This is the first study investigating two different treatment options in CAIS individuals with regard to metabolic effects in a randomised controlled fashion. Both treatments resulted in a less favorable lipid profile, as there was a significant increase in total and LDL-cholesterol and a significant decrease in HDL-cholesterol. In addition, there was a slight but significant increase in BMI in both groups. Changes in cholesterol levels were however independent of changes in weight. Although the observed changes may be due to a simple time effect this is unlikely in view of the relatively short time period of six months in each treatment phase. Changes might be clinically significant as in almost half of patients with initially normal LDL-cholesterol levels in both treatment arms, levels rose into the pathological range. There were no significant differences between both treatments in terms of metabolic and safety parameters. Although the study might be underpowered regarding the detection of more subtle changes in the investigated outcomes, the results do not indicate that there are major differences between both treatments.
Despite the fact that there was no significant change in lipid parameters in our study within the 2-months run-in phase, it might have played a role that not all patients had received hormonal treatment in the last months before inclusion in the study and also in those who did, steroid exposure might have been different from that provided in a controlled fashion during participation in the trial where medication use was well monitored.
As being evident by the sexually dimorphic fat distribution pattern emerging with onset of puberty [18], sex steroids and in particular estradiol have significant implications in fat and lipid metabolism and regulation [19]. Ovariectomy in rodent models as well as natural menopause in women results in increases in adipose tissue [20] preferably in the abdominal region. These changes can be reversed by estrogen replacement [21, 22]. Also, in men, aromatization of testosterone to estradiol seems to be crucial in terms of energy homeostasis and lipid distribution [23]. Men with aromatase deficiency [24] as well as estrogen receptor defects [25] e.g., present with features of the metabolic syndrome despite normal testosterone levels. In men with aromatase deficiency estradiol substitution results in a significant increase in HDL- and decrease in LDL- cholesterol [24].
That estradiol as well as testosterone treatment in our study resulted in worsening of lipid parameters was unexpected and resemble the effects seen in hyperandrogenism in women [26]. There is further evidence that there might be a U-shaped relationship between androgen levels and cardiometabolic risk in women [27]. However, his is not a uniform finding [28] and it cannot conclusively explain the detrimental effects of both treatments in our study.
High concentrations of testosterone e.g., as seen in gender-affirming hormone treatment (GAHT) in transgender men (=male gender identity) result in a decrease in subcutaneous fat with unchanged visceral fat depots [9, 29]. In addition, GAHT usually results in an unfavorable lipid profile with a decrease in HDL- and an increase in LDL-cholesterol [9]. In contrast, aside from CAIS, a clear human model for isolated hypoandrogenism without estradiol deficiency is missing. We can therefore only speculate on the cause of the changes observed in our study.
Unfortunately, there is a paucity of studies on metabolic characteristic in women with CAIS in general and on the effects of hormone replacement in particular that could help us to classify our results. An unfavorable metabolic profile in women with CAIS receiving estrogen replacement was also documented in a small cross-sectional study by Dati and colleagues [12]. CAIS women presented with lower fat free mass and elevated total and LDL-cholesterol levels and increased insulin resistance in comparison to control women. While overall mean BMI was not significantly increased, prevalence of obesity was higher in women with CAIS than expected for the corresponding Italian reference population. The comparability with our study is however limited by the fact that patients in the aforementioned study received various regimens of estrogen replacement and 23% of patients included had not undergone gonadectomy and therefore preserved testosterone secretion.
In another study by Tsimaris and colleagues [30] the effects of six months sex hormone replacement on cardiometabolic parameters were investigated in 23 patients with 46,XY DSD. Hormone therapy was initiated either after gonadectomy, or after a 3-month wash-out period of prior HT. In contrast to our study, the authors could show that hormone replacement therapy resulted in a raise in HDL-cholesterol and a decrease in triglyceride levels, while there were no changes in total and LDL-cholesterol. It has however to be highlighted that this study also included eight patients with XY gonadal dysgenesis and results were not reported separately for the remaining CAIS patients. Furthermore, all patients received 1mg norethisterone acetate in addition to 2mg of 17b- estradiol orally. It is known that progestins can have independent effects on metabolism [31, 32] and similar effects should be expected for women with CAIS with an intact progesterone receptor (PR). But progestins usually result in a decrease in HDL-cholesterol in the general population.
In addition, it has been demonstrated that there is a difference in the effects of estrogens on lipid levels depending on the application route [33]. While in postmenopausal women, both oral and transdermal application have beneficial effects on the HDL/LDL-cholesterol-ratios, this effect is more pronounced when taken orally [33, 34], while transdermal application is considered to have more favorable effects on triglyceride levels [34].
Finally, as expected, there were no significant differences in any other investigated parameters, including hemoglobin levels. Hemoglobin production is highly sensitive to testosterone [35] and its effects are also preserved on a female genetic background, as having been demonstrated by the fact that a few months of treatment in transmen are usually sufficient to increase its levels to those of the general male population [36]. That the effects of testosterone on hematopoiesis do not depend on aromatization [37] is in line with unchanged hemoglobin levels found in our study.
A strength of our study is that it is the first of its kind investigating two treatment options in a molecularly well-defined cohort of CAIS patients in a randomized controlled fashion. A limitation of our study is that the groups were rather small due to the relatively high drop-out rates and that power-analysis was not performed for secondary outcomes. This might have resulted in a bias. However, subjects who dropped out of the study did not differ in the investigate baseline characteristics in comparisons to those who successfully completed the study.
While the effects on cholesterol levels were quite strong, the study may be underpowered for the detection of more subtle group effects in other parameters. In addition, the subjects in our study were put on a fixed dosage of transdermal sex steroids. Although the corresponding dosage was selected on the basis of providing according to the manufacturer’s average serum levels within the reference range of men and women from the general population, there is a high inter-individual variance due to differences in transdermal absorption. A dose titration scheme-based protocol might therefore have helped to average out these differences.
Nevertheless, given the rarity of the disease and the paucity of studies on metabolic effects of hormone treatment in CAIS in general, in our opinion, this study adds valuable information to the understanding of the action of sex steroids on metabolism in this unique condition.
In addition, anthropometric measures such as body composition and waist/hip circumference were not recorded, therefore we cannot determine if the observed changes in BMI were due to an increase in fat or lean mass or simple water retention.