid's incretory capacity, maximum thyroid hormone output or SPINA-GT) is the maximum stimulated amount of [[thyroxine]] that the [[thyroid]] can produce in a given time-unit (e.g. one second).<ref name=dietrich2002>{{Cite book
|publisher = Logos-Verlag Berlin
|isbn = 978-3-89722-850-4
|publication-place = Berlin, Germany
|title = Der Hypophysen-Schilddrüsen-Regelkreis
|url = http://openlibrary.org/books/OL24586469M/Der_Hypophysen-Schilddrüsen-Regelkreis
|author = Dietrich, J. W.
|publication-date = 2002
|oclc = 50451543
|id = 3897228505
|postscript = {{inconsistent citations}}
}}</ref>

Experimentally, GT is determined by stimulating the thyroid with high thyrotropin concentration and measuring its output.

''In vivo'', GT can also be estimated from equilibrium levels of TSH and T4 or free T4. In this case it is calculated with

<math>\hat G_T = {{\beta _T (D_T + [TSH])(1 + K_{41} [TBG] + K_{42} [TBPA])[FT_4 ]} \over {\alpha _T [TSH]}}</math>

or

<math>\hat G_T = {{\beta _T (D_T + [TSH])[TT_4 ]} \over {\alpha _T [TSH]}}</math>

<math>\hat G_T </math>: Theoretical (apparent) secretory capacity 
<math>\alpha _T</math>: [[Dilution]] factor for T4 (reciprocal of apparent volume of distribution, 0.1 l−1) 
<math>\beta _T</math>: [[Clearance (medicine)|Clearance]] exponent for T4 (1.1e-6 sec−1) 
''K''41: [[Dissociation constant]] T4-TBG (2e10 l/mol) 
''K''42: Dissociation constant T4-TBPA (2e8 l/mol) 
''D''''T'': EC50 for TSH (2.75 mU/l)<ref name=dietrich2002/><ref>Dietrich JW, Stachon A, Antic B, Klein HH, Hering S. The AQUA-FONTIS study: protocol of a multidisciplinary, cross-sectional and prospective longitudinal study for developing standardized diagnostics and classification of non-thyroidal illness syndrome. BMC Endocr Disord. 2008 Oct 13;8:13. PMID 18851740.</ref>

{| class="wikitable"
|'''Lower limit'''||'''Upper limit'''||'''Unit'''
|-
| 1.41<ref name=dietrich2002/> || 8.67<ref name=dietrich2002/> || pmol/s
|-
|}

== Clinical significance ==
GT is elevated in [[hyperthyroidism]] and reduced in [[hypothyroidism]]<ref name=dietrich1999>Dietrich, J., M. Fischer, J. Jauch, E. Pantke, R. Gärtner und C. R. Pickardt (1999). "SPINA-THYR: A Novel Systems Theoretic Approach to Determine the Secretion Capacity of the Thyroid Gland." European Journal of Internal Medicine 10, Suppl. 1 (5/1999): S34.</ref><ref>Dietrich JW. [Thyroid storm]. Med Klin Intensivmed Notfmed. 2012 Sep;107(6):448-53. Epub 2012 Aug 11. German. PMID 22878518</ref> and it has been observed to correlate with thyroid volume.<ref name=dietrich2002/> In longitudinal evaluation GT shows lower intraindividual variation than [[Thyroid-stimulating hormone|TSH]], [[free thyroxine|FT4]] or [[Triiodothyronine|FT3]].<ref>Dietrich JW, Landgrafe, G, Fotiadou, EH. [http://www.hindawi.com/journals/jtr/2012/351864/ TSH and Thyrotropic Agonists: Key Actors in Thyroid Homeostasis] Journal of Thyroid Research, vol. 2012, Article ID 351864, 29 pages, 2012. [[doi:10.1155/2012/351864]]. PMID 23365787</ref>
Correlation of GT with [[creatinine clearance]] suggested a negative influence of [[Uremia|uremic]] toxins on thyroid biology.<ref>Rosolowska-Huszcz D, Kozlowska L, Rydzewski A. Influence of low protein diet on nonthyroidal illness syndrome in chronic renal failure. Endocrine. 2005 Aug;27(3):283-8. PMID 16230785.</ref> In the initial phase of major [[Euthyroid sick syndrome|non-thyroidal illness]] GT may be temporarily elevated.<ref name="Liu2012">{{cite journal | pmid = 22874844 | title=Nonthyroidal Illness Syndrome: ist it Far Away From Crohn's Disease? | year=2013 | author=Liu S, Ren J, Zhao Y, Han G, Hong Z, Yan D, Chen J, Gu G, Wang G, Wang X, Fan C, Li J | journal=J Clin Gastroenterol. | volume = 47 | issue = 2 | pages = 153–9 | doi = 10.1097/MCG.0b013e318254ea8a}}</ref>

==See also==
* [[Thyroid function tests]]
* [[Sum activity of peripheral deiodinases]]

== External links ==
* [http://spina.sf.net SPINA Thyr: Open source software for calculating GT and GD]

==References==
{{reflist}}

[[Category:Chemical pathology]]
[[Category:Blood tests]]
[[Category:Endocrine procedures]]
[[Category:Thyroidological methods]]

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