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Relationship: 2126


A descriptive phrase which clearly defines the two KEs being considered and the sequential relationship between them (i.e., which is upstream, and which is downstream). More help

Decrease, testosterone levels leads to Decrease, DHT level

Upstream event
The causing Key Event (KE) in a Key Event Relationship (KER). More help
Downstream event
The responding Key Event (KE) in a Key Event Relationship (KER). More help

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes.Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

AOP Name Adjacency Weight of Evidence Quantitative Understanding Point of Contact Author Status OECD Status
Decreased testosterone synthesis leading to short anogenital distance (AGD) in male (mammalian) offspring adjacent Moderate Low Terje Svingen (send email) Under development: Not open for comment. Do not cite Under Development

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KER.In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER.  More help
Term Scientific Term Evidence Link
Vertebrates Vertebrates High NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help
Sex Evidence
Mixed High

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER.  More help
Term Evidence
During development and at adulthood High

Key Event Relationship Description

Provides a concise overview of the information given below as well as addressing details that aren’t inherent in the description of the KEs themselves. More help

Testosterone (T) and dihydrotestosterone (DHT) are androgens that are involved in numerous developmental and functional processes across animal taxa. In vertebrates, testosterone can be aromatized into estrogens catalyzed by the enzyme aromatase (CYP19) or be metabolized to DHT by the enzyme 5α-reductase (Azzouni et al., 2012; Naamneh Elzenaty et al., 2022; Swerdloff et al., 2017). Both T and DHT binds to the androgen receptor (AR), but with different affinities. DHT has a higher affinity for the AR than T. DHT also has a longer half-life and slower dissociation rate than T and cannot be aromatized into estrogens (Gerald & Raj, 2022; Naamneh Elzenaty et al., 2022; Swerdloff et al., 2017).

During mammalian development, T is primarily produced by the fetal testes and is needed for differentiation of the Wolffian ducts, the epididymis, and the ejaculatory duct. In pubertal and adult mammals, T is produced by the testes, the ovaries (although at a much lower level), and the adrenal glands (Ogino et al., 2011; Rey, 2021). In peripheral tissues (i.e. relative to the testes), DHT is converted from T by 5α-reductase to induce the differentiation of the urogenital sinus and genital tubercle to form the prostate, penis, scrotum and urethra (Swerdloff et al., 2017). Both androgens are essential for masculinization, sexual development, and fertility.

Evidence Collection Strategy

Include a description of the approach for identification and assembly of the evidence base for the KER. For evidence identification, include, for example, a description of the sources and dates of information consulted including expert knowledge, databases searched and associated search terms/strings.  Include also a description of study screening criteria and methodology, study quality assessment considerations, the data extraction strategy and links to any repositories/databases of relevant references.Tabular summaries and links to relevant supporting documentation are encouraged, wherever possible. More help

This KER is considered canonical knowledge and supporting literature was mainly sourced from key review articles from the open literature.

Evidence Supporting this KER

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help
Biological Plausibility
Addresses the biological rationale for a connection between KEupstream and KEdownstream.  This field can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured.   More help

The biological plausibility for this KER is considered high

It is well established that DHT is synthesized from circulating T. 5α-reductase is the enzyme responsible for the conversion of T into DHT. Multiple isoforms of this enzyme are expressed in different tissues. Expression of 5α-reductase in peripheral tissues dictates where DHT will be formed from circulating T (Azzouni et al., 2012; Swerdloff et al., 2017).

Since T can be converted to DHT, it stands to reason that a lack of T can lead to a lack of DHT. Therefore, if there is a marked reduction in the availability of T, it can be surmised that DHT levels are consequently affected. However, to what extent T needs to be diminished in tissues before there is a functionally relevant decrease in DHT is largely unknown. In addition, the quantitative relationship between substrate (T) availability and levels of synthesized DHT is not well characterized in tissues in vivo. Notably, DHT can be produced via other steroid intermediates through the ‘backdoor pathway’ in mammals such as marsupials and humans (Renfree & Shaw 2023).

Uncertainties and Inconsistencies
Addresses inconsistencies or uncertainties in the relationship including the identification of experimental details that may explain apparent deviations from the expected patterns of concordance. More help

The levels of T do not always reflect the levels of DHT. T is also converted to estradiol (Naamneh Elzenaty et al., 2022). Therefore, the decrease in T may lead to a decrease in estradiol while DHT levels remain unchanged.  

Several studies have shown the existence of an alternative (‘backdoor’) pathway for DHT synthesis that is independent of T in marsupials and humans, but not in rodents (Marilyn B. Renfree et al., 1995). Instead of proceeding through the canonical pathway, progesterone or 17-OH progesterone, can be converted into allopregnanolone and 17OH-allopregnanolone. 17-OH allopregnanolone is then converted into androsterone leading to androstanediol that can finally be oxidized to produce DHT. Therefore, through this pathway, DHT can be synthesized without the presence of T (Auchus, 2004; Miller & Auchus, 2019).

Known modulating factors

This table captures specific information on the MF, its properties, how it affects the KER and respective references.1.) What is the modulating factor? Name the factor for which solid evidence exists that it influences this KER. Examples: age, sex, genotype, diet 2.) Details of this modulating factor. Specify which features of this MF are relevant for this KER. Examples: a specific age range or a specific biological age (defined by...); a specific gene mutation or variant, a specific nutrient (deficit or surplus); a sex-specific homone; a certain threshold value (e.g. serum levels of a chemical above...) 3.) Description of how this modulating factor affects this KER. Describe the provable modification of the KER (also quantitatively, if known). Examples: increase or decrease of the magnitude of effect (by a factor of...); change of the time-course of the effect (onset delay by...); alteration of the probability of the effect; increase or decrease of the sensitivity of the downstream effect (by a factor of...) 4.) Provision of supporting scientific evidence for an effect of this MF on this KER. Give a list of references.  More help
Response-response Relationship
Provides sources of data that define the response-response relationships between the KEs.  More help

The response-response relationship is not clearly established.

Information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). More help

Different time scales have been observed in the studies above, the shortest one found being 48h. With Ibuprofen treatment, a decrease in both testosterone and DHT was observed after 48h in human fetal explant’s exposure media (Ben Maamar et al., 2017). However, it is not evident that this effect is direct and only due to a decrease in T.

Known Feedforward/Feedback loops influencing this KER
Define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits. More help

Activity of 5α-reductase type 1 and 2: The activity of this enzyme determines how much T is converted into DHT. There are two isomers, with type 2 being the primary isomer expressed in DHT target organs. In deficiencies of this enzyme, there are studies that observe maintained DHT levels. This indicates that the type 1 enzyme can take over if needed (Azzouni et al., 2012).

Conversion of T to estradiol (E2): Aromatase can convert T into estrogens. The activity of this enzyme may push towards a decrease of T levels and an increase in estrogen levels without necessarily affecting DHT levels (Naamneh Elzenaty et al., 2022).

Hypothalamus-pituitary-gonadal (HPG) axis: Like most sex steroids, T production is controlled by the HPG axis during puberty and adulthood, but also during certain periods of development. For humans, the HPG axis is active following birth between 1-3 months in both males and females. Increase of LH and FSH are observed in infants up to 4-6months old. This stage is also known as the minipuberty (Lanciotti et al., 2018; Renault et al., 2020). Once GnRH is released from the hypothalamus, the pituitary gland secretes LH in pulses, which then stimulates the cells in the testes to produce T. A negative feedback loop can then occur, where testosterone then inhibits the release of GnRH and LH, in turn slowing down T production (Gerald & Raj, 2022; Naamneh Elzenaty et al., 2022; Nef & Parada, 2000).

Domain of Applicability

A free-text section of the KER description that the developers can use to explain their rationale for the taxonomic, life stage, or sex applicability structured terms. More help

T and DHT are androgens present in all vertebrates. They play a role in development and fertility in both males and females (Ogino et al., 2011; Prizant et al., 2014; Rey, 2021; Swerdloff et al., 2017). All tissues expressing 5α-reductase are applicable to this KER (Azzouni et al., 2012).


List of the literature that was cited for this KER description. More help

Auchus, R. J. (2004). The backdoor pathway to dihydrotestosterone. Trends in Endocrinology & Metabolism, 15(9), 432–438.

Azzouni, F., Godoy, A., Li, Y., & Mohler, J. (2012). The 5 Alpha-Reductase Isozyme Family: A Review of Basic Biology and Their Role in Human Diseases. Advances in Urology, 2012, 1–18.

Ben Maamar, M., Lesné, L., Hennig, K., Desdoits-Lethimonier, C., Kilcoyne, K. R., Coiffec, I., Rolland, A. D., Chevrier, C., Kristensen, D. M., Lavoué, V., Antignac, J.-P., Le Bizec, B., Dejucq-Rainsford, N., Mitchell, R. T., Mazaud-Guittot, S., & Jégou, B. (2017). Ibuprofen results in alterations of human fetal testis development. Scientific Reports, 7(1), 44184.

Culty, M., Thuillier, R., Li, W., Wang, Y., Martinez-Arguelles, D. B., Benjamin, C. G., Triantafilou, K. M., Zirkin, B. R., & Papadopoulos, V. (2008). In Utero Exposure to Di-(2-ethylhexyl) Phthalate Exerts Both Short-Term and Long-Lasting Suppressive Effects on Testosterone Production in the Rat1. Biology of Reproduction, 78(6), 1018–1028.

Gerald, T., & Raj, G. (2022). Testosterone and the Androgen Receptor. Urologic Clinics of North America, 49(4), 603–614.

Lanciotti, L., Cofini, M., Leonardi, A., Penta, L., & Esposito, S. (2018). Up-To-Date Review About Minipuberty and Overview on Hypothalamic-Pituitary-Gonadal Axis Activation in Fetal and Neonatal Life. Frontiers in Endocrinology, 9.

Marilyn B. Renfree, Jenny L. Harry, & Geoffrey Shaw. (1995). The marsupial male: a role model for sexual development. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 350(1333), 243–251.

Marty, M. S., Crissman, J. W., & Carney, E. W. (2001). Evaluation of the Male Pubertal Assay’s Ability to Detect Thyroid Inhibitors and Dopaminergic Agents. Toxicological Sciences, 60(1), 63–76.

Miller, W. L., & Auchus, R. J. (2019). The “backdoor pathway” of androgen synthesis in human male sexual development. PLOS Biology, 17(4), e3000198.

Moore, R. W., Potter, C. L., Theobald, H. M., Robinson, J. A., & Peterson, R. E. (1985). Androgenic deficiency in male rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin. Toxicology and Applied Pharmacology, 79(1), 99–111.

Naamneh Elzenaty, R., du Toit, T., & Flück, C. E. (2022). Basics of androgen synthesis and action. Best Practice & Research Clinical Endocrinology & Metabolism, 36(4), 101665.

Nef, S., & Parada, L. F. (2000). Hormones in male sexual development. Genes & Development, 14(24), 3075–3086.

Ogino, Y., Miyagawa, S., Katoh, H., Prins, G. S., Iguchi, T., & Yamada, G. (2011). Essential functions of androgen signaling emerged through the developmental analysis of vertebrate sex characteristics. Evolution & Development, 13(3), 315–325.

Prizant, H., Gleicher, N., & Sen, A. (2014). Androgen actions in the ovary: balance is key. Journal of Endocrinology, 222(3), R141–R151.

Renault, C. H., Aksglaede, L., Wøjdemann, D., Hansen, A. B., Jensen, R. B., & Juul, A. (2020). Minipuberty of human infancy – A window of opportunity to evaluate hypogonadism and differences of sex development? Annals of Pediatric Endocrinology & Metabolism, 25(2), 84–91.

Renfree, M., & Shaw G. (2023). The alternate pathway of androgen metabolism and window of sensitivity. Journal of Endocrinology, JOE-22-0296,

Rey, R. A. (2021). The Role of Androgen Signaling in Male Sexual Development at Puberty. Endocrinology, 162(2).

Swerdloff, R. S., Dudley, R. E., Page, S. T., Wang, C., & Salameh, W. A. (2017). Dihydrotestosterone: Biochemistry, Physiology, and Clinical Implications of Elevated Blood Levels. Endocrine Reviews, 38(3), 220–254.

Thibaut, R., & Porte, C. (2004). Effects of endocrine disrupters on sex steroid synthesis and metabolism pathways in fish. The Journal of Steroid Biochemistry and Molecular Biology, 92(5), 485–494.

Vierhapper, H., Nowotny, P., & Waldhäusl, W. (2003). Reduced production rates of testosterone and dihydrotestosterone in healthy men treated with rosiglitazone. Metabolism, 52(2), 230–232.

Xu, Y., Chen, Y., Li, N., Hu, X., Li, G., Ding, Y., Li, J., Shen, Y., Wang, X., & Wang, J. (2018). Novel compound heterozygous variants in the LHCGR gene identified in a subject with Leydig cell hypoplasia type 1. Journal of Pediatric Endocrinology and Metabolism, 31(2), 239–245.