First off, I am so sorry that this hit you out of left field...not sure why she didn't say anything to you sooner but sometimes kids just have their reasons for keeping things to themselves. (it is not a commentary on you as a parent but them and a teen)
I know a lot of people who are 'just built that way' and who never have formed stools. Period. That's it. Never did and never will and that is their normal. 3xs/day is not abusive either when stool is soft or liquidy and may never cause her any major problems other than a greater loss of mineral salts or trace elements to her system...Which may be the cause of all of her pseudo-thyroid symptoms.
She might wish to try taking a supplement with the following trace elements: iron, iodine, fluoride, copper, zinc, chromium, selenium, manganese and of course potassium and magnesium. I am not sure if any of these are counter-indicated with thyroid problems...Check with the doc first.
I have a k pouch so my output is liquidy. I empty out my pouch 8xs/day +/- so I understand the problem. When I do not supplement I get weak, tired, achy, depressed, impatient or teary, exhausted etc.
This fall, coming back from the U.S. I had a killer teaching schedule...teaching over 13 different classes per semester with upwards of 750 students in all.
I barely made it through the first week and then cried myself to sleep the whole next weekend...exhaustion.
Then I hit my vitamin cabinet...pulled down the Calcium/magnesium/potassium/zinc (not sure if she is allowed zinc with thyroid problems) plus extra iron and B complex.
Within 1 week I was bouncing to school and home at night. Sleeping like a baby too...
And I had had my blood work done twice in 6 months to be told that I was in perfect chemical condition...so sometimes 'perfect' for everyone else is not good enough for us...
Wishing her good luck