Upstream eventThyroperoxidase, Inhibition
T4 in serum, Decreased
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Inhibition of Thyroperoxidase and Subsequent Adverse Neurodevelopmental Outcomes in Mammals||non-adjacent||High||Moderate|
|Thyroperoxidase inhibition leading to reduced young of year survival via anterior swim bladder inflation||non-adjacent|
|Xenopus laevis||Xenopus laevis||High||NCBI|
Life Stage Applicability
|All life stages||High|
Key Event Relationship Description
TPO is the enzyme that catalyzes iodine organification of thyroglobulin to produce Tg-bound T3 and T4 in the lumen of thyroid follicles. TG-bound THs are endocytosed across the apical lumen-follicular cell membrane, undergo thyroglobulin proteolysis, followed by hormone section into the blood stream (see Taurog, 2005 for review). This indirect KER describes the relationship of TPO inhibition to reduced circulating levels of thyroid hormone (TH) in the serum.
Evidence Supporting this KER
The weight of evidence linking thyroperoxidase inhibition to reductions in circulating serum TH is strong. Many studies support this basic linkage. There is no inconsistent data.
It is a well-accepted fact that inhibition of the only enzyme capable of synthesizing THs, TPO, results in subsequent decrease in serum TH concentrations. A large amount of evidence from clinical and animal studies clearly support the commonly accepted dogma that inhibition of TPO leads to decreased serum THs.
The majority of research in support of this KER involve exposure to known TPO inhibitors and measurement of serum hormones. There are a many in vivo studies that link decreases in serum TH concentrations with exposure to xenobiotics that inhibit thyroperoxidase (TPO) (Brucker-Davis, 1998; Hurley, 1998; Boas et al., 2006; Crofton, 2008; Kohrle, 2008; Pearce and Braverman, 2009; Murk et al., 2013).
While these studies support the connection between exposure to a known TPO inhibitor and decreased TH, many of these studies do not empirically measure TPO inhibition or decreased TH synthesis. Thus, many studies support the indirect linkage between TPO inhibition (for chemicals identified as TPO inhibitors in in vivo or ex vivo studies) and decreased TH, with the well accepted theory that this proceeds via decreased TH synthesis. That exposure to TPO inhibitors leads to decreased serum TH concentrations, via decreased TH synthesis is strongly supported by decades of mechanistic research in a variety of species.
This indirect relationship is also evidenced by the use of clinically-relevant anti-hyperthyroidism drugs, MMI and PTU (Laurberg & Anderson, 2014; Sundaresh et al., 2013). These drugs are both recognized TPO inhibitors and are part of a standard drug-based regimen of care for clinically hyperthyroid patients including those with Grave's disease. Serum THs are measured as the bioindicator of successful treatment with anti-hyperthyroidism drugs; the actual decrease in TH synthesis in the thyroid gland is implied in the efficacious use of these drugs (Trepanier, 2006).
In rats, MMI and PTU are often used as control chemicals to decrease serum THs to study biological phenomena related to disruption of TH homeostasis (many examples, including Zoeller and Crofton, 2005; Morreale de Escobar et al, 2004; Schwartz et al., 1997; Herwig et al., 2014; Wu et al., 2013; Pathak et al., 2011). Further, MMI is recommended as a positive control for use in the Amphibian Metamorphosis (Frog) Assay within Tier 1 of the U.S. EPA Endocrine Disruptor Screening Program (US EPA, 2009; Coady et al., 2010), an assay used to evaluate the potential for chemicals to disrupt TH homeostasis. PTU has been suggested a positive control chemical in the guidance for the Comparative Developmental Thyroid Assay (US EPA, 2005), a non-guideline assay used to evaluate the potential for chemicals to disrupt TH homeostasis during gestation and early neonatal development.
Thus, an indirect key event relationship between TPO inhibition and decreased serum THs is strongly supported by a large database of clinical medicine and investigative research with whole animals (with a great deal of supporting evidence in rats and frogs).
Temporal Evidence: The temporal nature of this KER is applicable to all life stages, including development (Seed et al., 2005). The qualitative impact of thyroperoxidase inhibition on serum hormones is similar across all ages. The temporal nature of the impact on serum THs by TPO inhibitors in developmental exposure studies is evidenced by the duration of exposure and developmental age (Goldey et al., 1995; Ahmed et al., 2010; Tietge et al., 2010), as well as recovery after cessation of exposure (Cooke et al., 1993; Goldey et al., 1995; Sawin et al., 1998; Axelstad et al., 2008; Shibutani et al., 2009; Lasley and Gilbert, 2011). The temporal relationship between TPO inhibitor exposure duration and serum hormone decreases in adult organisms has been widely demonstrated (e.g., Hood et al., 1999; Mannisto et al., 1979). In addition, MMI and PTU induced decreases in serum T4 are alleviated by thyroid hormone replacement in both fetal and postnatal age rats (Calvo et al., 1990; Sack et al., 1995; Goldey and Crofton, 1998). Computational modeling of the thyroid also provides evidence for the indirect temporal relationship between these two KEs (e.g., Degon et al., 2008; Fisher et a., 2013).
Dose-Response Evidence: Empirical data is available from enough studies in animals treated with TPO inhibitors during development to make it readily accepted dogma that a dose-response relationship exists between TPO inhibition and serum TH concentrations. Again, these studies do not empirically measure TPO inhibition or decreased TH synthesis, but rely on the strong support of decades of mechanistic research in a variety of species of the causative relationship between these KEs. Examples of dose-responsive changes in TH concentrations following developmental exposure to TPO inhibitors include studies a variety of species, including: rodents (Blake and Henning, 1985; Goldey et al., 1995; Sawin et al., 1998); frogs (Tietge et al., 2013); fish tissue levels (Elsalini and Rohr, 2003.); and, chickens (Wishe et al., 1979). Computational modeling of the thyroid also provides evidence for the indirect dose-response relationship between these two KEs (e.g., Leonard et al., 2016; Fisher et a., 2013).
Uncertainties and Inconsistencies
There are no inconsistencies in this KER, but there are some uncertainties. The predominant uncertainty regarding the indirect key event relationship between inhibition of TPO activity and decreased serum T4 is the quantitative nature of this relationship, i.e., to what degree must TPO be inhibited in order to decrease serum T4 by a certain magnitude. Many animal (rat) studies typically employ relatively high exposures of TPO-inhibiting chemicals that result in hypothyroidism (severe decrements in T4 and T3). Thus, a dose-response relationship between TPO inhibition and decreased serum T4 is not typically defined. However, there are numerous publications demonstrating clear dose- and duration- dependent relationships between TPO inhibitors dose and reduced serum T3 and T4 in rodent models (see for example: Cooper et al., 1983; Hood et al., 1999; Goldey et al., 2005; Gilbert, 2011). The relationship between maternal and fetal levels of hormone following chemically-induced TPO inhibiton has not been well characterized and may differ based on kinetics. Reductions in serum TH in the fetus, in rat and human is derived a chemical’s effect on the maternal thyroid gland as well as the fetal thyroid gland.
Quantitative Understanding of the Linkage
The indirect linkage between exposure to known TPO inhibitors and decreased serum TH has not been defined quantitatively. The two key event relationships that mediate this relationship (TPO inhibition leading to decreased TH synthesis, and decreased TH synthesis leading to decreased serum TH) have been incorporated into some quantitative models. A quantitative biologically-based dose-response model for iodine deficiency in the rat includes relationships between thyroidal T4 synthesis and serum T4 concentrations in developing rats Fisher et al. (2013). Ekerot et al. (2012) modeled TPO, T3, T4 and TSH in dogs and humans based on exposure to myeloperoxidase inhibitors that also inhibit TPO and was has recently adapted for rat (Leonard et al., 2016). While the original model predicted serum TH and TSH levels as a function of oral dose, it was not used to explicitly predict the relationship between serum hormones and TPO inhibition, or thyroidal hormone synthesis. Leonard et al. (2016) recently incorporated TPO inhibition into the model. Degon et al (2008) developed a human thyroid model that includesTPO but does not make quantitative prediction of organification changes due to inhibition of the TPO enzyme.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Use of TPO inhibitors as anti-hyperthyroidism drugs, in humans and pets (Emiliano et al., 2010; Trepanier, 2006), in amphibian and avian species (Coady et al., 2010; Grommen et al., 2011; Rosebrough et al., 2006; Tietge et al., 2012), demonstrate decreased serum TH concentrations in vivo in rats (US EPA, 2005) and strongly supports a causative linkage between inhibition of TPO and decreased serum T4 across species.
Ahmed OM, Abd El-Tawab SM, Ahmed RG. Effects of experimentally induced maternal hypothyroidism and hyperthyroidism on the development of rat offspring: I. The development of the thyroid hormones-neurotransmitters and adenosinergic system interactions. Int J Dev Neurosci. 2010 28(6):437-54
Axelstad M, Hansen PR, Boberg J, Bonnichsen M, Nellemann C, Lund SP, Hougaard KS, Hass U. Developmental neurotoxicity of propylthiouracil (PTU) in rats: relationship between transient hypothyroxinemia during development and long-lasting behavioural and functional changes. Toxicol Appl Pharmacol. 2008 232(1):1-13.
Blake HH, Henning SJ. Effect of propylthiouracil dose on serum thyroxine, growth, and weaning in young rats. Am J Physiol. 1985 248(5 Pt 2):R524-30.
Boas M, Feldt-Rasmussen U, Skakkebaek NE, Main KM. Environmental chemicals and thyroid function. Eur J Endocrinol. 2006 154:599-611.
Brucker-Davis F. Effects of environmental synthetic chemicals on thyroid function. Thyroid. 1998 8:827-56.
Calvo R, Obregón MJ, Ruiz de Oña C, Escobar del Rey F, Morreale de Escobar G. Congenital hypothyroidism, as studied in rats. Crucial role of maternal thyroxine but not of 3,5,3'-triiodothyronine in the protection of the fetal brain. J Clin Invest. 1990 Sep;86(3):889-99.
Coady K, Marino T, Thomas J, Currie R, Hancock G, Crofoot J, McNalley L, McFadden L, Geter D, Klecka G. 2010. Evaluation of the amphibian metamorphosis assay: exposure to the goitrogen methimazole and the endogenous thyroid hormone L-thyroxine. Environmental toxicology and chemistry / SETAC. Apr;29:869-880.
Cooke PS, Kirby JD, Porcelli J. Increased testis growth and sperm production in adult rats following transient neonatal goitrogen treatment: optimization of the propylthiouracil dose and effects of methimazole. J Reprod Fertil. 1993 97(2):493-9
Cooper DS, Kieffer JD, Halpern R, Saxe V, Mover H, Maloof F, Ridgway EC (1983) Propylthiouracil (PTU) pharmacology in the rat. II. Effects of PTU on thyroid function. Endocrinology 113:921-928.
Crofton KM Thyroid disrupting chemicals: mechanisms and mixtures. Int J Androl. 2008 31:209-23
Degon, M., Chipkin, S.R., Hollot, C.V., Zoeller, R.T., and Chait, Y. (2008). A computational model of the human thyroid. Mathematical Biosciences 212: 22–53.
Ekerot P, Ferguson D, Glämsta EL, Nilsson LB, Andersson H, Rosqvist S, Visser SA. Systems pharmacology modeling of drug-induced modulation of thyroid hormones in dogs and translation to human. Pharm Res. 2013 Jun;30(6):1513-24.
Fisher JW, Li S, Crofton K, Zoeller RT, McLanahan ED, Lumen A, Gilbert ME. Evaluation of iodide deficiency in the lactating rat and pup using a biologically based dose-response model. Toxicol Sci. 2013 132(1):75-86.
Elsalini OA, Rohr KB.: Phenylthiourea disrupts thyroid function in developing zebrafish. Dev Genes Evol 212, 593-8, 2003.
Emiliano, A.B., Governale, L., Parks, M., Cooper, D.S., 2010. Shifts in propylthiouracil and methimazole prescribing practices: antithyroid drug use in the United States from 1991 to 2008. J. Clin. Endocrinol. Metab. 95, 2227–2233.
Gilbert ME. 2011. Impact of low-level thyroid hormone disruption induced by propylthiouracil on brain development and function. Toxicol. Sci. 124:432-445.
Goldey ES, Kehn LS, Rehnberg GL, Crofton KM. Effects of developmental hypothyroidism on auditory and motor function in the rat. Toxicol Appl Pharmacol. 1995 135(1):67-76.
Goldey ES, Crofton KM. Thyroxine replacement attenuates hypothyroxinemia, hearing loss, and motor deficits following developmental exposure to Aroclor 1254 in rats. Toxicol Sci. 1998 Sep;45(1):94-105.
Grommen, S.V., Iwasawa, A., Beck, V., Darras, V.M., De Groef, B., 2011. Ontogenic expression profiles of thyroid-specific genes in embryonic and hatching chicks. Domest. Anim. Endocrinol. 40, 10–18.
Herwig A, Campbell G, Mayer CD, Boelen A, Anderson RA, Ross AW, Mercer JG, Barrett P. 2014. A thyroid hormone challenge in hypothyroid rats identifies T3 regulated genes in the hypothalamus and in models with altered energy balance and glucose homeostasis. Thyroid 24:1575-1593.
Hood A, Liu YP, Gattone VH, 2nd, Klaassen CD (1999) Sensitivity of thyroid gland growth to thyroid stimulating hormone (TSH) in rats treated with antithyroid drugs. Toxicol Sci 49:263-271.
Hurley PM. Mode of carcinogenic action of pesticides inducing thyroid follicular cell tumors in rodents. Environ Health Perspect. 1998 106:437-45.
Köhrle J. Environment and endocrinology: the case of thyroidology. Ann Endocrinol (Paris). 2008 69:116-22.
Lasley SM, Gilbert ME. Developmental thyroid hormone insufficiency reduces expression of brain-derived neurotrophic factor (BDNF) in adults but not in neonates. Neurotoxicol Teratol. 2011 33(4):464-72
Laurberg P, Andersen SL. 2014. Therapy of endocrine disease: antithyroid drug use in early pregnancy and birth defects: time windows of relative safety and high risk? Eur J Endocrinol. 2014 Jul;171(1):R13-20.
Leonard JA, Tan YM, Gilbert M, Isaacs K, El-Masri H. Estimating margin of exposure to thyroid peroxidase inhibitors using high-throughput in vitro data, high-throughput exposure modeling, and physiologically based pharmacokinetic/pharmacodynamic modeling. Toxicol Sci. 2016 151(1):57-70.
Männistö PT, Ranta T, Leppäluoto J. Effects of methylmercaptoimidazole (MMI), propylthiouracil (PTU), potassium perchlorate (KClO4) and potassium iodide (KI) on the serum concentrations of thyrotrophin (TSH) and thyroid hormones in the rat. Acta Endocrinol (Copenh). 1979 91(2):271-81.
Morreale de Escobar G, Obregon MJ, Escobar del Rey F (2004) Role of thyroid hormone during early brain development. Eur J Endocrinol 151 Suppl 3:U25-37.
Murk AJ, Rijntjes E, Blaauboer BJ, Clewell R, Crofton KM, Dingemans MM, Furlow JD, Kavlock R, Köhrle J, Opitz R, Traas T, Visser TJ, Xia M, Gutleb AC. Mechanism-based testing strategy using in vitro approaches for identification of thyroid hormone disrupting chemicals. Toxicol In Vitro. 2013 27:1320-46.
Pathak A, Sinha RA, Mohan V, Mitra K, Godbole MM. 2011. Maternal thyroid hormone before the onset of fetal thyroid function regulates reelin and downstream signaling cascade affecting neocortical neuronal migration. Cerebral Cortex. 21:11-21.
Pearce EN, Braverman LE. Environmental pollutants and the thyroid. Best Pract Res Clin Endocrinol Metab. 2009 23:801-1.
Rosebrough, R.W., Russell, B.A., McMurtry, J.P., 2006. Studies on doses of methimazole (MMI) and its administration regimen on broiler metabolism. Comp. Biochem. Physiol. A: Mol. Integr. Physiol. 143, 35–41.
Sack J, Weller A, Rigler O, Rozin A.A simple model for studying the correction of in utero hypothyroidism in the rat. Pediatr Res. 1995 37(4 Pt 1):497-501.
Sawin S, Brodish P, Carter CS, Stanton ME, Lau C. Development of cholinergic neurons in rat brain regions: dose-dependent effects of propylthiouracil-induced hypothyroidism. Neurotoxicol Teratol. 1998 20(6):627-35
Schwartz HL, Ross ME, Oppenheimer JH (1997) Lack of effect of thyroid hormone on late fetal rat brain development. Endocrinology 138:3119-3124.
Seed J, Carney EW, Corley RA, Crofton KM, DeSesso JM, Foster PM, Kavlock R, Kimmel G, Klaunig J, Meek ME, Preston RJ, Slikker W Jr, Tabacova S, Williams GM, Wiltse J, Zoeller RT, Fenner-Crisp P, Patton DE. Overview: Using mode of action and life stage information to evaluate the human relevance of animal toxicity data. Crit Rev Toxicol. 2005 35(8-9):664-72.
Shibutani M, Woo GH, Fujimoto H, Saegusa Y, Takahashi M, Inoue K, Hirose M, Nishikawa A. Assessment of developmental effects of hypothyroidism in rats from in utero and lactation exposure to anti-thyroid agents. Reprod Toxicol. 2009 Nov;28(3):297-307
Sundaresh V, Brito JP, Wang Z, Prokop LJ, Stan MN, Murad MH, Bahn RS. 2013. Comparative effectiveness of therapies for Graves' hyperthyroidism: a systematic review and network meta-analysis. The Journal of clinical endocrinology and metabolism. 98:3671-3677.
Taurog A. 2005. Hormone synthesis. In: Werner and Ingbar’s The Thyroid: A Fundamental and Clinical Text (Braverman LE, Utiger RD, eds). Philadelphia:Lippincott, Williams and Wilkins, 47–81
Taurog, a, Dorris, M. L., & Doerge, D. R. (1996). Mechanism of simultaneous iodination and coupling catalyzed by thyroid peroxidase. Archives of Biochemistry and Biophysics, Taurog A. Molecular evolution of thyroid peroxidase. Biochimie. 1999 May;81(5):557-62
Tietge JE, Butterworth BC, Haselman JT, Holcombe GW, Hornung MW, Korte JJ, Kosian PA, Wolfe M, Degitz SJ. Early temporal effects of three thyroid hormone synthesis inhibitors in Xenopus laevis. Aquat Toxicol. 2010 Jun 1;98(1):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., Hornung, M.W., 2012. Inhibition of the thyroid hormone pathway in Xenopus laevis by 2- mercaptobenzothiazole. Aquat. Toxicol. 126C, 128–136.
Tietge JE, Degitz SJ, Haselman JT, Butterworth BC, Korte JJ, Kosian PA, Lindberg-Livingston AJ, Burgess EM, Blackshear PE, Hornung MW. Inhibition of the thyroid hormone pathway in Xenopus laevis by 2-mercaptobenzothiazole. Aquat Toxicol. 2013 15;126:128-36
Trepanier, L.A., 2006. Medical management of hyperthyroidism. Clin. Tech. Small Anim. Pract. 21, 22–28.
U.S. Environmental Protection Agency. 2005. Guidance for Thyroid Assays in Pregnant Animals, Fetuses, and Postnatal Animals, and Adult Animals. Office of Pesticide Programs, Health Effects Division, Washington, DC
U.S. Environmental Protection Agency. 2009. Endocrine Disruptor Screening Program Test Guidelines OPPTS 890.1100: Amphibian Metamorphosis (Frog). Washington, DC.
Wishe H I, Rolle-Getz G K, and Goldsmith E D.: The effects of aminotriazole (ATZ) on the thyroid gland and the development of the white leghorn chick. Growth 43, 238-251, 1979
Wu S, Tan G, Dong X, Zhu Z, Li W, Lou Z, Chai Y. 2013. Metabolic profiling provides a system understanding of hypothyroidism in rats and its application. PloS one.8:e55599.
Zoeller RT, Crofton KM (2005) Mode of action: developmental thyroid hormone insufficiency--neurological abnormalities resulting from exposure to propylthiouracil. Crit Rev Toxicol 35:771-781.