Hi, Welcome to HCM.
Based on the information you have given, underweight
itself can be a cause of amenorrhoea independent of any other causes. Secondly, as you have indicated high prolactin levels can be another cause. While high prolactin levels can be due to multiple causes, they have to be ruled out systematically. One of the common causes can be due to adenoma at the level of pituitary gland
, particularly micro-adenoma in your daughter's case as her other hormone levels are normal. MRI or CT scan of the brain can help in the diagnosis. Also, high prolactin levels are found in patients with polycystic ovarian syndrome
. I will advise ultrasound
of the abdomen preferentially trans-vaginal. Lastly dopamine suppresses prolactin secretion, in this regard I will advise that you consult her primary physician about prescribing her dopamine agonists.
Overall, I will advise that you have your daughter examined thoroughly from an endocrinologist
as there are multiple causes of amenorrhoea.
Hope this helps.