Key Event Title
|Level of Biological Organization|
Key Event Components
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP|
|TPO inhib alters metamorphosis||KeyEvent|
|NIS inhib alters metamorphosis||KeyEvent|
|IYD inhib alters metamorphosis||KeyEvent|
|Pendrin inhib alters metamorphosis||KeyEvent|
|DUOX inhib alters metamorphosis||KeyEvent|
|Hepatic nuclear receptor activation alters metamorphosis||KeyEvent|
|African clawed frog||Xenopus laevis||Moderate||NCBI|
Key Event Description
Thyroxine (T4) uptake from serum into tissues plays a substantial role in thyroid hormone action, as it can then be available for enzymatic conversion to the active hormone, triiodothyronine (T3). Uptake of T4 into cells/tissues is mediated by active transport proteins that exhibit unique expression profiles depending on tissue type and timing of development (Hennemann et al., 2001; Visser et al., 2011, 2007; Connors et al., 2010; Friesema et al., 2005). Specific regulation of transporter profiles plays a role in timing of thyroid hormone uptake into specific tissues for proper sequencing of development and protection against metabolism and clearance of thyroid hormone during critical developmental periods. Although several different classes of proteins have been identified as capable of transporting T4 (and T3) across plasma membranes, three proteins in particular have been shown to have high affinity and specificity toward thyroid hormone; namely, MCT8, MCT10 and OATP1c1 (Friesema et al., 2003, 2008; Pizzagalli et al., 2002; Jansen et al., 2007; Mayer et al., 2014). These transport proteins have primarily been studied in relation to mammalian brain development, so the details of their role in other species and tissues during vertebrate development are not well-understood.
Decreases in tissue T4 could potentially occur in several ways individually or in combination, (1) circulating levels of T4 decrease to critical levels that even compensatory increases in active transport cannot overcome, (2) TH-specific transporters are non-functional either due to mutation, inhibited by a xenobiotic or their transcriptional expression is repressed, (3) enhanced T4 catabolism by type III deiodinase in peripheral tissues or by phase II metabolic enzymes in the liver.
How It Is Measured or Detected
Methods that have been previously reviewed and approved by a recognized authority should be included in the Overview section above. All other methods, including those well established in the published literature, should be described here. Consider the following criteria when describing each method: 1. Is the assay fit for purpose? 2. Is the assay directly or indirectly (i.e. a surrogate) related to a key event relevant to the final adverse effect in question? 3. Is the assay repeatable? 4. Is the assay reproducible?
Tissues typically contain low levels of thyroid hormones and are generally complex sample matrices, making analyses of thyroid hormones in tissues challenging. This requires special consideration for the composition of the specific tissue in order to employ the proper sample preparation technique. Ideally, analyses of tissue TH is a two-step process starting with extraction and then subsequent analyses. Extraction procedures for thyroid hormones, their precursors and analogues have been demonstrated in whole body homogenates of fish or tadpoles using ethanol extractions, tadpole and rat thyroid gland tissue using proteolytic digestions and rat peripheral tissues using methanol-chloroform extractions (see reference table:  for details). Analyses of sample extracts have been performed using radioimmunoassay (RIA), liquid-chromatography inductively coupled mass spectrometry (HPLC-ICP/MS) and liquid chromatography tandem mass spectrometry (UPLC/HPLC-MS/MS). These methods generally require addition of radioisotope labeled compounds (RIA) or stable isotope labeled compounds (LC-MS/MS) to samples prior to the extraction procedure in order to correct for recovery, so the extraction and analysis steps are not entirely independent of each other. Another consideration for the technique employed to measure thyroid hormones is that RIA methods are only capable of measuring a single analyte at a time and is typically either T3 or T4 whereas LC-ICP/MS and LC-MS/MS methods allow quantitation of thyroid hormones, precursors and metabolites all in the same sample (see reference table:  for details).
Domain of Applicability
Ackermans, M.T., Kettelarij‐Haas, Y., Boelen, A. and Endert, E., 2012. Determination of thyroid hormones and their metabolites in tissue using SPE UPLC‐tandem MS. Biomedical Chromatography, 26(4), pp.485-490.
Bastian, T.W., Prohaska, J.R., Georgieff, M.K. and Anderson, G.W., 2010. Perinatal iron and copper deficiencies alter neonatal rat circulating and brain thyroid hormone concentrations. Endocrinology, 151(8), pp.4055-4065.
Bastian, T.W., Anderson, J.A., Fretham, S.J., Prohaska, J.R., Georgieff, M.K. and Anderson, G.W., 2012. Fetal and neonatal iron deficiency reduces thyroid hormone-responsive gene mRNA levels in the neonatal rat hippocampus and cerebral cortex. Endocrinology, 153(11), pp.5668-5680.
Bastian, T.W., Prohaska, J.R., Georgieff, M.K. and Anderson, G.W., 2013. Fetal and neonatal iron deficiency exacerbates mild thyroid hormone insufficiency effects on male thyroid hormone levels and brain thyroid hormone-responsive gene expression. Endocrinology, 155(3), pp.1157-1167.
Crane, H.M., Pickford, D.B., Hutchinson, T.H. and Brown, J.A., 2004. Developmental changes of thyroid hormones in the fathead minnow, Pimephales promelas. General and comparative endocrinology, 139(1), pp.55-60.
Donzelli, R., Colligiani, D., Kusmic, C., Sabatini, M., Lorenzini, L., Accorroni, A., Nannipieri, M., Saba, A., Iervasi, G. and Zucchi, R., 2016. Effect of Hypothyroidism and Hyperthyroidism on Tissue Thyroid Hormone Concentrations in Rat. European thyroid journal, 5(1), pp.27-34.
ESCOBAR, G.M.D., Pastor, R., Obregón, M.J. and REY, F.E.D., 1985. Effects of Maternal Hypothyroidism on the Weight and Thyroid Hormone Content of Rat Embryonic Tissues, before and after Onset of Fetal Thyroid Function*. Endocrinology, 117(5), pp.1890-1900.
Gilbert, M.E., Hedge, J.M., Valentín-Blasini, L., Blount, B.C., Kannan, K., Tietge, J., Zoeller, R.T., Crofton, K.M., Jarrett, J.M. and Fisher, J.W., 2013. An animal model of marginal iodine deficiency during development: the thyroid axis and neurodevelopmental outcome. toxicological sciences, p.kfs335.
Hornung, M.W., Kosian, P.A., Haselman, J.T., Korte, J.J., Challis, K., Macherla, C., Nevalainen, E. and Degitz, S.J., 2015. In vitro, ex vivo, and in vivo determination of thyroid hormone modulating activity of benzothiazoles. Toxicological Sciences, 146(2), pp.254-264.
Kunisue, T., Fisher, J.W., Fatuyi, B. and Kannan, K., 2010. A method for the analysis of six thyroid hormones in thyroid gland by liquid chromatography–tandem mass spectrometry. Journal of Chromatography B, 878(21), pp.1725-1730.
Kunisue, T., Fisher, J.W. and Kannan, K., 2011. Determination of six thyroid hormones in the brain and thyroid gland using isotope-dilution liquid chromatography/tandem mass spectrometry. Analytical chemistry, 83(1), pp.417-424.
Lavado-Autric, R., Calvo, R.M., de Mena, R.M., de Escobar, G.M. and Obregon, M.J., 2012. Deiodinase activities in thyroids and tissues of iodine-deficient female rats. Endocrinology, 154(1), pp.529-536.
Pinna, G., Hiedra, L., Prengel, H., Broedel, O., Eravci, M., Meinhold, H. and Baumgartner, A., 1999. Extraction and quantification of thyroid hormones in selected regions and subcellular fractions of the rat brain. Brain Research Protocols, 4(1), pp.19-28.
Simon, R., Tietge, J., Michalke, B., Degitz, S. and Schramm, K.W., 2002. Iodine species and the endocrine system: thyroid hormone levels in adult Danio rerio and developing Xenopus laevis. Analytical and bioanalytical chemistry, 372(3), pp.481-485.
Saba, A., Donzelli, R., Colligiani, D., Raffaelli, A., Nannipieri, M., Kusmic, C., Dos Remedios, C.G., Simonides, W.S., Iervasi, G. and Zucchi, R., 2014. Quantification of thyroxine and 3, 5, 3′-triiodo-thyronine in human and animal hearts by a novel liquid chromatography-tandem mass spectrometry method. Hormone and Metabolic Research, 46(09), pp.628-634.
Tietge, J.E., Butterworth, B.C., Haselman, J.T., Holcombe, G.W., Hornung, M.W., Korte, J.J., Kosian, P.A., Wolfe, M. and Degitz, S.J., 2010. Early temporal effects of three thyroid hormone synthesis inhibitors in Xenopus laevis. Aquatic Toxicology, 98(1), pp.44-50.
Tietge, J.E., Degitz, S.J., Haselman, J.T., Butterworth, B.C., Korte, J.J., Kosian, P.A., Lindberg-Livingston, A.J., Burgess, E.M., Blackshear, P.E. and Hornung, M.W., 2013. Inhibition of the thyroid hormone pathway in Xenopus laevis by 2-mercaptobenzothiazole. Aquatic toxicology, 126, pp.128-136.