What is the appropriate cycle day to check FSH levels, and why?

The appropriate cycle day to check FSH levels is Day 3 of the menstrual cycle. This timing is crucial because, during the early follicular phase (which encompasses days 1-3 after menstruation starts), estradiol levels are typically low. As a result, measuring FSH levels during this time helps to avoid the potential suppression of FSH that can occur when estradiol levels are elevated.

At Day 3, the body is transitioning from the follicular phase, and it is an optimal time to assess the function of the hypothalamic-pituitary-ovarian axis. Elevated levels of FSH at this time can indicate diminished ovarian reserve or impending menopause. Thus, measuring FSH on this specific day provides the most accurate assessment needed for diagnosing conditions related to menopause and fertility.

While measuring FSH at the beginning of the cycle (Day 1) might seem appealing to some, it may not provide an accurate picture of ovarian function because FSH could be influenced by rising estradiol levels later in the follicular phase. Checking on Day 14 does not align with identifying ovarian reserve either, since by this phase, ovulation is anticipated, and hormone levels are different from those at Day 3. The idea that