Impact of Tamoxifen on Endometrial Thickness in Women With Thin Endometrium Following Estrogen Therapy During Frozen-Thawed Embryo Transfer.

This pilot study was conducted at Zeinabieh Hospital, affiliated to Shiraz University of Medical Sciences in Shiraz, Iran, during the years 2019 and 2021. The study received approval from the Ethics Committee of Shiraz University of Medical Sciences, with the approval ID IR.SUMS.REC.1395.25. The participants were three consecutive infertile patients aged 31–40 who were referred to our center for IVF and had not responded to standard and high doses of estrogen therapy in their previous IVF‐FET cycles. Inclusion criteria specified that participants must have a body mass index (BMI) between 18.5 and 25 kg/m 2 , and at least one frozen 3‐days embryo with a normal uterine cavity, confirmed through hysterosalpingography (HSG), saline infusion sonography (SIS), or hysteroscopy. Exclusion criteria included patients with drug sensitivities, drug or alcohol addiction, and systemic diseases such as renal disease, diabetes mellitus, or autoimmune disorders. Additionally, patients with any space‐occupying lesions in the uterine cavity were also excluded from the study. Demographic and medical characteristics of patients were documented, including age, BMI, history of any systemic diseases, previous pelvic surgeries, and current medications. Additionally, the type of infertility (primary or secondary), the cause of infertility (such as polycystic ovary syndrome, tubal factors, male factors, and unexplained infertility), and the duration of infertility were also recorded. Upon admission, patients had a comprehensive physical and pelvic examinations. ET was measured using ultrasonography prior to initiating tamoxifen treatment. Tamoxifen was administered starting from the second day of the menstrual cycle for a duration of 5 days, at a daily dose of 20 mg. On the seventh day of menstruation, a second transvaginal sonography was performed to evaluate the ET and the trilinear pattern. If the endometrium was deemed adequate, defined as an ET of 7 mm or greater, progesterone was started based on the age of the embryos, and embryos were transferred. Following the embryo transfer, luteal support with progesterone were continued according to the standard protocols.

Masoumeh Zaree: investigation, project administration. Bahia Namavar Jahromi: formal analysis, validation, visualization. Mohammad Ebrahim Parsanezhad: software, validation, visualization, writing – review and editing. Zahra Mosallanezhad: conceptualization, data curation, investigation. Seyedeh‐Sara Hashemi: conceptualization, data curation, methodology, resources, supervision. Fatemeh YaghoobiGorgi: conceptualization, data curation, formal analysis, investigation, methodology, resources, software, validation, visualization. Alireza Salehi: conceptualization, data curation, formal analysis, investigation, methodology, project administration, resources, software, validation, visualization, writing – original draft, writing – review and editing.

Before submission, a written consent was obtained from each patient. The consent forms are available from the corresponding author upon reasonable request.

A total of three patients with an ET of 2 mm or less, as determined by the initial TVS, were enrolled in this study after they had been unable to achieve a sufficient increase in their ET during previous IVF‐ET cycles. The protocols used to attain the desired ET for these patients are outlined below. The first patient was a 40‐year‐old woman who has been experiencing primary male factor infertility for 7 years. Her BMI was 24.25 kg/m 2 , and she had no history of systemic or autoimmune diseases. Her medication history was unremarkable, and she had not undergone any previous pelvic surgeries. A SIS evaluation of her uterine cavity yielded normal results. The patient had regular menstrual cycles and showed no gynecological abnormalities. The patient underwent four cycles of high‐dose estrogen therapy previously; however, the ET always remained below 6 mm. In the current cycle, oral tamoxifen was initiated at a dosage of 20 mg daily, starting on the second day of her menstrual cycle. A transvaginal sonography was performed 5 days later, on the seventh day of menstruation, which showed an ET of 5.3 mm. Tamoxifen therapy continued until the eighth day of her cycle, and a second sonography was conducted on the 11th day. At that time, the endometrial lining appeared trilinear and thickness increased to 7.4 mm. FET was performed according to the standard protocol; however, the patient did not achieve pregnancy. Baseline evaluation demonstrated a normal ovarian reserve and endocrine profile: day‐3 FSH was 7 mIU/mL (reference: 2.8–11.3 mIU/mL) [ 16 ], LH was 8 mIU/mL (1.1–11.6 mIU/mL), estradiol was 60 pg/mL (30–100 pg/mL), TSH was 3.5 mIU/mL (0.4–4.0 mIU/mL) [ 17 ], prolactin was 14 μg/L (5–20 μg/L), and day‐21 progesterone was 10 ng/mL (6–11 ng/mL) [ 18 ] (Table  1 ). Estimated parameters. Note: Progesterone was obtained at the 21st day of the menstrual cycle, while others were obtained at the third day. Our second patient was a 31‐year‐old woman with a BMI of 25 kg/m 2 and regular menstrual cycles. She had been experiencing secondary infertility for 12 years and had a history of fibroids, that had been surgically removed 15 years ago. Additionally, she underwent a uterine curettage 12 years ago to remove retained placental tissue following the loss of a 6‐week‐old fetus. Her medical history was otherwise unremarkable, with no systemic medical disorders reported. Despite receiving a high dose of estrogen therapy in her previous cycle (4 mg of estradiol twice a day until the 12th day of the cycle, which was later increased to 6 mg twice a day), her endometrial thickness did not improve. In this current cycle, we initiated treatment with 20 mg of oral tamoxifen daily starting on the second day of her menstrual cycle. Initially, her ET measured about 2 mm before beginning the tamoxifen. By the seventh and nineth days of the cycle, her ET increased to 5.8 and 6.2 mm, respectively. After this, we discontinued the tamoxifen therapy and introduced conjugated estrogen at a dose of 1.25 mg daily for 3 days. As a result, her ET improved to 7.6 mm by the 11th day of the cycle. We then administered two additional doses of conjugated estrogen (1.25 mg each) over the next 3 days and FET was performed on the 18th day of her menstrual cycle; however, the patient did not achieve a pregnancy. This patient with secondary infertility presented with a normal endocrine profile on day 3 of her cycle—FSH 3 mIU/mL, LH 5.5 mIU/mL, estradiol 110 pg/mL, TSH 1.5 mIU/mL, and prolactin 12 μg/L—all well within established 2.5th–97.5th percentile reference intervals (FSH 2.59–15.12 mIU/mL; LH 2.75–20.68 mIU/mL; E 2 52–439.91 pmol/L; TSH 0.52–4.16 mIU/mL; prolactin 5.13–37.35 ng/mL). Serum progesterone on day 21 measured 18 ng/mL, consistent with the midluteal phase threshold (> 21 nmol/L or ~6.6 ng/mL) [ 19 ] (Table  2 ). Estimated parameters. Note: Progesterone was obtained at the 21th day of menstrual cycle, while others were obtained at third day. The patient was a 38‐year‐old woman with a BMI of 23.43 kg/m 2 and had 6 years of primary male factor infertility. She was referred to our center for IVF‐FET. She had no systemic disorders, and her menstrual cycle was completely regular. The patient previously had four cycles of high‐dose estrogen therapy, receiving upto 12 mg of estradiol daily, but did not achieve the desired ET. On the second day of her cycle, oral tamoxifen was initiated at a dose of 20 mg per day and continued until the seventh day. A TVS performed on the seventh day showed an endometrial thickness of 4.51 mm. From the 7th to the 11th day of the cycle, 1.25 mg of conjugated estrogen was added to her treatment regimen. By the 12th day of the cycle, her endometrial thickness increased to approximately 5.32 mm, exhibiting a trilinear pattern. However, due to the thin endometrium, embryo transfer was not performed. In this patient with primary male factor infertility and a normal BMI, baseline day‐3 endocrine parameters (FSH 5.5 mIU/mL; LH 7 mIU/mL; estradiol 80 pg/mL; TSH 3 mIU/mL; prolactin 15 μg/L) and midluteal progesterone (15 ng/mL) were all within established reference intervals [ 20 , 21 ] (Table  3 ). Estimated parameters. Note: Progesterone was obtained at the 21th day of menstrual cycle, while others were obtained at third day.

In this study, we evaluated the effect of tamoxifen on achieving optimal endometrial thickness for IVF‐FET. It is known that endometrial thickness below 7 mm in frozen–thawed embryo transfer (FET) cycles is linked to significantly lower implantation and live birth rates [ 22 ], whereas achieving ≥ 8–10 mm is associated with optimal clinical outcomes [ 23 ]. Addition of ifen, a SERM, exerts agonist effects on endometrial receptors and has been shown to increase endometrial thickness even more effectively than hormone replacement therapy (HRT) alone in some studies, which increased thickness from 6.2 ± 1.0 to 8.2 ± 1.7 mm in tamoxifen cycles ( p  < 0.05) [ 19 , 24 , 25 ]. Nevertheless, the failure to achieve pregnancy despite a trilaminar, 7.4 mm endometrium in our case suggests that thickness alone may not fully predict implantation success; additional factors such as endometrial receptivity markers and uterine perfusion warrant optimization. For refractory thin endometrium, adjunctive strategies, such as intrauterine platelet‐rich plasma or vaginal sildenafil, had been studied to further enhance endometrial growth and receptivity. It was observed that optimal live birth rates in HRT‐FET cycles correlate with endometrial thickness between 8.7 and 14.5 mm, with thinner linings yielding significantly lower success rates [ 24 , 26 ]. Moreover, combining tamoxifen with HRT protocols further enhances endometrial proliferation, supporting tamoxifen + HRT as a viable preparation for thin endometrium in FET cycles [ 25 ]. Pure estrogen augmentation alone can achieve thickness > 7 mm in refractory cases, yet may still fall short of the ≥ 8 mm target for optimal implantation [ 25 ]. For persistently thin linings, emerging adjuvant therapies such as intrauterine platelet‐rich plasma infusion have demonstrated significant thickness gains (to ~8.7 ± 0.6 mm) and improved clinical pregnancy rates in randomized trials [ 27 ]. Additionally, vaginal sildenafil combined with estradiol valerate has been shown to enhance uterine perfusion and further increase endometrial thickness in unexplained infertility [ 28 ]. In the case of the third patient, a large body of evidence indicates that an ET ≥ 8–10 mm on the day of progesterone initiation or embryo transfer is associated with significantly higher implantation and live birth rates in FET cycles [ 29 ]. Tamoxifen, acting as an SERM in the endometrium, has been shown to improve ET more effectively than estrogen alone, with studies reporting mean thickness increases to ≥ 8 mm and enhanced live birth rates [ 19 ]. In our case, however, tamoxifen plus adjunctive conjugated estrogen produced only a modest ET gain, underscoring that even combined SERM–HRT protocols may fall short in refractory thin endometrium. Emerging adjuvant therapies have demonstrated ET improvements to approximately 8.7 ± 0.6 mm and higher clinical pregnancy rates in randomized studies of women with previous thin‐endometrium FET failures [ 30 , 31 ]. Briefly, in our first patient, the administration of tamoxifen alone resulted in an ideal endometrial thickness characterized by a trilinear pattern. However, in the second patient, tamoxifen was not effective in increasing the endometrial thickness. It appears that continuing treatment with conjugated estrogen significantly improved the endometrial thickness in this case. For the third patient, we administered tamoxifen until the 11th day of the menstrual cycle, in combination with conjugated estrogen, but the desired endometrial thickness was not achieved, even though the trilinear pattern was present. We suggest that the minimal response in this case, despite the combination treatment of tamoxifen and high‐dose estrogen, may be attributed to the patient's intrinsic uterine factors. Additionally, previous studies have reported that the mean ET in patients with endometriosis is lower than that of control subjects [ 32 ]. So, It is possible that the patients with a history of endometriosis might show limited improvements in ET following extensive treatments. Tamoxifen is a SERM that is primarily used to treat breast cancer [ 13 , 33 ]. Several studies suggest that this drug has a complex hormonal action and induces mild estrogenic effects [ 13 , 14 , 34 ]. Tamoxifen resembles clomiphene, which decreases the amount of circulating estrogen available to the hypothalamus, thereby triggering the secretion of GnRH [ 34 ]. As a result, estradiol levels are elevated in the presence of tamoxifen. On the other hand, some studies indicate that tamoxifen can directly affect the ovaries, leading to excessive follicular growth and an increase in serum estradiol levels. Additionally, research has shown that tamoxifen can enhance the effects of estrogen on the endometrium, and this effect may persist even after the drug is discontinued [ 13 , 14 , 33 , 34 ]. Several studies have evaluated the effect of tamoxifen on ET with varying results [ 35 ]. A recent study reported that tamoxifen significantly improved endometrial proliferation compared to hormone replacement therapy in patients undergoing IVF‐FET [ 35 ]. Additionally, earlier research found that administering tamoxifen to patients with an ET of less than 7 mm after ovulation induction with clomiphene citrate led to a significant improvement in endometrial proliferation [ 36 ]. These findings, however, contrast with our observations in two of our cases (Cases 2 and 3), where single‐drug treatment with tamoxifen did not result in any improvement in ET for these patients. Tamoxifen is utilized in the treatment of thin endometrium, as evidenced by various studies. In one study conducted by Chen et al., tamoxifen was administered to three women with unresponsive thin endometrial thickness, resulting in an endometrial thickness increase to at least 7.7 mm in all cases [ 37 ]. Additionally, another study focusing on women undergoing FET found that the endometrial thickness increased from 6.5 to 8.8 mm following tamoxifen administration [ 25 ]. In a study involving 226 women with ET of less than 7.5 mm [ 38 ], various endometrial preparation protocols were evaluated. Across all protocols, the addition of tamoxifen effectively improved ET. Research indicates that tamoxifen is associated with an increase in total estrogen metabolites, suggesting that it enhances local estrogen biosynthesis in endometrial cells. This increase in estrogen biosynthesis could be attributed to tamoxifen's stimulation of the GPR30 estrogen receptor (GPER), which activates the steroidogenic factor 1 (SF‐1). SF‐1 is a transcription factor that induces the expression of aromatase in endometrial cells, thereby promoting endometrial proliferation mediated by tamoxifen [ 39 , 40 ]. Additionally, tamoxifen upregulates the expression of Ki‐67, a marker of cell proliferation, as well as insulin‐like growth factor 1 (IGF‐1) [ 41 ]. Therefore, tamoxifen increases endometrial thickness through both estrogenic and non‐estrogenic pathways. As observed in our second and third patients, the combination therapy of tamoxifen and estrogen appears to improve ET and endometrial pattern more effectively than tamoxifen alone in certain cases. In another study, three IVF patients experienced recurrent thin endometrium measuring less than 6 mm, despite undergoing extended estrogen therapy and aspirin administration during their FET cycles. All three patients had previously undergone at least three other unsatisfactory treatment cycles that resulted in cancellations. However, after receiving tamoxifen, all of them achieved appropriate endometrial thicknesses [ 37 ]. Some studies indicate that the impact of tamoxifen on the endometrium may be influenced by factors such as treatment duration, cumulative dose, and possibly the daily dosage [ 42 , 43 ]. Previous findings, along with our observations in Cases 1 and 2, suggest that a daily dose of 20 mg of tamoxifen might be sufficient to treat thin endometrium. However, in Case 3, despite administering the same dosage of tamoxifen, we were unable to achieve the desired endometrial thickness. Despite its advantages, tamoxifen does have some risks, particularly its effects on the endometrium. Prolonged use has been associated with endometrial hyperplasia and, in rare cases, endometrial cancer. However, regular gynecological monitoring, including ultrasound evaluations and biopsies, can help detect and manage these risks early. For postmenopausal individuals, aromatase inhibitors may be considered as an alternative to tamoxifen to reduce endometrial concerns [ 44 ].

We had non‐ reassuring and non consistant results with tomoxifen alone 20 mg daily for the development of ET. Based on the varying results from previous studies and our own cases, it appears that several unkown factors might influence the effectiveness of tamoxifen on endometrial proliferation. Further well‐designed studies with higher and different tomoxifen doses, longer duration of tomoxifen administration, larger populations and longer follow‐up periods are needed to identify any effectoveness of tomoxifen usage for IVF‐ET. Additionally, the effectiveness of tamoxifen in combination with different FET protocols also seems to vary that should be taken into account in the future studies. The limits of the current study were the limited available cases due to strict criteria, consisting of BMI, age, nonresponsiveness to standard and high estrogen therapy in the previous cycles, and external factors such as genetic predisposition. Our study indicated that administering tamoxifen alongside high‐dose estrogen therapy may assist some patients with thin endometria in achieving a trilinear pattern and sufficient endometrial thickness.

Endometrial thickness (ET), as assessed by transvaginal ultrasonography (TVS), is an important factor in determining endometrial receptivity and the likelihood of successful implantation during IVF with frozen–thawed embryo transfer (FET) cycles [ 1 ]. Although previous studies have not established a specific cut‐off value for ET below which successful implantation is impossible, many investigations indicate that an ET greater than 7 mm on the day of the luteinizing hormone (LH) surge or human chorionic gonadotropin (HCG) administration is recommended for embryo transfer [ 2 , 3 , 4 ]. In most infertility centers, an ET of less than 7 mm typically leads to the cancellation of the embryo transfer [ 4 ]. Some of applied strategies in clinic for improving the endometrial growth and achieving more than 7 mm thickness includes estrogen supplementation, vaginal sildenafil, aspirin and tocopherol [ 5 , 6 , 7 ]. Also, administration of pentoxifylline and intrauterine granulocyte colony‐stimulating factor (GCSF) seems to have positive effect on endometrial growth in the previous studies [ 7 ]. Among these strategies, the efficacy of exogenous estrogen seems to be more than others. Today, exogenous estrogen is widely used to achieve the appropriate ET during IVF‐FET [ 8 , 9 ]. Despite the desirable effects in administration of estrogen, this method had some adverse effects. Increasing the rate of excess amounts of vaginal bleeding, deep vein thrombosis (DVT) and some types of gynecological malignancies (especially endometrial cancer) are increased with long‐term supra‐physiological estrogen levels in the serum [ 10 , 11 , 12 ]. In another hand, estrogen insensitivity occurred in some patients for whom ET did develope well in response to the estrogen therapies [ 13 ]. Tamoxifen is a medication commonly used for the prevention and treatment of breast cancer. It is classified as a selective estrogen receptor modulator (SERM). Similar to clomiphene citrate, which is used for ovulation induction in certain cycles [ 13 , 14 ], tamoxifen has distinct effects on the reproductive system. While clomiphene citrate is known to cause a thinning of the endometrium due to its anti‐estrogenic effects, tamoxifen is thought to enhance endometrial thickness and effectively stimulate ovulation induction [ 14 ]. There are only a few published studies examining the impact of tamoxifen on endometrial thickness in patients with estrogen insensitivity during IVF‐FET cycles [ 13 , 14 , 15 ]. This project aimed to investigate the effect of tamoxifen on endometrial growth in women with thin endometrium who did not respond to standard and high doses of estrogen therapy in previous IVF‐FET cycles.

The authors declare no conflicts of interest.