Re: Hashi's questions
Boy, we had a similar mess with one of our kids. Very low energy teen, low body temps (96 degrees-ish); pediatrician thought depressed, and the current clinical thinking is that depression in kids & teens means bipolar.
Because neither psychiatrists (several wishywashy evals, differing dxs, no bipolar) nor peds would test thyroid (despite extensive family history), we ended up doing it ourselves through HealthCheck USA. We had all the antibodies run, plus the frees (free T3 & free T4), plus TSH. It was a package deal, but not on their standard menu, if I recall correctly.
It turned out our son DID have a bargain-basement free T4 level. I do blame myself -- I had cooked low sodium for years, & I believe my son had an iodine deficiency. Once we started adding iodine back into his diet(not excessively), there was major energy improvement.
I'm not saying that iodine insufficiency is your young relative's problem, but her family could follow a similar route with the testing at HCUSA if they wanted to. Even if testing had proven perfectly normal, for us, it was worth it to spend the $150 or so. We just slept better knowing one more important base was covered.
If you are a reasonably close blood relative (aunt or grandma), your own thyroid problem is part of familial medical history.
Your question about the bobbing around of test results is a good one. IME, the TSH bobs around more than the frees. For a Hashi's dx, typically you need antibodies, or biopsy findings. 'Course, you don't find those unless the specific tests are done. It's good to test for all the antibodies. If she does indeed have any type of thyroid antibodies, that is a situation that calls for attention.