In the present study, when using spot urine samples collected on the same day as a blood sample, urinary BPA concentrations were associated with serum levels of FSH, inhibin B, FSH:inhibin B ratio, and E2
:T ratio. When one or two subsequent urine samples collected in the weeks or months following the collection of the serum sample were taken into consideration, the associations involving FSH and inhibin B weakened, but the inverse relationship with E2:T ratio remained consistent. In these analyses involving multiple urine samples, we also found associations between urinary BPA concentrations and SHBG, FAI, estradiol, and TSH. Based on our report of temporal within-individual variability in urinary BPA concentrations (14
), the use of multiple urinary BPA measures may reflect an individual’s exposure level over time more adequately than a single measure. Therefore, while the subsequent urine samples were collected weeks or months after the collection of the serum samples used for hormone level measurement, we included these urine samples in the analysis. However, because multiple urinary BPA concentrations were not available for all participants, the additional samples were collected outside of the exposure time window likely to be of most interest (i.e. days or weeks leading up to measurement of serum hormone levels as opposed to after hormone levels were measured), and the number of samples and the time interval between their collection varied among the men with multiple BPA measures, we analyzed the data using four different approaches, as presented in .
The interpretation of the differences in effect estimates and confidence intervals between approaches 3) and 4) needs to be done cautiously because of the small sample size. Also, because of the large number of comparisons made some of the observed associations may be chance findings. Overall, the magnitude and direction of the effect estimates for both approaches were similar, though there were large differences in the magnitude of estimates for inhibin B (which also results in differences for the FSH:Inhibin B ratio). From a comparison of these analyses it is not possible to determine why there are differences in effect estimates. However, based on these results and our earlier work on urinary temporal variability in urinary BPA concentrations, we recommend collecting more than one sample per person to better estimate exposure, preferably within the most relevant time window of interest (e.g. prior to serum collection for the measurement of hormones as opposed to after serum collection).
Despite the variable findings obtained with these different statistical analysis approaches, our findings can be compared with the in vitro
, animal, and limited human data on BPA exposure and endocrine function. In humans, serum BPA concentrations were associated with increased androgen levels, but not estradiol, LH, or FSH levels, among women in two small studies (n=41 and n=74) (12
). In a study of 42 male workers exposed to bisphenol A diglycidyl ether and mixed organic solvents and 42 male matched control workers, urinary BPA was inversely associated with plasma FSH levels, but not LH or free testosterone (11
). These reports are inconsistent with our observation of a positive association between urinary BPA and FSH. However, our finding, along with the inverse association between BPA and inhibin B and the positive association between BPA and FSH:inhibin B ratio, may be consistent with animal and in vitro
studies showing reduced testicular inhibin levels (20
) and adverse effects on Sertoli cell function and sperm production in relation to BPA exposure (21
). FSH and inhibin B are the two hormones most highly predictive of semen quality, where elevated levels of FSH and/or low levels of inhibin B are associated with poorer semen quality (16
). FSH, a gonadotropin produced and secreted by the anterior pituitary, acts on Sertoli cells in the seminiferous tubules to initiate spermatogenesis. In addition to nurturing and protecting developing germ cells during spermatogenesis, Sertoli cells also produce and secrete inhibin B, a protein hormone, which then exerts negative feedback on the anterior pituitary to inhibit FSH secretion (34
). Thus, BPA may be associated with adverse effects on the Sertoli cells or their FSH receptors to alter inhibin B production. This may, in turn, through reduced negative inhibin B feedback lead to increased pituitary FSH production and secretion, potentially resulting in reduced sperm production and semen quality. However, additional studies, including human studies with larger sample sizes and measures of semen and sperm quality, are needed to further examine this hypothesis.
Our finding of an inverse association between urinary BPA and the serum E2
:T ratio using all four statistical analysis approaches is also consistent with previous reports. Since estradiol is produced through the aromatization of testosterone, a reduction in the E2
:T ratio is considered a marker for declined aromatase activity. BPA has been shown to suppress aromatase in rat Leydig cells (29
), rat ovarian granulosa cells (36
), and placental JEG-3 cells (37
). While our findings are consistent with BPA suppression of aromatase activity, an alternative explanation for a decreased E2
:T ratio may involve differential effects of BPA exposure on testosterone and estradiol metabolism.
When considering multiple urinary BPA measures from individual participants, we found inverse associations between BPA and FAI, estradiol, and TSH. With regards to a reduced FAI, BPA has demonstrated anti-androgenic activity in a number of studies (5
), although it was also found to be a strong SHBG ligand (44
). The inverse association with circulating estradiol in this analysis was also consistent with a study involving both in vivo
and in vitro
experiments of rats and rat Leydig cells that reported declines in Leydig cell estradiol biosynthesis and circulating levels estradiol following environmentally relevant doses of BPA; these effects were attributed to BPA-induced inhibition of Leydig cell aromatase activity (29
). Finally, perhaps consistent with our finding of an inverse relationship between BPA and TSH, with no corresponding positive association between BPA and free T4 or total T3, is a recent report showing that BPA suppresses TSH release from amphibian pituitary in a manner that is independent of both the thyroid hormone feedback mechanism and BPA’s estrogenic activity (9
This study has several limitations. First, the present study is cross-sectional in nature due to the availability of only a single hormone measure from each participant, as well as only one urine sample from over half of the men. Thus, we cannot rule out reverse causation to explain our findings. For example, hormonal status could be associated with altered BPA metabolism and result in associations between hormone levels and urinary BPA concentrations (45
). If this were the case, information on predictors of altered BPA metabolism would also be of great interest given the broad range of health concerns surrounding BPA exposure. Another limitation is the likelihood of exposure measurement error due to the high within-individual temporal variability in BPA exposure and the availability of multiple BPA measures from only a subset of participants. However, measurement error would be expected to be non-differential, which would tend to reduce the ability to detect associations between exposure and outcome. In addition, when using broad exposure categories (e.g. quartiles as presented in ), a single measure may adequately predict an individual’s exposure category over a longer period of time such as several months (14
). Finally, because the present study was conducted among men recruited through an infertility clinic our ability to generalize the results to the general population may be limited. However, because to date no evidence exists that men from an infertility clinic are differentially affected by BPA exposure we expect our results to be generalizable.
In conclusion, human exposure to BPA may be associated with alterations in circulating hormone levels. Additional studies assessing the relationship between BPA exposure and endocrine function and downstream clinical implications are needed. These studies should be designed to utilize BPA exposure biomarkers from multiple time points in the exposure time-window that is most relevant to the health outcomes of interest.