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Figure 2. Follicular phase diagram of hormones and their origins. Follicle-stimulating hormone (FSH) is secreted by the anterior pituitary gland (Figure 2). FSH secretion begins to rise in the last few days of the previous menstrual cycle, [3] and is the highest and most important during the first week of the follicular phase [4] (Figure 1).
With the concomitant rise in inhibin B, FSH levels then decline in the late follicular phase. This seems to be critical in selecting only the most advanced follicle to proceed to ovulation. At the end of the luteal phase, there is a slight rise in FSH that seems to be of importance to start the next ovulatory cycle.
A rise in pituitary FSH caused by the disintegration of the corpus luteum at the conclusion of a menstrual cycle precipitates the recruitment of five to seven class 5 follicles to participate in the next cycle. These follicles enter the end of the prior menstrual cycle and transition into the follicular phase of the next one. The selected ...
Estrogen levels peak towards the end of the follicular phase, around 12 and 24 hours. This, by positive feedback, causes a surge in levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This lasts from 24 to 36 hours, and results in the rupture of the ovarian follicles, causing the oocyte to be released from the ovary. [17]
During the early and mid-follicular phase, FSH is secreted to promote growth and proliferation of the granulosa cells, which increases GnSAF concentrations. [14] Once the dominant ovarian follicle has been selected at mid-follicular phase, the non-dominant follicles undergo atresia. [15]
FSH stimulates the secretion of inhibin from the granulosa cells of the ovarian follicles in the ovaries. In turn, inhibin suppresses FSH. Inhibin B reaches a peak in the early- to mid-follicular phase, and a second peak at ovulation. Inhibin A reaches its peak in the mid-luteal phase.
The diagnosis is based on age less than forty, amenorrhea, and two elevated serum follicle-stimulating hormone (FSH) and decreased estrogen measurements at one-month intervals. [10] The anterior pituitary secretes FSH and LH at high levels to try to increase the low estrogen levels that are due to the dysfunction of the ovaries.
FSH levels are elevated early in the cycle of women who have lower ovarian reserve, because their follicles do not produce enough estrogen to inhibit FSH production, [14] therefore high levels early on in a woman's menstrual cycle can indicate lower ovarian reserve and lower likelihood of retrieving eggs for IVF.