Thanks for your thoughtful response, I will get on to the next step then!
FT4 is below range. It may look like it's "barely" off, but when you consider that thyroid reference ranges are very flawed and most of us have symptoms until FT4 is around midrange, you can see that it's actually much worse than it looks.
TT4, though considered an obsolete test of little use, is just barely in range.
TT3 is in the same class as TT4...obsolete. Her doctor should be ordering FREE T3. TT3 is 32% of range, and the guideline for T3 is upper half to upper third of range.
TSH isn't agreeing with FT4 and TT3 levels. With those FT4 and TT3 levels, I'd expect her TSH to be much higher. However, TSH is a pituitary hormone and a very indirect measure of thyroid status. It's the least important of the tests.
Any other hypo symptoms? Fatigue and/or sleepiness? Depression? Constipation? Lowered HR and/or BP? Hair loss? Dry skin?
I wouldn't wait; I'd see an endo now. With FT4 below range, if she isn't feeling pretty bad now, she probably will be soon.
He should test your daughter for thyroid antibodies. Hashimoto's thyroiditis, autoimmune hypothyroidism, is the #1 cause of hypo in the developed world. TPOab (thyroid peroxidase antibodies) and TGab (thyroglobulin antibodies) are the two markers to test for. (It's a simple blood test.)
If her neck is enlarged, i.e. she has a goiter, the doctor will probably also do an ultrasound to get a better look at it. It's non-invasive and painless.
Probably best option is to run the tests again in a few months. If you see an endocrinologist that is probably what they would recommend anyways,
Any enlargement of the neck could be checked out with a CT scan. Though ultrasound is typically used for looking the the thyroid. CT scan is more general but it uses x-rays so isn't something you want to do for no good reason.
"Low or normal TSH, low free T3 or free T4
Common:
Non-thyroidal illness
Recent treatment for hyperthyroidism (TSH remains suppressed)
Rare:
Pituitary disease (secondary hypothyroidism)
Congenital TSH or thyrotropin-releasing hormone deficiency."
Excerpt from - Interpretation of thyroid function tests.