Amenorrhea x 4 months?

I have not had a period in 4 months. Here is some background:

-Was on birth control pill x 14 years. Stopped taking in March '06.
-Had a very regular cycle from April - Oct 2006.
-Have not had a period since Oct 2006.
-I have lost 35 lbs in two years.
-I excercise (cardio and/or lift) at least one hour per day.
-My body fat is 16%.
-Eat 2000 calories per day primarily veggies, whole grains, lean protein, fruit.
-Hair grows very fast, but not in unusual places.
-Diminished libido.
-Tests so far show normal TSH, not pregnant, normal pap and normal pituitary function.
-30 years old. No children. Married.

My doctor has prescribed Prometrium to induce period, but I haven't taken it and I don't want to. I went off BCP to rid my body of hormones, so I really don't want to add the Prometrium.

What tests would be helpful (hormone levels in the blood)?
What could be causing the amenorrhea?
What next?

I would be interested to know your height/weight ratio, but just from the info you've given I would suspect that your body is trying to conserve.
You're not having a menstrual cycle because of your low body fat, dietary limitations and the strenuous daily exercise.
If your first response to that was "but I eat a great diet!" and "an hour a day is not strenuous!" then you might have a problem in your head.
You are correct in that you have an excellent diet. And an hour a day is strenuous, and NOT excessive. However, your body is telling you as clearly as it can "speak"----"Hey, Houston, we've got a problem. I can't manufacture enough "extra" to produce a period for you. You aren't a good candidate for motherhood because you don't have enough extra to give a baby." [And yes, you could still get pregnant (just as the starving women of India do), but it's not "best possible case scenario."]
It's hard to believe your dr. didn't suspect this.
I think you are right to forego the Prometrium; hopefully you'll consult a Registered Dietician or a physician who specializes in eating disorders or who treats athletes.

Google PCOS. It stands for Polycystic ovarian syndrome. I have it and it has a lot of different problems. Many of them you have mentioned. I'm not a Dr. but I have PCOS and have had it for several years prior to being diagnosed this past fall. It causes problems with diabetes, fatigue, cycles, hair, and many many more and it effects different people different ways.

I agree with melissach up there...I also have PCOS and that is exactly what it sounds like to me.