Introduction
Being one of the key parameters of endometrial receptivity, endometrial thickness plays an important role in successful embryo implantation (1) .Although there has been no commonly accepted thickness below which successful implantation would be preclude, it has been recognized by many investigators that endometrial thickness ≥7mm on the day of luteinizing hormone(LH) surge or human chorionic gonadotrophin(HCG) administration is preferred for embryo transfer(2-5). Thin endometrium (<7 mm) affects about 2.4% patients in in vitro fertilization(IVF) cycles (6).It often results in cancelation of embryo transfer and cryopreservation of all the embryos.
Various strategies have been attempted to improve endometrial growth in these patients, including estrogen supplementation, vasoactive measures such as vaginal silde­nafil, low-dose aspirin, pentoxifylline, or tocopherol, intrauterine infusion of granulocyte colony‑stimulating factor, and regenerative medicine(7-11). Among them, exogenous estrogen treatment is the most commonly used method(12). Many factors may influence the effectiveness of estrogen treatment and the subsequent reproductive outcomes(12-14).Although high dose estrogen treatment sustaining a long time may be effective for some of these patients(12), supraphysiologic estrogen levels resulted from this treatment may have a negative impact on reproductive outcomes(15-19). Moreover, a higher risk of side-effects, such as breakthrough bleeding (20), some types of tumor such as endometrial cancer (21), deep venous thrombosis (22) may also be associated with this treatment. Furthermore, after pregnancy, exogenous estrogen and progesterone should be continued until 8-10 weeks of gestation to substitute for the absent corpus luteum. In addition, some patients were insensitive to estrogen treatment. Alternative methods should be considered for endometrium preparation in these women during their frozen embryo transfer (FET) cycles.
Tamoxifen (TAM) is a selective estrogen receptor modulator (SERM) with chemical properties similar to clomiphene citrate (CC).It has been used in clinic as a highly effective agent for the treatment of breast cancer(23)and anovulation(24-26). In contrast to CC, whose anti-estrogenic effects at the level of the endometrium sometimes result in thin endometrium(27), ovulation induction with tamoxifen improves endometrial thickness in women with a history of thin endometrium (28). However, little information is available regarding its use during FET cycles in that kind of patients. To the best of our knowledge, there has been three published study investigating the use of TAM for endometrium preparation (EP) in thin endometrium patients. The first one is only a case report (29).The obvious weakness of the other two studies is that there were no control groups (30, 31). Furthermore, they are limited by their small sample size. The aim of this study is to evaluate the efficacy of TAM in women with thin endometrium during their FET cycles through comparing it with the most commonly used protocol- hormone replacement treatment (HRT) protocol. We propose that the result of our study may contribute to the treatment of infertility patients with thin endometrium.