Cycle day 3 FSH is tested to check for 'ovarian reserve' -- the quality of your eggs or 'good eggs' and 'bad eggs.' An elevated FSH could be a problem, because it may mean that the ovary doesn't respond well to FSH hormone stimulation.An elevated FSH may indicate diminished ovarian reserve.
In other words, an elevated FSH may be indicative of 'bad eggs.'
FSH is reported in "units" (mIU/ml) and results from 2-7 mIU/ml are probably normal in just about any lab.
FSH levels above 25 mIU/ml are probably abnormal, and above 30-40 mIU/ml the FSH levels are clearly abnormal and probably indicative of ovarian failure.
A quicker way is to use the second method of extended ovarian reserve testing, called the clomiphene citrate challenge test or CCCT.