Editing Iodothyronine deiodinase (section)

== Quantifying enzyme activity ==

''In vitro'', including [[cell culture]] experiments, deiodination activity is determined by incubating cells or [[homogenate]]s with high amounts of labeled thyroxine (T<sub>4</sub>) and required [[cosubstrate]]s. As a measure of deiodination, the production of [[radioactive iodine]] and other physiological [[metabolite]]s, in particular T<sub>3</sub> or reverse T<sub>3</sub>, are determined and expressed (e.g. as fmol/mg protein/minute).<ref name="pmid9835613">{{cite journal | vauthors = Steinsapir J, Harney J, Larsen PR | title = Type 2 iodothyronine deiodinase in rat pituitary tumor cells is inactivated in proteasomes | journal = The Journal of Clinical Investigation | volume = 102 | issue = 11 | pages = 1895–9 | date = December 1998 | pmid = 9835613 | pmc = 509140 | doi = 10.1172/JCI4672 }}</ref><ref name="pmid18259611">{{cite journal | vauthors = Simonides WS, Mulcahey MA, Redout EM, Muller A, Zuidwijk MJ, Visser TJ, Wassen FW, Crescenzi A, da-Silva WS, Harney J, Engel FB, Obregon MJ, Larsen PR, Bianco AC, Huang SA | display-authors = 6 | title = Hypoxia-inducible factor induces local thyroid hormone inactivation during hypoxic-ischemic disease in rats | journal = The Journal of Clinical Investigation | volume = 118 | issue = 3 | pages = 975–83 | date = March 2008 | pmid = 18259611 | pmc = 2230657 | doi = 10.1172/JCI32824 }}</ref>

''In vivo'', deiodination activity is estimated from [[Chemical equilibrium|equilibrium]] levels of free T<sub>3</sub> and free T<sub>4</sub>. A simple approximation is T<sub>3</sub>/T<sub>4</sub> ratio,<ref name="pmid16982586">{{cite journal | vauthors = Mortoglou A, Candiloros H | title = The serum triiodothyronine to thyroxine (T3/T4) ratio in various thyroid disorders and after Levothyroxine replacement therapy | journal = Hormones | volume = 3 | issue = 2 | pages = 120–6 | year = 2004 | pmid = 16982586 | doi = 10.14310/horm.2002.11120 }}</ref> a more elaborate approach is calculating [[sum activity of peripheral deiodinases]] (SPINA-GD) from free T<sub>4</sub>, free T<sub>3</sub> and parameters for [[protein binding]], dissociation and hormone kinetics.<ref name=dietrich2002>{{cite book | publisher = Logos-Verlag Berlin | isbn = 978-3-89722-850-4 | location = Berlin, Germany | title = Der Hypophysen-Schilddrüsen-Regelkreis | author = Dietrich JW | year = 2002 | oclc = 50451543 | id = 3897228505 | ol = 24586469M }}</ref><ref name="pmid16230785">{{cite journal | vauthors = Rosolowska-Huszcz D, Kozlowska L, Rydzewski A | title = Influence of low protein diet on nonthyroidal illness syndrome in chronic renal failure | journal = Endocrine | volume = 27 | issue = 3 | pages = 283–8 | date = August 2005 | pmid = 16230785 | doi = 10.1385/ENDO:27:3:283 | s2cid = 25630198 }}</ref><ref name="Dietrich2016">{{cite journal |last1=Dietrich |first1=JW |last2=Landgrafe-Mende |first2=G |last3=Wiora |first3=E |last4=Chatzitomaris |first4=A |last5=Klein |first5=HH |last6=Midgley |first6=JE |last7=Hoermann |first7=R |title=Calculated Parameters of Thyroid Homeostasis: Emerging Tools for Differential Diagnosis and Clinical Research. |journal=Frontiers in Endocrinology |date=2016 |volume=7 |pages=57 |doi=10.3389/fendo.2016.00057 |doi-access=free |pmid=27375554|pmc=4899439 }}</ref><ref name="Chatzitomaris2017">{{cite journal |last1=Chatzitomaris |first1=A |last2=Hoermann |first2=R |last3=Midgley |first3=JE |last4=Hering |first4=S |last5=Urban |first5=A |last6=Dietrich |first6=B |last7=Abood |first7=A |last8=Klein |first8=HH |last9=Dietrich |first9=JW |title=Thyroid Allostasis-Adaptive Responses of Thyrotropic Feedback Control to Conditions of Strain, Stress, and Developmental Programming. |journal=Frontiers in Endocrinology |date=2017 |volume=8 |pages=163 |doi=10.3389/fendo.2017.00163 |doi-access=free |pmid=28775711|pmc=5517413 }}</ref> In atypical cases, this last approach can benefit from measurements of [[Thyroxine-binding globulin|TBG]], but usually only requires measurement of TSH, fT3 and fT4, and as such has no added laboratory requirements besides the measurement of the same.