L.M.
I have been reading about autoimmune disorders a lot in the last couple of years and the subject of absent periods comes up OFTEN.
This article is very informative and may help you look at some potential causes of your daughter's condition: http://adam.about.com/reports/000101_2.htm
These two paragraphs about thyroid and celiac issues caught my eye:
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"Thyroid Problems. Thyroid problems, either too much thyroid hormone (hyperthyroidism) or too little (hypothyroidism), can interrupt cycles. Hypothyroidism can result in excess prolactin. Most women with hypothyroidism fail to produce eggs, and they may receive a diagnosis of hypothyroidism for the first time during a fertility evaluation.
Celiac Sprue. Celiac sprue is an inability to tolerate gluten, a protein found in wheat, rye, oats, barley, and other grains. Exposure to gluten damages the lining of the intestinal tract. It is also associated with late puberty, early menopause, and amenorrhea. This disorder is now considered more common than previously believed and may even be linked to non-intestinal symptoms, such as depression, discolored teeth, and neurologic problems."
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Celiac disease is an autoimmune condition. It is an intolerance of gluten which is found in wheat, rye barley and some other grains. It is not as uncommon as the doctors seem to think and it can sometimes be hard to properly diagnose. The condition causes inflammation of and damage to the intestines and results in your body's inability to absorb the nutrients from your food resulting in serious vitamin and mineral deficiencies. This can lead to can lead to MANY problems including digestive issues, anemia, depression, skin rashes and hives, cancers (especially of the intestines), fatigue (especially after eating), hair loss, headaches, canker sores, constipation or diarrhea, weight loss or gain, numbness in hands or feet, etc.
Celiac diesease or gluten intolerance is also linked to autoimmune disorders such as diabetes, lupus, arthritis, Hashimotos's Thyroiditis (thyroid autoimmune condition), and eczema.
I hope that you have some time to do a little reading about Celiac disease and gluten intolerance. Research shows that approx 1 in 150 have it but most are undiagnosed. Researchers believe that approx 1 in 50 people are actually gluten intolerant which is a little different that celiac but causes virtually the same problems. You can go online and search "celiac gluten absent periods menstruation" and you should find some info. The condition often causes secondary problems (like hormone imbalances) which led to the problem you are describing. I hope her doctor finds you some answers soon. I just wanted to mention this to you so you can see if others in your family have any of the symptoms (it is a genetic condition) and maybe the doctors will consider testing her.
Take care.