This meta-analysis of 35 eligible studies and more than 3,700 patients receiving TRT is the largest consolidation of RCT data thus far. Our main finding is that no significant increase in CV event risk was noted among studies of various TRT administration routes when analyzed together. Further, when the risk of CV events was analyzed based on the mode of administration, only oral TRT was associated with elevated CV risk when compared with placebo. The increase in CV risk resulting from transdermal TRT and the decrease in CV risk seen with intramuscular TRT did not achieve statistical significance. A second important finding in this meta-analysis is that the oral and transdermal administration methods of TRT are associated with greater DHT elevations than intramuscular administration. Because there is emerging data demonstrating an association between elevated DHT (rather than serum T) and adverse CV events, these two findings may have important implications for our current understanding of the mechanisms of CV risk in TRT recipients.

Mode of administration and CV risk

Our finding that there are varying CV risks based on the type of TRT formulation helps reconcile seemingly disparate observations across various studies regarding testosterone’s CV effects. While three prior meta-analyses suggested no significant increase in CV risk across TRT RCTs [4]-[6], a more recent meta-analysis by Xu et al. [10] indicated higher CV risk with TRT. The present meta-analysis is the most extensive thus far. Although we included all reported CV AEs in this meta-analysis, we have included newer studies exclusive to this review which may reflect less publication bias, more rigorous patient screening practices and more attention to the reporting of hard CV endpoints rather than nonspecific CV events that may have driven AE rates in previous studies.

The increased CV risk of the oral formulation subgroup is a novel finding in our analysis. While no significant effects on CV risk were observed with either injected or transdermal TRT, the point estimates suggest that further research is needed to establish whether administration by these routes is protective or detrimental, respectively. To the best of our knowledge, differing CV risk specific to varying testosterone formulations has not been previously reported.

DHT elevation and increased CV risk

The greater elevation of DHT that occurs with oral or transdermal TRT may be due to the high expression of 5-α reductase in skin [20] and liver [21] in comparison to lower 5-α reductase in skeletal muscle [89]. The finding of differential DHT elevation may be critical to our understanding of adverse CV risk, because elevated serum DHT (not elevated T) has recently been found to be associated with CV risk in several observational studies. Shores et al. published two studies of 1,032 older men which reported significant associations between the serum DHT concentration and both the 10-year rate of incident ischemic stroke [23] and the 9-year rate of incident CV disease and all-cause mortality [16] (see Figure 4). Interestingly, similar relationships did not exist for serum total or free T, suggesting that CV risk resulting from TRT may result from the 5α-reduction of T to DHT. In both studies by Shores et al., the lowest risk was associated with a serum DHT concentration of approximately 60 ng/dL, while greater risk was associated with both higher and lower DHT concentrations.

Figure 4 Comparison of DHT levels after testosterone treatment with DHT levels that are associated with cardiovascular disease risk. Left panel. Testosterone-induced elevation of DHT in the eight RCTs of testosterone injection, twenty RCTs of transdermal administration and four RCTs of oral testosterone administration shown in Table 2. Transdermal administration causes a greater elevation of serum DHT. Center panel. Data from panel 1 is overlayed on observational data from Shores et al. showing the relationship between serum DHT and 10 year risk of incident ischemic stroke in older men. The solid line represents the estimated hazard ratio (HR) and the shaded area depicts the 95% confidence interval. All models are adjusted for age (reprinted with permission from Shores et al. Clin Endocrinol (Oxf) 2014. doi: 10.1111/cen.12452 [22]. Right panel. Data from panel 1 is overlayed on observational data from Shores et al. showing the relationship between serum DHT and incident cardiovascular disease risk. The solid line represents the estimated hazard ratio (HR) and the shaded area depicts the 95% confidence intervals. All models are adjusted for age. (Reprinted with permission from Shores et al. J Clin Endocrinol Metab 2014, 99:2061-2068. [23]). DHT, dihydrotestosterone; RCT, randomized controlled trial. Full size image

In Figure 4, the left panel represents data from our meta-analysis showing the elevation of serum DHT with intramuscular, transdermal and oral TRT. In the center and right panels, we have superimposed that data on top of the previously published data from the two papers by Shores et al. Taken together, these data appear to indicate that intramuscular TRT elevates the serum DHT concentration into a range that is associated with reduced CV disease (CVD) and stroke risks. In contrast, transdermal and oral TRT appear to elevate serum DHT into a range that is associated with unchanged CVD risk and increased ischemic stroke risk.

Limitations

Reporting of AEs may be open to interpretation and so may vary somewhat among trials. Using the most serious CV events (stroke, myocardial infarction, and CV-related death) might be more unambiguous. Because of very long follow-up periods, such events are common enough to assess in observational studies [16],[22],[23]. However, due to shorter study duration, serious CV events are not common enough to study in clinical trials of TRT. As a result, our analyses are based on all CV events, serious or not.

The data on oral TRT must be interpreted with caution, since only four studies met the inclusion criteria. Of those, two had very low rates of CV events in both the treated and placebo groups [61],[64] and one study had very high post-treatment serum T concentrations [32], possibly due to the presence of liver disease in the study subjects. The latter study was not included in the analysis of TRT-induced elevations of T/DHT because DHT was not measured. However, among the four studies analyzed for T/DHT, there was considerable variation in serum concentrations. Variation may result from the fact that serum T concentrations are not sustained following oral TRT and the time of blood acquisition is therefore critical.

Two studies included in the analysis of CV risk were stopped early. One study of oral TRT was stopped because of lack of evidence for efficacy unrelated to CV [32] and one study of gel TRT was stopped early for excess CV events in the group receiving testosterone [7]. The first study, whose stopping was uninfluenced by CV has no bias associated with early stopping. The second, may actually be associated with a slight bias estimate away from the null, actually strengthening the null conclusion. There is no way to adjust for this without serial patient level data and the exact stopping rules used.

Interpretation of the data on TRT-induced elevations of T and DHT may be limited by the fact that DHT was assayed by several methods in the included studies. The latter include mass spectroscopy (MS) based methods and various radioimmunoassays (RIAs). MS-based assays provide highly accurate measurements of DHT. RIAs are specific for DHT [90] but the values are somewhat higher than those obtained with MS-based assays [91]. The enzyme-linked immunosorbant assay (EIA) for DHT is not valid as we have recently shown [24] and studies using this method were excluded. The current analysis is based on clinical trials that have a high rate of compliance. An additional limitation in extending our findings to a clinical setting is that compliance may be lower. Schoenfeld et al. have shown that TRT gel adherence is only 37.4% at six months [92]. Similarly, Donatucci et al. [93] reported that at three months, adherence to transdermal TRT was 52% and adherence to injected TRT was 32%.

Potential cardiovascular benefits of testosterone