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The FSH test

Mr Anil Gudi, Mr Amit Shah & Prof Roy Homburg

The FSH test is a test of the number of eggs remaining in the ovary (ovarian reserve).

FSH (follicle stimulating hormone) originates in the pituitary gland at the base of the brain and is released into the blood stream in order to stimulate the growth of follicles containing eggs in the ovary, to achieve ovulation. FSH also encourages the production of hormones from the ovary, mainly estrogens and inhibin. These two hormones feed back to the pituitary and regulate the amount of FSH that is released. With increasing female age and the consequent depletion of the number of eggs in their follicles, the production of estrogen and, particularly, inhibin decreases and, in response, the levels of FSH increase. This can be seen as a compensatory mechanism to keep the ovary going. This is the reason why levels of FSH are used as a test of ovarian reserve; the higher the FSH, the lower the ovarian reserve or number of eggs remaining. From the age of about 35 years the number of eggs left in the ovaries starts to decline more rapidly with a consequent increase in the FSH levels. The nearer the approaching
menopause, the higher will be the levels of FSH and the lower the chances of becoming pregnant. Rather than predicting the chances of pregnancy, the FSH level is better at predicting the response of the ovary to stimulation in treatment such as in-vitro fertilization (IVF).

The ideal time to sample blood for FSH levels is on day 3 of a cycle although day 2-5 will probably suffice. Any testing outside of these days is irrelevant. When used as a screening test for ovarian reserve, levels of estradiol should also be measured. A combination of high FSH and high estradiol levels are bad news.

Precise levels of FSH for making decisions are difficult to define and individual laboratory guidelines should always be taken into consideration. However, as a rule of thumb, FSH levels on day 3 of higher than 40 IU/L usually indicate menopause, less than 10 IU/L are regarded as normal and more than 15 IU/L indicate a dwindling ovarian reserve. High FSH levels in younger women are less seriously regarded than high levels in older women, emphasizing yet again that age is the most significant factor in determining ovarian reserve and chances to conceive.

In addition to the drawback that blood sampling for FSH must be done around day 3 of the cycle, the variation of levels from cycle to cycle may be confusing. However, it has been found that the highest FSH value measured is the one that more accurately predicts the ovarian reserve. Although the FSH test has long been used routinely, it is now being superseded by testing for blood levels of anti-Mullerian hormone (AMH) and by the antral follicle count on ultrasound examination.

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