Follicle Stimulating Hormone should be measured on day 2 or 3 of the menstrual cycle, otherwise it is meaningless as a measurement of potential fertility.
Most clinics like to see FSH under 10 at any age. However, the FSH needs to be measured in conjunction with your E2 (Oestrogen) as well (on day 2 or 3). E2 should be below about 200. This will give you a more realistic measurement of your ovarian reserve.
Age is an important factor in fertility, obviously, but it is hard to say what is a normal FSH measurement for 36/37. However, it is a better prognosis to be young with high FSH than to be older with high FSH, as it is not only FSH but also the age of the eggs which impacts on fertility.
With the best will in the world, GP´s are not as clued up when it comes to fertility care and treatment. I would ask to have it repeated on day 2 or 3. The hormone to be checked on day 21 is your progesterone if you have a regular 28 day cycle.
I'd just like to point out that the reference range will differ from hospital to hospital so one place might say FSH below 10 and others below e.g. 12. In my lab we quote < 12.5.
As for oestradiol needing to be below 200, that is completely false. The oestradiol reference range is so wide that it's impossible to interpret except in a few situations.
Oestradiol does not give an accurate indication of ovarian reserve although if it is low with a high FSH (values relative to the reference range), then this would indicate ovarian insufficiency.
My GP got this wrong too - suggested I had my FSH tested on day 22. When I queried this with him a couple of days later (thanks to the girls on this board) he immediately said it was a mistake and I should do the FSH test on day 3.
I agree that different labs/clinics have different FSH cut-off points, however I believe you are wrong about E2 on day 2 and 3. If E2 is too high it can falsely suppress the FSH reading thereby giving the impression that FSH is lower than it is. In the US (REs in IVF clinics) they like to see E2 below 80 (approx. 240 in UK measures I think). In fact if FSH is LOW with HIGH E2, this can indicate that the E2 is suppressing the FSH number. Here's an example [urlhttp://haveababy.com/infert/eggsold.asp][/url].
Basically, what I am saying is that yes, FSH (coupled with age of woman) is the primary indicator of ovarian reserve BUT the E2 number is relevant.
My FSH and E2 (amongst others) were tested on day 2 of my cycle.
It seems that 8.4 is OK but not brilliant but that is probably due to my age.
I got the impression that <10 was were my clinic would do the cut-off.
Hopefully, I'll start my tx in March so it wont be a delay.