comparison, the clinical dosage range of spironolactone in humans is usually about 4 to 8 mg/kg. there is also mixed/conflicting evidence that spironolactone may inhibit 5α-reductase, and thus the synthesis of the potent androgen dht from testosterone, to some extent. however, the combination of spironolactone and the potent 5α-reductase inhibitor finasteride has been found to have significant improved effectiveness in the treatment of hirsutism relative to spironolactone therapy alone, suggesting that any inhibition of 5α-reductase by spironolactone is only weak or at best incomplete. spironolactone has been found not to have activity as an aromatase inhibitor. estrogenic activity spironolactone has been found to directly interact with the er. a study using human uterine tissue found that a 1,000-fold excess of spironolactone (0.3–2 μm) resulted in no displacement of estradiol from the er. however, a subsequent study found that the medication did interact with the human er at higher concentrations, albeit with very low affinity (ki = 20 μm). in the same study, spironolactone was administered to rats and found to produce mixed estrogenic and antiestrogenic or selective estrogen receptor modulator (serm)-like effects that were described as very similar to those of tamoxifen. in spite of the fact that tamoxifen had two orders of magnitude higher affinity for the er than spironolactone however, the two medications showed similar potency in vivo. the likelihood of spironolactone interacting with the er itself is remote in consideration of its very low affinity for the receptor in vitro. however, it has been hypothesized that metabolism of spironolactone might result in active metabolites with greater er affinity, which might potentially account for the activity. one of the major active metabolites of spironolactone, canrenone, likewise did not interact with the human uterine er at up to a more than 5,000-fold excess (25 μm). in other research, spironolactone has been found to have no estrogenic or antiestrogenic effects in the uterus when administered by subcutaneous injection in rodents even at very high doses. the authors of the study concluded that direct interaction of spironolactone (and/or its metabolites) with the er could be involved in the gynecomastia, feminization, and effects on gonadotropin levels that the medication is associated with. subsequently, it has also been suggested that, as a serm-like medication, er agonistic activity of spironolactone in the pituitary gland could be responsible for its antigonadotropic effects while er antagonistic activity of spironolactone in the endometrium could be responsible for the menstrual disturbances that are associated with it. such actions might explain these effects of spironolactone in light of the finding that it is not significantly progestogenic or antiprogestogenic in women even at high dosages. in accordance, a study found that in women treated with a gnrh analogue, spironolactone therapy almost completely prevented the bone loss that is associated with these medications, whereas treatment with the selective ar antagonist flutamide had no such effect. other studies have also found an inverse relationship between spironolactone and decreased bone mineral density and bone fractures in men. estrogens are well known for maintaining and having positive effects on bone, and it has been suggested that the estrogenic activity of spironolactone may be involved in its positive effects on bone mineral density. high levels of aldosterone have been associated with adverse bone changes, and so the antimineralocorticoid activity of spironolactone might partially or fully be responsible for these effects as a potential alternative explanation. in addition to potential direct interaction with the er, spironolactone also has some indirect estrogenic activity, which it mediates via several actions, including: by acting as an antiandrogen, as androgens can suppress both estrogen production and signaling (e.g., in the breasts). inhibition of the conversion of estradiol to estrone, resulting in an increase in the ratio of circulating estradiol to estrone. estradiol is far more potent than estrone as an estrogen, which is comparatively almost inactive. enhancement of the rate of peripheral conversion of testosterone into estradiol, thus decreasing the ratio of circulating testosterone to estradiol. spironolactone has been found to act as a reversible inhibitor of human 17β-hydroxysteroid dehydrogenase 2 (17β-hsd2), albeit with weak potency (ki = 0.25–2.4 μm; ic50 = 0.27–1.1 μm). c7α thioalkyl derivatives of spironolactone like the 7α-thioethyl analogue were found to inhibit the enzyme with greater potency, suggesting that the actual active metabolites of spironolactone like 7α-tms might be more potent inhibitors. 17β-hsd2 is a key enzyme responsible for inactivation of estradiol into estrone in various tissues, and inhibition of 17β-hsd2 by spironolactone may be involved in the gynecomastia and altered ratio of circulating testosterone to estradiol associated with the medication. spironolactone has also been associated with positive effects on bone, and it is notable that 17β-hsd2 inhibitors are under investigation as potential novel treatments for osteoporosis due to their ability to prevent estradiol inactivation in this tissue. in contrast to 17β-hsd2, spironolactone does not appear to inhibit 17β-hydroxysteroid dehydrogenase 1 (17β-hsd1) in vitro. in contrast to estrogens and serms, spironolactone does not increase sex hormone-binding globulin (shbg) levels or risk of venous thromboembolism. nor is there an increase in the risk of breast cancer or endometrial cancer with spironolactone. spironolactone is known to produce a high rate of menstrual irregularities in women at around the middle of the menstrual cycle, when ovulation occurs. a study investigated the effects of 100 mg/day spironolactone on the endometrium and hormone levels and the mechanism for the menstrual abnormalities associated with it in a group of women with polycystic ovary syndrome. they found that spironolactone, likely due to inhibition of steroidogenesis, blunted the surge in estradiol levels around ovulation, with estradiol levels of about 41 to 66% of normal at this time. conversely, periovulatory levels of the gonadotropins, luteinizing hormone and follicle-stimulating hormone, were unchanged. spironolactone significantly decreased endometrial thickness in the women (by 22 to 33%), and as estrogens stimulate endometrial growth, this effect of spironolactone may have been due to the decreased levels of estradiol. as such, spironolactone may have a functional antiestrogenic effect around ovulation in women, and this may be involved in its side effect of menstrual irregularities at high doses. progestogenic activity spironolactone has weak progestogenic activity in bioassays. its actions in this regard are a result of direct agonist activity at the pr, though with a very low half-maximal potency. spironolactone's progestogenic activity has been suggested to be involved in some of its side effects, including the menstrual irregularities seen in women and the undesirable serum lipid profile changes that are seen at higher doses. it has also been suggested to augment the gynecomastia caused by the estrogenic effects of spironolactone, as progesterone is known to be involved in mammary gland development. a major active metabolite of spironolactone, canrenone, has been found to interact with the human uterine pr with a ki of 300 nm. this is relatively weak at approximately 100-fold less than that of progesterone. in any case, levels of canrenone of up to 1,200 nm have been observed in some studies with 100 to 200 mg doses of spironolactone. it has been widely stated that the menstrual irregularities associated with spironolactone are due to its progestogenic activity, and animal studies, both in rabbits and rhesus monkeys, have shown clear progestogenic effects of spironolactone. however, the dosages of spironolactone used in animals to produce progestogenic effects were very high (50–200 mg/kg/day in rabbits, 400 mg/day in rhesus monkeys). in one study, the threshold dose by subcutaneous injection for endometrial transformation in rabbits was 0.003–0.01 mg for cyproterone acetate, 0.1–0.3 mg for drospirenone, 0.5 mg for progesterone, and 10–20 mg for spironolactone. spironolactone orally at 40 mg/kg/day failed to show an antigonadotropic effect or decrease testosterone levels in male cynomolgus monkeys, whereas oral drospirenone at 4 mg/day was effective and strongly suppressed testosterone levels. in addition, no evidence of progestogenic nor antiprogestogenic effects (as assessed by endometrial changes) have been observed in women even with high doses of spironolactone. as such, the progestogenic potency of spironolactone appears to be below the level of clinical significance in humans. moreover, the menstrual abnormalities associated with spironolactone must have a different cause. suggestions for other possible mechanisms for the menstrual disturbances of spironolactone include interference with the hypothalamic–pituitary–gonadal axis, inhibition of enzymatic steroidogenesis, and mixed estrogenic and antiestrogenic activity. antigonadotropic effects pure ar antagonists like flutamide and bicalutamide are potent progonadotropins with indirect estrogenic activity in males. this is because they block the ar in the pituitary gland and hypothalamus and thereby inhibit the negative feedback of androgens on the hypothalamic–pituitary–gonadal axis (hpg axis). this, in turn, results in increased gonadotropin secretion, activation of gonadal steroidogenesis, and an up to 2-fold increase in testosterone levels and 2.5-fold increase in estradiol levels. conversely, ar antagonists that are also progestogens, like cyproterone acetate, are not progonadotropic, as activation of the pr is antigonadotropic and preserves negative feedback on the hpg axis, and these medications are indeed potently antigonadotropic in clinical practice. although spironolactone is an ar antagonist with no significant progestogenic effects in women even at high dosages and hence is a pure-like ar antagonist, many studies have not found it to be progonadotropic in men, nor to increase testosterone or estradiol levels. moreover, spironolactone is also said to possess very little or no antigonadotropic activity (in terms of lowering gonadotropin levels to below normal) even at high dosages, although some conflicting reports exist. nonetheless, since spironolactone does not generally increase gonadotropin levels in spite of inhibition of androgen signaling, it must have some degree of antigonadotropic activity sufficient to at least keep the hpg axis from being upregulated. as estrogens are antigonadotropic similarly to progestogens, and as serm-like activity has been described for spironolactone, the antigonadotropic effects of spironolactone may be due to estrogenic activity. glucocorticoid activity spironolactone weakly binds to and acts as an antagonist of the gr, showing antiglucocorticoid properties, but to a significant degree only at very high concentrations that are probably not clinically relevant. most studies have found that spironolactone has no effect on cortisol levels. however, some studies have observed increased cortisol levels with spironolactone treatment. it is thought that this is due to the antimineralocorticoid activity of spironolactone, which prevents negative feedback of mineralocorticoids like aldosterone on the hypothalamic–pituitary–adrenal axis (hpa axis), and thereby upregulates adrenocorticotropic hormone (acth) and adrenal gland synthesis of corticosteroids such as cortisol and aldosterone. although spironolactone usually doesn't affect cortisol levels, it does significantly increase concentrations of aldosterone. spironolactone has been shown to weakly inhibit 11β-hydroxylase in vitro. this enzyme is responsible for the transformation of 11-deoxycorticosterone into corticosterone and of 11-deoxycortisol into cortisol. the clinical significance of this action, if any, is unclear. spironolactone notably does not decrease cortisol or aldosterone levels. other activities spironolactone is an agonist of the pregnane x receptor (pxr), a xenobiotic-sensing receptor. by activating the pxr, spironolactone induces the expression of cytochrome p450 cyp3a enzymes and of the atp-binding cassette transporter (abc transporters) p-glycoprotein (multidrug resistance protein 1; mrp1; abcb1) and multidrug resistance protein 2 (mrp2; abcc2). these proteins are involved in the metabolism and elimination of xenobiotics, and as a result, their induction by spironolactone may contribute to drug interactions of spironolactone. examples of such interactions include oral digoxin and estradiol. there is evidence that spironolactone may block voltage-dependent ca2+ channels. it has also been found to block voltage-gated potassium (k+) channels. spironolactone has been identified as an inhibitor of nrg1‐erbb4 signaling. spironolactone has been found to act as a potent inhibitor of the pannexin 1 channel, and this action appears to be involved in its antihypertensive effects independently of mr antagonism. spironolactone has been found to block herg channels. hormone levels a 2 sysreview of 18 studies found that spironolactone shows no significant effects on levels of estrogen, estradiol, testosterone, androstened, dehydroep sulfate, luteinizing hormone, or follicle-stimulating hormone in women. a 2017 hybrid systematic review reported that data from 50 studies on androgen levels with spironolactone in women were equivocal. a 1993 review reported that changes in hormone levels with spironolactone were very heterogeneous, with most changes not achieving significance. levels of cortisol were unchanged in all studies (four total) and levels of dehydroepiandrosterone sulfate were unchanged in all but one of seven studies. however, it was reported that testosterone levels were decreased in 81% of the studies (13 of 16). however, neither of the two placebo-controlled trials in the review found a significant