The role of testosterone in chronic kidney disease and kidney function in men and women: a bi-directional Mendelian randomization study in the UK Biobank

Chronic kidney disease (CKD) represents a major burden of morbidity and mortality globally, affecting 10% of the population worldwide [1]. Renal replacement therapy (dialysis or kidney transplantation), which is the treatment to enable survival during end-stage renal disease, has become the leading cost in health care systems in developed countries [2]. Notably, CKD has an apparent disparity in men and women [3]. Specifically, kidney function declines faster in men than in women, and the mortality rate is higher in men at all levels of pre-dialysis CKD [3]. Understanding the reasons underlying men’s disadvantage may provide new clues about etiology, with relevance to identification of new modifiable targets in primary care and development of clinical treatment for CKD. It may also provide information that can be used to address the sexual disparity in life expectancy.

Accumulating evidence suggests androgens may play a key role in differences in longevity by sex [4]. From the perspective of evolutionary biology, growth and reproduction trade off against longevity, with men and women having different investments in these trade-offs and as a result have different vulnerabilities to disease [4‐6]. Correspondingly, hypothalamic-pituitary axis dysfunction is closely related to infertility and to CKD [7, 8], supporting a role of reproductive factors in CKD. Animal experiments have repeatedly shown that testosterone damages renal function [9, 10]. For example, in vitro experiments show testosterone induced renal tubular epithelial cell death in a dose-response manner [10]. Correspondingly, castration in males increases renal clearance [11], lowers proteinuria, and protects against glomerular injury assessed from renal morphology [11, 12]. Nevertheless, evidence, especially experimental evidence, in humans is limited. A clinical case report showed testosterone therapy induced renal impairment [13]. A randomized controlled trial in 48 men showed 6-month testosterone treatment lowered estimated glomerular filtration rate (eGFR) [14]. In contrast, some observational studies have found higher testosterone associated with lower mortality in CKD patients [15] and higher eGFR [16]. These observed associations in patients are difficult to interpret because observational studies, essentially of survivors, are open to selection bias [17]. No large-scale RCT has ever been implemented to assess the long-term effect of testosterone on CKD. Moreover, no study has specifically examined the sex-specific role of testosterone in CKD and kidney function.

In this situation where a large RCT is not available, using naturally occurring testosterone-related genetic variants as proxies of exposure, i.e., a Mendelian randomization (MR) study design, enables us to obtain unconfounded associations without any potentially harmful interventions. Genetic variants resulting in lifelong differences in endogenous exposures are unlikely to be associated with socioeconomic position or other confounders as they are determined at conception [18]. In this study, for the first time, we examined the association of genetically predicted testosterone with CKD events and kidney function using an MR study in the UK Biobank. For validation of the genetic instrument, we similarly examined the effect of genetically predicted testosterone on hemoglobin and high-density lipoprotein (HDL)-cholesterol, because meta-analysis of RCTs has clearly shown testosterone increases hemoglobin and lowers HDL-cholesterol [19]. As serum testosterone may also be affected by kidney function [20], we used a bi-directional MR to examine the effect of genetically predicted eGFR on serum testosterone.

In an MR study using univariable and multivariable MR, our study for the first time suggests testosterone affects CKD and kidney function in a sex-specific way. Testosterone might be an underlying cause of CKD in men but not in women. Meanwhile, genetically predicted kidney function did not affect serum testosterone. Our finding may be relevant to the sex disparity in CKD and kidney function, consistent with the sex-specific role of testosterone in CVD [40], and with expectations from evolutionary biology [4‐6].

Our findings did not corroborate a beneficial association of testosterone with kidney function sometimes seen in observational studies [15, 16], possibly due to poor health lowering both testosterone and kidney function [41, 42]. However, dietary factors, such as a high protein diet, which boost testosterone in men [43], increase CKD. In contrast, some medications lowering testosterone show a benefit for kidney function. For example, statins lower testosterone [44] and can slow down the progression of CKD [45]. Although the benefits may be due to lipid lowering, recent MR studies do not support a role of LDL-cholesterol in CKD [46]. Our findings are also consistent with changing disease patterns with economic development. Improvements in living conditions that enable higher levels of testosterone [47], with corresponding effects on kidney function, may be relevant to the rising rates of CKD that emerge with economic development, such as in China [1].

To our knowledge, this MR study is the first to examine the role of testosterone in CKD and kidney function. Despite the novelty, our study has several limitations. First, MR relies on three assumptions, i.e., relevance, independence, and exclusion-restriction [23]. To satisfy these assumptions, we used genetic variants from the largest sex-specific published GWAS of testosterone based on the UK Biobank and with replication in three independent studies [21]. We also checked the randomization from their associations with potential confounders, such as socioeconomic position and lifestyle. To address the assumption of exclusion-restriction, we tested and corrected for potential pleiotropy using MR-PRESSO. We also used multivariable MR to control for pleiotropic associations with SHBG. Second, measurement error might exist in the classification of CKD or the single time-point assay of testosterone. However, any measurement error should be non-differential, thus bias toward a null association, which should not affect the directions of associations. Third, our study could be affected by survivor bias [30] and by competing risk, i.e., by an event whose occurrence precludes the occurrence of CKD [29]. For example, if testosterone leads to death from ischemic heart disease at 74 years [48], death from CKD which usually occurs at 85 years will not be observed [49]. As such, the potential harm for CKD will be underestimated due to competing events. To control for the bias arising from competing risk, we adjusted for common causes of CKD and CVD [29], specifically BMI, smoking, and blood pressure. Fourth, the participants in the UK Biobank are healthier than the general population [50], and the majority do not suffer from CKD, so the estimates may not be applicable to CKD patients; however, the directions of associations should be consistent. Fifth, the associations in Europeans may not apply to other populations, such as Asians. However, causal effects should be consistent across settings, for example the effect on HDL-cholesterol was also reported in our previous MR study in Chinese [51]. Nevertheless, the effect size might vary by population according to testosterone levels; as such, replication in Asians would be worthwhile. Sixth, the genetic effects might be buffered by compensatory processes or feedback mechanisms. Such compensation would be expected to mitigate the genetic effects, thus biasing toward the null. As such, the estimates for CKD and kidney function might be underestimated. Finally, the wide age range in the UK Biobank participants may increase the variation in testosterone levels. However, this will only lower the precision of the testosterone GWAS and MR estimates, rather than affect the directions of association.

Despite the consistency with evolutionary biology [4‐6], the known sex disparity [3], and evidence from animal experiments [9, 10], these novel findings need to be interpreted cautiously. First, the association with CKD needs to be replicated in a larger sample with more power. Second, due to the limited number with follow-up measures of kidney function, we used baseline rather than decline in kidney function. It would be valuable to assess the role of testosterone in CKD progression, specifically the relevance to decline in kidney function, when sufficient follow-up data in the UK Biobank and suitable analytic techniques are available [52]. Third, the role of endogenous testosterone might be different from the role of testosterone supplementation or other lifestyle factors that modulate testosterone. However, the findings for hemoglobin and HDL-cholesterol in MR showed consistency with meta-analysis of RCTs [19]. Moreover, MR estimates lifetime effects rather than the effects of a short-term exposure. As such, the effect on CKD and kidney function might not be comparable to the acute renal effect of testosterone supplementation, although the use of testosterone gel lowers eGFR in RCT [14]. Finally, the underlying pathways from testosterone to kidney function remain to be clarified. The inverse association with eGFRcr might be due to or partly due to testosterone increasing muscle mass and thereby increasing serum creatinine. However, the inverse association remained for eGFRcr_cys. The similar pattern of associations for CKD and albuminuria also provides consistent evidence concerning the renal effects of testosterone. The effect size is small and might not be clinically significant; however, an MR study is more useful in determining the direction of causation than the magnitude of an effect size [53]. Moreover, a small effect size may be highly relevant at the population level [54]. Several mechanisms might underlie the renal effects of testosterone, including an effect on cellular apoptosis that may impact renal disease progression, an effect on glomerular matrix accumulation, an influence on the synthesis and activity of several cytokines and vasoactive agents, an interaction with the renin-angiotensin system [55], an increase in the generation of reactive oxygen species [55, 56], and a pro-inflammatory effect in the kidney [57]. Inflammation and immune function may underlie the pathology of CKD [58], and testosterone is known to be immunomodulatory [59]. In a clinical case report, testosterone directly modulated kidney perfusion [13], but the specific pathway is unclear. Clarifying these pathways, especially as regards any sex differences in the response to testosterone, such as immune function [59], would be valuable.

From the perspective of clinical and public health practice, our findings suggest that testosterone and drivers of testosterone are potential targets for lowering the burden of kidney disease in men rather than in women, consistent with the dominant role of testosterone in men’s reproduction and health [4]. Medications or lifestyle factors that modulate testosterone might be effective for the prevention and treatment of CKD and may be expected to play a sex-specific effect. As such, our study highlights the importance of considering sex-specific causes and treatments for CKD and raises the question as to whether any causes of CKD specific to women exist. Exploration of such factors and the underlying pathways would give insights to the re-positioning of existing drugs, new drug development, and lifestyle recommendations.

Genetically predicted testosterone was associated with CKD and unfavorable kidney function in men, which might contribute to the sex disparity in CKD. Serum testosterone was not affected by kidney function. Clarifying the underlying pathways could provide new insights for prevention and treatment strategies in both men and women. Replication in other studies, especially in understudied populations, is needed.