To the best of our knowledge, the present study is the largest clinical study to investigate the association between serum T and AMH levels in infertile women. In this large retrospective cross-sectional study of 1,935 infertile women, higher serum T concentrations were associated with higher serum AMH levels after adjustment for potential confounders. Consistently, infertile women in the lowest T quartile had a 11.44-fold higher odds for the risk of DOR than those in the highest T quartile. The odds for the risk of EOR were 10.41-fold higher in infertile women in the highest T quartile than in those in in the lowest T quartile.
Androgens play important roles in the regulation of ovarian function. AR, expressed in oocytes, GCs and theca cells, is pivotal for normal follicular development 10,11. AR is most highly expressed in the GCs of preantral and early antral follicles, and its expression decreases as the follicles grow 13. Via the AR, androgens increase the FSH receptor and synergize with FSH to enhance follicle growth 19–21. Moreover, androgens support follicle health by decreasing follicle atresia and GC apoptosis, and stimulating the proliferation and differentiation of GC 20,22,23. Although AR is not expressed in primordial follicles, androgens promote primordial follicle initiation 24,25 via indirect mechanisms, such as upregulation of insulin-like growth factor 1 expression 25. The above information supports our results that lower T levels were associated with a higher risk of DOR. Studies have shown that women with DOR or POI demonstrated significantly lower serum T levels than controls 26,27. On the other hand, androgen excess may lead to impaired ovarian function and dysregulated follicle development, displaying irregular cycles, oligo-ovulation and polycystic ovaries 28,29. These findings agree with our results that infertile women with higher T levels had a higher risk of EOR. Thus, an optimal balance in androgenic actions is necessary for maintaining normal ovarian function. Serum T concentrations decline with age 14. Thus, our study demonstrated an age-specific normal reference range for serum T levels to aid in identifying women who suffer from androgen insufficiency or excess (Table 4).
As mentioned above, androgens enhance FSH activity through increased FSH receptor expression 20,21. FSH stimulates AMH expression 30,31, which could inhibit the sensitivity of preantral follicles to FSH to avoid premature selection by FSH in the gonadotrophin-independent stage 32,33. Therefore, Dewailly et al. proposed that androgens may promote AMH generation via enhancement of FSH-stimulated AMH expression 34. Elevated AMH could attenuate FSH-induced aromatase activity, leading to an increase in androgen levels 33. Moreover, via AMH receptor type 2 on the hypothalamus and pituitary, elevated AMH may boost GnRH-dependent LH pulsatility and secretion which stimulates androgen production in theca cells 35,36. Taken together, it seems that androgens and AMH mutually stimulate each other. These results support our results that serum T concentrations positively correlated with serum AMH levels. Some studies also showed a positive correlation between serum androgens and AMH 37–39. However, some studies revealed contradictory results, which indicated that androgens or FSH may have an inhibitory effect on AMH expression 40–42. Thus, the accurate relationship between androgens and AMH remains unclear. Further studies with ideal experimental models are needed to clarify the relationship.
Serum T levels have been suggested to be positively associated with ovarian response 43,44 and even pregnancy outcomes 44,45 in women undergoing IVF cycles. Although some conflicting studies have shown that serum T levels do not predict IVF outcomes 43,46, available data have indicated that T supplementation may improve ovarian response and IVF outcomes in PORs 47,48. In a randomized controlled trial of 110 PORs undergoing IVF cycle, Kim et al. reported that pretreatment with transdermal T gel significantly increased AFC and reduced the day of stimulation and total dosage of gonadotropins. In addition, the numbers of oocytes retrieved, mature oocytes, fertilized oocytes, and good-quality embryos were significantly higher in the T pretreatment group than in the control group 16. A meta-analysis of 7 randomized controlled trials conducted by Noventa and colleagues revealed that PORs receiving T therapy demonstrated higher numbers of total oocytes, MII oocytes and total embryos, as well as a higher clinical pregnancy rate and live birth rate than controls 47. On the other hand, the addition of insulin sensitizing agents to suppress insulin resistance and excess androgen may ameliorate the results of ovulation induction in PCOS patients 49,50. In the present study, we provided an age-specific normal reference range for serum T levels to help determine whether infertile women require agents for androgen enhancement or suppression (Table 4). Further large-scale, well-defined randomized controlled trials are still mandatory to confirm the effectiveness of androgen supplementation and androgen suppression by agents.
Several potential limitations should be taken into consideration when interpreting the data. First, the retrospective design of this study presented the major limitation. Second, since this is a cross-sectional study, a causal relationship could not be determined between serum T and AMH levels. Third, our study population only consisted of infertile women. We cannot be sure that our results would be applicable to the general population. Fourth, measurement of testosterone levels was not reliable in the lower range of values, especially in women. This may be a relevant source of bias. Fifth, serum concentrations of free T and sex hormone-binding globulin (SHBG) were not measured in this cohort because the two items were not included in our routine infertility evaluation.
In conclusion, our data reveal an obvious positive association between serum T and AMH levels in infertile women. Additionally, the risk of DOR was significantly increased in a dose-dependent manner across decreasing T quartile categories; the risk of EOR dose-dependently increased across increasing T quartile categories. Long-term longitudinal studies are required to confirm our results.