Influence of Testosterone in COVID-19


The Beneficial Role of Testosterone in COVID-19

It has been reported that testosterone serum level is reduced by aging and cardio-metabolic diseases including T2DM, obesity, dyslipidemia, heart failure, and atherosclerosis, which are common risk factors for the development of COVID-19 severity (Millar et al., 2016; Maddaloni et al., 2020).

Different studies illustrated that testosterone has a protective role in the respiratory system; it improves forced expiratory volume, vital capacity, oxygen consumption, and respiratory muscle contraction (Montano et al., 2014). Marques et al. (2020) illustrated that testosterone replacement therapy in orchiectomized male rats improves oxygenation and attenuates tissue hypoxia and hypercapnia. Thus, low testosterone serum levels in patients with hypogonadism may increase severity of obstructive pulmonary disease (Novković et al., 2019).

Testosterone and Acute Lung Injury

Testosterone has a crucial pulmo-protective effect through the modulation of lung inflammations, and a reduction of testosterone by aging may predispose the old age for chronic inflammatory pulmonary disorders and viral infections (Keilich et al., 2019). Redente et al. (2011) illustrated that testosterone’s defending role against bleomycin-induced ALI is through inhibition of pro-inflammatory mediated neutrophil alveolitis.

It has been observed that testosterone inhibits the production of pro-inflammatory cytokines, including IL-1β, IL-6, and TNF-α, and inflammatory adipocytokines with a cumulative effect on the anti-inflammatory adiponectin (Bianchi, 2019). Wang et al. (2021) revealed that testosterone therapy reduces lung inflammation and fibrosis by inhibiting nuclear respiratory factor 1 (NRF1) and the NF-κB signaling pathway.

Pro-inflammatory cytokines, mainly IL-6, are involved in the pathogenesis of ALI, ARDS, and cytokine storm-induced multi-organ damage in COVID-19 (Sun et al., 2020). Typically, testosterone inhibits the synthesis and release of IL-6 and downregulates the expression of IL-6 receptors (Sun et al., 2020).

Thus, IL-6 serum level is augmented in hypogonadism patients that increase their susceptibility for COVID-19 severity (Papadopoulos et al., 2020). Besides, adiponectin, through its anti-inflammatory effects, may reduce SARS-CoV-2-induced pro-inflammatory hypercytokinemia and associated ALI (Messina et al., 2020).

Of note, activation of nod-like receptor pyrin 3 (NLRP3) inflammasome is linked to over-activation of NF-κB signaling-induced inflammatory reactions in COVID-19.

Activation of NLRP3 inflammasome is also related with COVID-19 severity and associated complications like ALI and ARDS (van den Berg and Te Velde, 2020). Chen et al. (2020) in their experimental study confirmed that testosterone therapy reduces atherogenesis by suppressing NLRP3 inflammasome.

However, Alves et al. (2020) showed that supra-physiological testosterone level leads to oxidative stress-induced endothelial dysfunction and vascular injury through stimulation of NLRP3 inflammasome. Previously, Vignozzi et al. (2012) disclosed that testosterone through its derivative dihydrotestosterone suppresses NF-κB signaling activation and associated pro-inflammatory activations.

Therefore, testosterone therapy may improve the clinical outcomes in COVID-19 patients through NLRP3 inflammasome/NF-κB-dependent activation of anti-inflammatory cytokines and suppression of pro-inflammatory cytokines.

A prospective study involving 221 hospitalized men with COVID-19 pneumonia aged > 18 years illustrated that testosterone serum levels are reduced and correlated with COVID-19 severity and mortality (Çayan et al., 2020). Similarly, low baseline total and free testosterone in 31 COVID-19 patients recovered from ARDS in Italy are negatively correlated with inflammatory risk factors (ferritin, CRP procalcitonine, and D-dimer) and linked to COVID-19 severity (Rastrelli et al., 2020).

Likewise, an observational study in Germany involving 45 COVID-19 patients revealed that 70% of them have low testosterone at the time of admission with subsequent reduction through ARDS development and admission to the intensive care unit (ICU) (Schroeder et al., 2020). Furthermore, a prospective study composed of 358 men with COVID-19 compared with 92 negative for COVID-19 illustrated that testosterone levels are reduced in COVID-19 patients and linked to poor clinical outcomes (Cinislioglu et al., 2021).

In addition, a retrospective study showed that low testosterone level is connected with COVID-19 severity and risk of death (Okçelik, 2021). Low testosterone serum level is associated with reduction of body anti-inflammatory capacity with augmentation of pro-inflammatory axis. High pro-inflammatory cytokines in turn suppress release and action of testosterone in a vicious cycle manner (Bianchi, 2019).

Furthermore, the receptor for advanced glycation end-product (RAGE) is a member of immunoglobulin superfamily protein, which presents in two forms, membrane RAGE (mRAGE) and soluble RAGE (sRAGE) (Papadopoulos et al., 2020). mRAGE has an inflammatory effect through activation of NF-kB, while sRAGE has anti-inflammatory effects through upregulation of ACE2 and anti-inflammatory cytokines (Serveaux-Dancer et al., 2019). RAGE pathway is mainly expressed in lung tissue and linked to the development of acute and chronic lung injuries (Wang et al., 2018).

It has been shown that SARS-CoV-2 activates mRAGE at pulmonary alveolar cells, leading to severe inflammatory reactions (Yalcin Kehribar et al., 2020). It has been confirmed that the concentration of sRAGE is reduced with aging, which might explain the susceptibility of old age to COVID-19 (Evens et al., 2020). However, in young and asymptomatic COVID-19 patients, the concentration of sRAGE is high. In severe COVID-19, sRAGE level is significantly reduced, so low sRAGE level is associated with progression of ALI and ARDS (Abbasi-Oshaghi et al., 2021).

Therefore, COVID-19-induced reduction in circulating testosterone may induce ALI due to increase of pro-inflammatory and reduction anti-inflammatory effects.

Testosterone and Testicular Injury

In COVID-19, SARS-CoV-2 may bind testicle ACE2, leading to Sertoli and Leydig cells’ damage with subsequent inhibition of testicular testosterone synthesis (Illiano et al., 2020). Also, local inflammatory reaction in the testes due to SARS-CoV-2 infection and deregulation of the testicular renin–angiotensin system (RAS) may also impair testicular testosterone synthesis leading to hypogonadism (Yang et al., 2020).

Analysis of testicular biopsies in patients with COVID-19 illustrated that the histopathological changes like hypoxic injury and microthrombosis are similar to that observed in COVID-19-induced ALI. However, SARS-CoV-2 was not detected in the injured testes, suggesting oxidative stress; coagulation disorders might mediate this damage as evident in COVID-19 pneumonia (Flaifel et al., 2021).

Therefore, preexistence or SARS-CoV-2-induced hypogonadism may reduce the protective effect of testosterone against SARS-CoV-2 infection, suggesting a link between testicular injury and development of ALI and ARDS in COVID-19 patients (Yang et al., 2020; Zaim et al., 2020). Moreover, high pro-inflammatory cytokines in SARS-CoV-2 infection may induce endothelial dysfunction and coagulopathy, a hallmark in COVID-19.

The pro-thrombotic status and risk of thromboembolism are highly aggravated in hypogonadism (Fei et al., 2020). Local testicular thrombosis during SARS-CoV-2 infection is associated with diffuse damage of Leydig and Sertoli cells (Duarte-Neto et al., 2021). However, testosterone supplementation improves endothelial function via activation of nitric oxide release, inhibiting platelet activations and pro-thrombotic cascades (Hotta et al., 2019).

These clinical studies illustrated that reduction in the testosterone level is due to testicular injury with a subsequent reduction in the synthesis and release of testosterone from Leydig cells. This simple explanation is not acceptable since testicular injury is not frequently involved during SARS-CoV-2 infections (Schroeder et al., 2020). However, total testosterone may reduce in COVID-19 in the absence of testicular injury, as 90% of COVID-19 patients have a negative test for SARS-CoV-2 in the testes (Yang et al., 2020).

A recent study illustrated that hypogonadism is developed in the early phase of COVID-19 due to SARS-CoV-2-induced testicular injury (Dutta and Sengupta, 2021). Higher expression of ACE2 in the testes makes them a potential target for SARS-CoV-2 with subsequent progression of male infertility. Excessive production of reactive oxygen species by SARS-CoV-2 may disrupt sperm function and morphology leading to early- or late-onset infertility (Esteves et al., 2021). Xu et al. (2021) showed that despite testicular injury during acute SARS-CoV-2 infection, male sex hormones remain unchanged even after recovery from COVID-19.

Herein, extensive molecular studies are recommended to observe the implication of SARS-CoV-2 infections in reducing testosterone levels in COVID-19 patients. Zhao et al. (2016) illustrated that activation of mRAGE is correlated with inhibition of Leydig cell function with reduction of testosterone biosynthesis. This finding might explain low testosterone levels in patients with severe COVID-19.

In COVID-19, downregulation of lung ACE2 by SARS-CoV-2 is associated with high circulating AngII level, which is linked to development and progression of ALI and ARDS (Zhang et al., 2020). It has been confirmed that AngII inhibits Leydig cell function and testosterone synthesis (Reis and Reis, 2020). Add to these findings, the testes have full RAS, which is involved in the regulation function of Leydig cells and testosterone biosynthesis (Reis and Reis, 2020). Thus, systemic or testicular AngII levels are augmented due to downregulation of ACE2 in COVID-19. Local and circulating AngII activate harmful AT1R on the Leydig cells leading to the inhibition of testosterone biosynthesis (Pascolo et al., 2020). The deregulation of the protective AT2R and Mas receptors within the testes provokes inflammatory cascades that also contribute to Leydig cells’ dysfunction (Aitken, 2020; Pascolo et al., 2020). From the above considerations, AngII might be the potential biomarker linking ALI and testicular injury in patients with severe COVID-19.

Testosterone and Oxidative Stress

Additional studies illustrated that SARS-CoV-2 infection leads to oxidative stress injury and oxidative storm due to membrane lipid and protein peroxidations (Ntyonga-Pono, 2020). The high neutrophil ratio in SARS-CoV-2 infection is linked to high oxidative stress due to the production of reactive oxygen species (ROS) by neutrophils. These changes provoke a cascade of immuno-biological events that the human body responds to (Ntyonga-Pono, 2020). ROS causes various pathological events related to COVID-19, such as endothelial dysfunction, erythrocyte injury, platelet activation, and thrombosis (Laforge et al., 2020). High ROS in COVID-19 promotes neutrophil extracellular traps (NETs) and induction release of pro-inflammatory cytokines (Laforge et al., 2020). NETs activate NLRP3 inflammasome, NF-κB, and induction of coagulopathy (Schönrich et al., 2020).

It has been reported that oxidative stress inhibits testosterone biosynthesis through activation of mitogen-activated protein kinase p38 (MAPK), which alter the metabolic process and gene expression (Shi and Dansen, 2020). Therefore, severe oxidative stress upregulates the p38MAPK pathway in the Leydig cells causing significant inhibition of testicular testosterone biosynthesis (Han et al., 2018). Recently, Jing et al. (2020) confirmed that oxidized low-density lipoprotein (oxLDL) inhibits testosterone synthesis through induction of p38MAPK pathway in the Leydig cells.

Oxidative stress inhibits Leydig and adrenal cells to synthesize testosterone through upregulation of cyclooxygenase 2 (COX2), induced by the p38MAPK pathway (Martin and Touaibia, 2020). Both p38MAPK pathway and COX2 are activated in COVID-19; Grimes and his colleague (Grimes and Grimes, 2020) illustrated that SARS-CoV-2 might directly or indirectly activate the p38MAPK pathway through downregulation of ACE2 and augmentation of AngII.

Besides, activation of pro-inflammatory cytokines in COVID-19 induces upregulation of COX2 (Ong et al., 2020). Zhoa et al. (2019) confirmed that the NF-κB signaling pathway mediates the interaction between the p38MAPK pathway and COX2 in reducing testicular testosterone biosynthesis. In addition, activated p38MAPK provokes blood-testes barrier injury by suppressing testicular spermatogenesis and testosterone biosynthesis (Liu et al., 2018). Therefore, SARS-CoV-2 infection may reduce circulating testosterone and induces hypogonadism through activation of the p38MAPK/COX2 axis.

Into the bargain, testosterone inhibits neutrophil oxidative stress by reducing the production of superoxide anion, inhibition of lipid peroxidation, and improvement of glutathione reductase activity (Marin et al., 2010). In addition, an experimental study revealed that testosterone improves testes antioxidant potential by which it may attenuate oxidative stress-induced testicular injury (Aydilek et al., 2004).

Therefore, testosterone may reduce COVID-19 severity through mitigation of SARS-CoV-2-induced oxidative stress and associated complications.

Testosterone and Macrophage Function

Moreover, SARS-CoV-2 infection may lead to macrophage activation syndrome (MAS), which is characterized by hemophagocytosis, pancytopenia, coagulopathy, and disseminated intravascular coagulation (DIC). The MAS is developed in different viral infections including SARS-CoV-2 due to imbalanced release of pro-inflammatory cytokines (McGonagle et al., 2021).

Of note, testosterone has an important regulatory role on the macrophage, monocyte, and T-cell functions. Testosterone inhibits release of pro-inflammatory and inflammatory cytokines from immune cells (Bereshchenko et al., 2018). Testosterone therapy was shown to prohibit release of pro-inflammatory cytokines from monocytes mainly in hypogonadal men compared with eugonadal one (Bianchi, 2019). In addition, testosterone decreases the expression and sensitivity of macrophage TLR4 for its ligand (Rettew et al., 2008).

Of interest, TLR4 mediates early immunological interaction of SARS-CoV-2 with macrophage and other immune cells (Aboudounya and Heads, 2021). Therefore, testosterone therapy in COVID-19 patients may interrupt macrophage activation, exaggerated immune response, and development of MAS.

Furthermore, testicular macrophages (TMs) have immunoregulatory and immunotolerant functions as well as control of testicular steroidogenesis and spermatogenesis (Meinhardt et al., 2018). During sepsis and pathogen invasion, the classical type macrophage (M1) is activated and induces release of local pro-inflammatory cytokines. These cytokines impair spermatogenesis with significant inhibition of testicular steroidogenesis. The alternative type macrophage (M2) has local anti-inflammatory action supporting spermatogenesis and release of testosterone from Leydig cells (Chen et al., 2018; Meinhardt et al., 2018).

In SARS-CoV-2 infection, macrophage polarization is toward M1 phenotype resulting in testicular injury with impairment of testicular steroidogenesis and spermatogenesis (Lv et al., 2021). Becerra-Diaz et al. (2018) illustrated that testosterone and other androgens through macrophage androgenic receptor (AR) enhance M2 polarization with domination of macrophage anti-inflammatory effect. Taken together, testosterone modulates macrophage functions in general and more specifically TMs, by which it reduces the harmful effects of SARS-CoV-2 infection on the testes.

Testosterone Versus Estrogen in Men

In general, women have a robust immune system as compared to men due to the protective effect of estrogen against immunological dysregulation during different viral infections (Priyanka and Nair, 2020). It has been shown that estrogen has complex immunomodulating effects, and its effect on the inflammatory milieu in COVID-19 has been suggested (Ma et al., 2021).

High estrogen serum level in premenopausal women might be a protective factor against COVID-19 severity, though older post-menopausal women are of high risk for development of COVID-19 severity compared to elderly men (Ciarambino et al., 2021). Nevertheless, reduction of estrogen level in later life in women does not appear to play a harmful role regarding COVID-19 severity in elderly women (Papadopoulos et al., 2021).

In elderly men, there is significant reduction of testosterone with elevation of estrogen level due to increasing aromatization of adrenal and testicular androgens (Jardí et al., 2018). However, during sepsis in men, there is a noteworthy reduction of testosterone level with parallel increase of estrogen that reflects negative outcomes in septic men (Bech et al., 2020).

Thus, administration of estrogen in men with COVID-19 may offer a potential protective effect against COVID-19 severity (Suba, 2020). Bukowska et al. (2017) confirmed from experimental data that estrogen is able to regulate expression of the ACE/ACE2 axis, which is highly distorted in COVID-19.

Also, estrogen inhibits propagation of cytokine storm and can activate B cells for antibody production. Besides, estrogen reduces expression of TMPRSS2, thereby reducing the entry of SARS-CoV-2 to the susceptible cells (Bennink et al., 2021). So, estrogen treatment is suggested to be an effective treatment against COVID-19 (Bennink et al., 2021).

These findings highlighted the potential protective effects of testosterone against SARS-CoV-2 infection (Table 1). However, reduction of total testosterone level in COVID-19 is due to complex interactions between SARS-CoV-2 with oxidative stress, pro-inflammatory cytokines, and systemic and local RAS (Figure 2).

Table 1 Beneficial effects of testosterone in COVID-19.
Figure 2 The potential role of SARS-CoV-2 infection in the reduction of testosterone and associated COVID-19 severity. SARS-CoV-2 induces oxidative stress, activation of nod-like receptor pyrin-3 (NLRP3) inflammasome and abnormal immune activation, downregulation of angiotensin converting enzyme 2 (ACE2), and activation of receptor for advanced glycation end-product (mRAGE). Downregulation of ACE2 with activation of mRAGE increases angiotensin II (AngII) and reduces Mas receptor (MasR). Activation of NLRP3 inflammasome triggers release of nuclear factor kappa B (NF-κB) and together with cyclooxegenase-2 (COX-2) and p38 mitogen-activated protein kinase (p38MAPK) stimulate release of pro-inflammatory cytokines, which cause testicular injury. These pathophysiological changes reduce production and release of testosterone from injured testes. Reduction in the level of testosterone provokes releases of pro-inflammatory cytokines and reduces anti-inflammatory cytokines with immune deregulation. These changes lead to induction of cytokine storm with consequent augmentation of COVID-19 severity.

Harmful Role of Testosterone in COVID-19
Testosterone and TMPRSS2 in COVID-19

Various studies illustrated that men’s higher predisposition to develop severe and serious COVID-19 complications is related to sex hormones, mainly testosterone and sociocultural factors (Lipsky and Hung, 2020). It has been confirmed that TMPRSS2 is required for proteolytic activation and priming of SARS-CoV-2 spike protein to bind ACE2 (Rahman et al., 2020). The expression of the TMPRSS2 gene is promoted by testosterone hormone, which might explain the severity of COVID-19 in men due to facilitating the entry of SARS-CoV-2 (Stopsack et al., 2020). TMPRSS2 is a cellular enzyme encoded by the human TMPRSS2 gene involved in prostatic cancer (Mehra et al., 2007) and cleaving of hemagglutinin viral antigen and infectivity of HIN1 and H7N9 influenza virus (Cheng et al., 2015). The TMPRSS2 gene is expressed in different tissues including lung and testes (Shen et al., 2020).

In addition to the androgen, nicotine smoking increases the expression of the TMPRSS2 gene, which might explain the severity of COVID-19 severity in nicotine smoker patients (Voinsky and Gurwitz, 2020). However, various studies reported the protective effect of nicotine smoking against SARS-CoV-2 due to different mechanisms, including upregulation of lung ACE2, anti-inflammatory, and immunosuppressive effects through activation of nicotinic acetylcholine receptor type 7 alpha (nAChR7α) on the macrophage (Farsalinos et al., 2020).

Likewise, Donlan et al. (2020) observed that the expression of the TMPRSS2 gene is activated by IL-13, a highly expressed cytokine in COVID-19 and regarded as a predictive factor for mechanical ventilation independent of gender, age, and comorbidities. Besides, TMPRSS2 is highly co-expressed with furin, cathepsin L and B, CD209, and CD147 in men only, while co-expression with ACE2 is similar in both sexes (Piva et al., 2021). Certainly, TMPRSS2 co-expression with CD147 is important since CD147 is regarded as an entry point for SARS-CoV-2 (Radzikowska et al., 2020). Furthermore, Cao et al. (2017) in their experimental study confirmed that testosterone therapy increases the expression of CD147.

Therefore, overexpression of TMPRSS2 by androgen may implicate the testosterone in the pathogenesis of SARS-CoV-2 infection and COVID-19 severity. Thereby, TMPRSS2 inhibitors such as bromhexine, aprotinin, camostat, and nafamostat are useful in managing COVID-19 through attenuation of TMPRSS2-dependent lung inflammation, coagulopathy, and development of ARDS (Azimi, 2020; Breining et al., 2020).

Indeed, the population-based study of Montopoli et al. (2020) that involved 9,280 COVID-19 patients with or without prostatic cancer illustrated that patients receiving androgen deprivation therapy (ADT) are at a lower risk for COVID-19-related complications compared to patients who did not receive ADT. This finding suggests that the anti-androgen agents reduce testosterone’s activation role on the expression of TMPRSS2, and thus high testosterone level may increase COVID-19 severity. Adamowicz et al. (2020) showed that high dihydrotestosterone level is linked to poor pulmonary outcomes in COVID-19 patients, though use of 5-α reductase inhibitors may aggravate COVID-19 severity due to disturbance of intra-pulmonary androgen metabolism. However, McCoy et al. (2020) showed that using 5-α reductase inhibitors is associated with good clinical outcomes in COVID-19 patients.

Moreover, different therapeutic modalities such as dexamethasone, nitric oxide, and chloroquine, which are effective in managing COVID-19, are reported to have anti-androgenic effects and suppression of TMPRSS2 (Cronauer et al., 2007; Guo et al., 2018; Chi et al., 2020). Taken together, based on the current findings, testosterone is implicated in the facilitation of SARS-CoV-2 infection through upregulation of TMPRSS2 and androgen receptor (AR) activation.

Androgen Sensitivity and COVID-19

The role of androgen sensitivity and polymorphism in COVID-19 is explained by different studies. It has been reported that a low mortality rate in pre-pubescent compared to the high mortality rate in adult men during the COVID-19 pandemic is due to low androgen sensitivity (Wambier et al., 2020). In addition, men with androgenic alopecia and women with polycystic ovary syndrome are at a higher risk for SARS-CoV-2 infection and COVID-19 severity due to higher androgen sensitivity. Therefore, the higher mortality rate for COVID-19 in the African American population is related to the polymorphism and higher sensitivity of androgenic receptors (Goren et al., 2020).

It has been known that the polyglutamine (poly-Q) tract of the AR affects the physiological response of circulating testosterone (Callewaert et al., 2003). Longer poly-Q of AR reduces the sensitivity to testosterone and is associated with high testosterone serum level because of impairment of negative feedback inhibition (Mohamad et al., 2018). In addition, longer poly-Q of AR is linked to activation of pro-inflammatory axis (Pierotti et al., 2010), although AR with short poly-Q has protective and anti-inflammatory roles in COVID-19 regardless of testosterone serum levels (Baldassarri et al., 2021). Therefore, testosterone may have bidirectional effects depending on the underlying length of AR poly-Q tract.

The distribution of poly-Q allele differs among diverse populations: longer in Asians, medium in Caucasians, and shorter in Africans (Ackerman et al., 2012). This might explain the high mortality in the first wave of SARS-CoV-2 infection in both China and Italy (Pereira et al., 2020). Of interest, African populations are more prone to the SARS-CoV-2 infection due to higher sensitivity of AR and higher expression of the TMPRSS2 gene (de Lusignan et al., 2020).

Therefore, AR sensitivity and length of poly-Q tract of AR seem to be more important than testosterone level in the prediction of COVID-19 severity. Besides, testosterone therapy in patients with COVID-19 may improve or worsen the clinical outcomes depending on patient AR sensitivity (Mukherjee and Pahan, 2021).

Immunological Effects of Testosterone in COVID-19

It has been reported that both adaptive and innate immune systems are crucial for contrasting viral infections and enhancing viral clearance and tissue repair (Kikkert, 2020). Giagulli et al. (2021) illustrated that circulating testosterone has immunosuppressive effects by inhibiting B and T cells, impairing immune response and immunoglobulin generations in different viral infections (Ghosh and Klein, 2017). In COVID-19, natural killer, B, and T cells are reduced; specifically reducing CD8 T cell is regarded as an independent predictor for severe COVID-19-related complications (Wang et al., 2020). Kissick et al. (2014) revealed that testosterone inhibits differentiation of CD4 T cells, providing a basis for targeting testosterone and other androgenic receptors to mitigate CD4 T-cell response in various forms of autoimmune disorders.

Several lines of evidence from various studies point to the immunosuppressive potential role of testosterone on various components of the immune system (Gubbels Bupp and Jorgensen, 2018), although the basic molecular mechanism is still not elucidated. Testosterone mediated downregulation of systemic immune response through cell-type-specific effects in many immunological disorders (LaVere et al., 2021).

The precise immunological effects of testosterone and other androgens are through inhibition of antibody response to the viral infections and vaccines, suppression of macrophages and dendritic cells, promotion of immunological tolerance via activation of regulatory T cells, and inhibition of functions and developments of B and T cells (Trigunaite et al., 2015). Regarding these considerations, men are more vulnerable for COVID-19 severity as compared with women due to the immunosuppressive effects of testosterone (Bwire, 2020). Testosterone enhances both secretion and production of Th1-to-Th2 cytokine ratio via stimulated T cells and reduces humoral response and B-cell proliferation (Roved et al., 2017).

Moreover, the lysophosphatidyl serine receptor (GPR174) encoded by X-chromosome gene is highly expressed on B and T cells in women compared with men (Barnes et al., 2015). GPR174 regulates and controls the release of pro-inflammatory cytokines, B-cell migration, and macrophage polarization in septic shock and in response to chemokines (Qiu et al., 2019). These verdicts and results highlighted testosterone’s potential immunosuppressive effect in the progression of SARS-CoV-2 infection and COVID-19 severity (Salonia et al., 2021). Therefore, ADT might be a therapeutic opportunity against COVID-19 by reversal of immunosuppression status (Montopoli et al., 2020).

Metabolic Effects of Testosterone in COVID-19

Testosterone has a permissive effect for circulating AngII by expressing AT1R and downregulation of ATR2 with a higher ATR1R/AT2R ratio. However, castration reverses this ratio (Mishra et al., 2019). High circulating AngII and ATR1R expression are linked to ALI and ARDS development in COVID-19 (Wu et al., 2020). Besides, a high AngII level induces testicular injury and cell apoptosis with the reduction of Leydig cells for the synthesis of testosterone (Wang et al., 2017). Thus, in SARS-CoV-2 infection, there is a vicious cycle conflict in the interaction between testosterone and AngII concerning the lung–testis axis in severe COVID-19.

To date, dipeptidyl peptidase 4 (DPP4), which is highly expressed in different tissues, mainly in lung type II alveolar cells, is regarded as an entry point for SARS-CoV-2 and is associated with poor clinical outcomes in COVID-19 patients (Solerte et al., 2020). Blauschmidt et al. (2017) observed that testosterone upregulates the expression of DPP4 receptors in women with polycystic ovary syndrome. DPP4 inhibitors effectively manage COVID-19 through modulation of the anti-inflammatory/pro-inflammatory axis (Mirani et al., 2020). Thus, testosterone may increase COVID-19 severity through the DPP4/CD26 pathway; however, there is no study related to DPP4/CD26 and testosterone in SARS-CoV-2 infection.

Moreover, obesity is associated with low circulating testosterone due to aromatization of testosterone to estrogen by adipose tissue and abnormal hypothalamic–pituitary axis (Haring et al., 2010). Obesity is regarded as an independent risk factor for COVID-19 severity despite low testosterone levels (Yang et al., 2021), although ample evidence from experimental, preclinical, and clinical studies revealed that low testosterone level promotes development of obesity (Fui et al., 2014).

Testosterone improves catecholamine-induced lipolysis and inhibits uptake of triglyceride by suppressing the activity of adipose tissue lipoprotein lipase (Grossmann, 2011). It has been reported that patients with prostatic cancer on ADT had increased fat mass and visceral adipose tissue by about 22% within 6 months of established therapy (Hamilton et al., 2011). Likewise, experimental hypogonadism in young men induces obesity within 10 weeks (Mauras et al., 1998).

Therefore, low testosterone-induced obesity may aggravate the clinical course of COVID-19 severity. Sarver and Wong (2021) showed that obesity increases the expression of TMPRSS2 and DPP4 with alteration of the ACE/ACE2 ratio. Thereby, obesity may increase the risk of SARS-CoV-2 infection and abnormal immune response by underlying high pro-inflammatory cytokines (Seidu et al., 2021). Therefore, testosterone’s harmful effects in COVID-19 are related to TMPRSS2, ATR1, CD147, DPP4, and AngII expression that are mutually interrelated in facilitating SARS-CoV-2 entry and associated inflammatory reactions (Table 2 and Figure 3).

Table 2 Harmful effects of testosterone in COVID-19.
Figure 3 The harmful effects of testosterone in SARS-CoV-2 infection. Dihydrotestosterone from testosterone leads to immunosuppression and increases expression of Dipeptidyl peptidase 4 (DPP4), cluster differentiation 147 (CD147), and Transmembrane protease serine 2 (TMPRSS2). Also, Dihydrotestosterone increases activity of Angiotensin II (AngII). CD147 and DPP4 increase entry of SARS-CoV-2, while TMPRSS2 facilitates SARS-CoV-2 entry. Immunosuppression reduces viral clearance that increases SARS-CoV-2 entry. These changes lead to cytopathic effects with release of pro-inflammatory cytokines with subsequent development of acute lung injury (ALI), acute respiratory distress syndrome (ARDS), and testicular injury.

SARS-CoV-2 affects the testicles – reducing hormones and sperm quality

While following up on male patients as they recovered from COVID-19 since the start of last year, andrologist Jorge Hallak, a professor at the University of São Paulo’s Medical School (FM-USP) in Brazil, noticed that their fertility and hormone tests continued to display alterations for many months after they recovered from the disease.

Several patients’ semen analysis results, for example, showed that sperm motility fell to 8%-12% (compared with more than 50% for healthy men) and remained at this level for almost a year after they were infected by SARS-CoV-2.

Hormone tests showed testosterone levels also sharply down after the disease in many patients. Normal testosterone levels are 300-500 nanograms per deciliter of blood (ng/dL). In these patients, they were below 200 ng/dL, and in some cases as low as 70-80 ng/dL.

“We’re increasingly seeing long-lasting alterations in semen and hormone quality in COVID-19 patients, even when they had mild or no symptoms of the disease,”.

Studies conducted in recent months by Hallak, in collaboration with colleagues in FM-USP’s Department of Pathology, have helped elucidate these observations made in clinical practice. 

The researchers found that SARS-CoV-2 infects the testicles, impairing their production of sperm and testosterone. “It’s a cause of concern that the virus affects the testicles even in asymptomatic patients or patients with mild symptoms of the disease. Of all the agents that harm the testicles I’ve studied to date, SARS-CoV-2 appears to be the most active,” Hallak said. “Every pathology has peculiarities that experience and practice reveal to us. SARS-CoV-2 affects spermatogenesis [sperm production] in ways we’re discovering now, such as persistently low progressive motility without significant alterations to sperm concentration.”

Good sperm motility is critical to fertilization because it assures passage through the female reproductive tract and into the egg. Abnormally motile sperm cannot fertilize.

Ultrasound scans of 26 COVID-19 patients showed half to have severe inflammation of the epididymis, a long, coiled tube that transports sperm from each testicle. Sperm acquires its fertilizing ability and forward motility properties during epididymal transit. 

The patients, whose age averaged 33, were admitted to Hospital das Clínicas, a hospital complex run by FM-USP, and tested by Instituto Androscience.

An article reporting the results is published in the journal Andrology.

“In contrast with classic bacterial infections or other viral infections such as mumps, which causes testicular swelling and pain in a third of those infected, the epididymitis caused by SARS-CoV-2 is painless and can’t be diagnosed by visual inspection or palpation,” Hallak said, adding that testicular self-examination should be recommended as a public health policy in response to the pandemic.

“Ideally, adolescents, young adults and men at a reproductive age or who want to have children should consult a urologist or andrologist after being infected by SARS-CoV-2 to have all the relevant tests – measurement of testicular volume, testosterone and other hormones, semen analysis, and a sperm function test – followed by a color Doppler ultrasound scan to see if they have any kind of testicular abnormality that could affect fertility and hormone production,” Hallak said. “They should be followed up for at least one or two years after infection by the virus. We don’t yet know enough about these complications of the disease.”

Invasion of testicular cells

Another study recently published by the same group of researchers showed that SARS-CoV-2 invades all types of testicular cell, causing lesions that can impair hormone function and fertility.

Under the auspices of a project led by Paulo Saldiva and Marisa Dolhnikoff, both also professors at FM-USP, the researchers used a minimally invasive autopsy technique to extract testicular tissue samples from 11 men aged 32-88 who died at Hospital das Clínicas as a result of severe COVID-19. Analysis of the samples showed a number of testicular lesions possibly due to inflammatory alterations that reduce spermatogenesis and hormone production.

“What immediately drew our attention in these patients who died from COVID-19 was the drastic reduction in spermatogenesis. Even the younger ones at a reproductive age had practically no sperm,” said Amaro Nunes Duarte Neto, who heads the study. Duarte Neto is an infectious disease specialist and pathologist at FM-USP and Adolfo Lutz Institute.

According to Duarte Neto, the probable causes of reduced spermatogenesis in these patients include lesions caused by the virus in the vessels of the testicular parenchyma, with the presence of blood clots leading to hypoxia (low oxygen in the tissue), and fibrosis obstructing the seminiferous tubules, the part of the testicle in which sperm is produced.

A probable reason for reduced hormone production is loss of Leydig cells, interstitial cells that are located next to the seminiferous tubules and produce testosterone.

“The functions performed by the testicles, producing sperm and the main male sex hormone, are independent but connected because fertility declines if hormone production by Leydig cells is impaired,” Duarte Neto said.

Some symptoms of testosterone deficiency (hypogonadism), including loss of muscle mass, fatigue, irritability, memory loss and weight gain, can be confused with the long-term effects of COVID-19.

“An important part of this clinical condition is undoubtedly due to low testicular function, but the connection had never been made before because these patients don’t complain of pain and it isn’t routine practice to measure hormone production or analyze sperm after recovery from COVID-19,” Hallak said.

The researchers plan to conduct a follow-up study of male patients who have had the disease in order to find out if testicular lesions can heal naturally or be cured with medication. “We don’t know whether the lesions can be reversed or how long it would take,” he said.

His main concerns relate to men of reproductive age, adolescents and prepubertal children, given the lack of data on the testicular lesions caused by COVID-19. Nothing is known about the effects of the disease on puberty in terms of fertile capacity, whether hormone production is affected temporarily, for a long time or definitively, and the degree of irreversible residual damage.

Because there is no data on the condition of these male COVID-19 patients before they were infected, prospective studies should include a control group for comparison, Hallak suggested. “These subjects may have infertility problems and hormone alterations in future, and be unaware that this was caused by COVID-19 because they had mild symptoms or none at all,” he said.

Increase in male infertility

Hallak believes COVID-19 can cause an increase in male infertility. Between 15% and 18% of couples have difficulty conceiving, caused by male problems in 52% of cases. This trend may lead to an increase in demand for assisted reproduction, which in his view is sometimes rushed in Brazil, and provided without a properly standardized initial assessment to diagnose the cause. There has to be enough time to work out and propose the approach with the best cost-benefit ratio, including specific treatment to address the cause or restore natural fertile capacity.

“We’ll need to take great care with assisted reproduction in the wake of the pandemic. We don’t know much about the consequences of COVID-19 in the months following infection,” Hallak said.

Because SARS-CoV-2 has been detected in all types of testicular cells, which participate in all stages of spermatogenesis, scientists do not know whether the virus is also present in the sperm of COVID-19 patients months after they recover from the disease. Their sperm may have been affected by the virus, and in Hallak’s view medical teams should wait around 90 days, the time taken by a spermatogenesis cycle, to perform a fresh andrological examination.

“We’ve seen DNA lesions caused by the virus at very high levels of around 80%, whereas up to 25% is considered normal and up to 30% acceptable,” he said.

Another concern is unnecessary testosterone replacement. “If a patient has had COVID-19 and a decline in hormone production is detected, testosterone replacement will further inhibit testicular function,” Hallak stressed. “The testicles have repair mechanisms that restore hormone production, and there are medications that increase natural production of steroid hormones, gradually reestablishing intrinsic testicular function, although we don’t yet know to what extent will it depend on whether Leydig cells have been damaged and how badly.”

“At FMUSP, we are bringing together specialists from various medical specialties to study a group of 749 male patients who had COVID-19 who will undergo a first evaluation over the next four years to obtain more knowledge about the post-COVID-19 syndrome,” says Hallak.

The article “Radiological patterns of incidental epididymitis in mild-to-moderate COVID-19 patients revealed by color Doppler ultrasound” (doi: 10.1111/and.13973) by Felipe Carneiro, Thiago A. Teixeira, Felipe S. Bernardes, Marcelo S. Pereira, Giovanna Milani, Amaro N. Duarte-Neto, Esper G. Kallas, Paulo H. N. Saldiva, Maria C. Chammas and Jorge Hallak is at:

The article “Testicular pathology in fatal COVID-19: a descriptive autopsy study” (doi: 10.1111/andr.13073) by Amaro N. Duarte-Neto, Thiago A. Teixeira, Elia G. Caldini, Cristina T. Kanamura, Michele S. Gomes-Gouvêa, Angela B. G. dos Santos, Renata A. A. Monteiro, João R. R. Pinho, Thais Mauad, Luiz F. F. da Silva, Paulo H. N. Saldiva, Marisa Dolhnikoff, Katia R. M. Leite and Jorge Hallak is at:

The article “SARS-CoV-2 and its relationship with the genitourinary tract: Implications for male reproductive health in the context of the COVID-19 pandemic” (doi: 10.1111/andr.12896) by Jorge Hallak, Thiago A. Teixeira, Felipe S. Bernardes, Felipe Carneiro, Sergio A. S. Duarte, Juliana R. Pariz, Sandro C. Esteves, Esper Kallas and Paulo H. N. Saldiva is at:

The article “Viral infections and implications for male reproductive health” (doi: 10.4103/aja.aja_82_20) by Thiago A. Teixeira, Yasmin C. Oliveira, Felipe S. Bernardes, Esper G. Kallas, Amaro N. Duarte-Neto, Sandro C. Esteves, Joël R. Drevet and Jorge Hallak is at:

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