If you have irregular ovulation, recurrent miscarriages, or endocrine disorders, your doctor will perform reproductive hormone tests to understand your problem by measuring hormone levels. Common hormone tests include: follicle producing hormone (FSH), luteinizing hormone (LH), estradiol (E2), progesterone (P), testosterone (T), and prolactin (PRL), and in some cases, thyroid hormones. When in the menstrual cycle should the tests be done? What hormones should be checked when? Reproductive hormones are secreted periodically during a woman’s cycle, and the relationship between which hormone we test at which point in time varies greatly. Specialized knowledge of reproductive endocrinology is necessary to tell us which hormone to test at which point in time and what it means. It is never as simple as just checking the normal values against the labs. We set three standardized time points in the cycle for hormone testing. I. Basic Endocrine Hormone Test (Day 2-3 of the menstrual cycle) Women’s basic endocrine hormone levels are best checked on the 2nd-3rd day of the menstrual cycle, which is the early follicular phase, which can reflect the functional status of the ovaries but cannot predict whether ovulation will occur in the current cycle. It is usually necessary to test the 3 sex hormones, namely: follicle-producing hormone (FSH), luteinizing hormone (LH) and estradiol (E2). The basal values of FSH and LH are 5-10 IU/L. During a normal menstrual cycle, FSH and LH levels remain low during the early follicular phase (days 2-3 of menstruation), and monitoring FSH and LH levels during the early follicular phase can be used to initially determine the function of the gonadal axis in women, with FSH being more valuable in determining ovarian reserve than LH. Estradiol (E2) level reflects the follicular growth status, which is at a low level in the early stage of normal menstrual cycle and then rises steadily to reach the peak before ovulation. Ovulation test (11th~15th day of menstrual cycle) Hormone measurement before ovulation is an indicator to determine whether the follicle is mature, whether ovulation will occur, and whether the development of the lining is synchronized. Generally, E2, LH and P are checked. Estradiol (E2) reaches its first peak before ovulation and declines rapidly after ovulation. Checking estradiol levels during ovulation can be one of the indicators of follicular development and maturation. When follicles are mature (≥16 mm in diameter), the average blood E2 per follicle reaches 1,100 pmol/L. If the low E2 value does not correspond to the diameter of the follicle, it suggests the possibility of poor egg quality. LH rises rapidly before ovulation to form a peak value, which can be as high as 3-8 times the basal value, and after ovulation LH rapidly returns to the follicular phase level. the peak LH value can be estimated at the time of ovulation about 36 hours later. Under normal circumstances, blood P in the follicular phase has been at a low level, before ovulation P secretion began to increase, after ovulation, the ovarian corpus luteum produces a large amount of P, the blood P concentration rises rapidly, usually at the level of 10-15 ng/ml. Third, luteal phase examination (the 22nd to 25th day of the menstrual cycle) P elevation is the gold standard to prove that there is ovulation. The best time to measure is one week after the ultrasound monitoring of ovulation or 6~7 days after the rise of basal body temperature. At this time, theoretically, progesterone is at the highest level in the luteal phase to determine ovulation; blood progesterone level in the luteal phase is lower than physiological values, suggesting that the luteal function is insufficient. Testosterone (T) and prolactin (PRL) These two hormones are also important at any time of the menstrual cycle.T is mildly to moderately elevated in patients with PCOS; T levels are significantly elevated in patients with ovarian or adrenal tumors, or hirsutism. PRL and with the menstrual cycle has a small fluctuation, prolactin secretion is not stable, mood, exercise, sexual intercourse, hunger and eating can affect its secretion status, with sleep-related rhythmicity; short-term PRL secretion increases after sleep, and is elevated in the afternoon compared with the morning. Therefore, according to the characteristics of this rhythmic secretion, blood should be drawn in the morning at 9 ~ 10 a.m. fasting. PRL significantly elevated, a test can be determined; PRL mildly elevated, a second examination should be carried out.