Thyroxine and triiodothyronine can be measured as free thyroxine and free triiodothyronine , which are indicators of thyroxine and triiodothyronine activities in the body. They can also be measured as total thyroxine and total triiodothyronine , which also depend on the thyroxine and triiodothyronine that is bound to thyroxine-binding globulin . A related parameter is the free thyroxine index , which is total thyroxine multiplied by thyroid hormone uptake , which, in turn, is a measure of the unbound thyroxine-binding globulins.  Additionally, thyroid disorders can be detected prenatally using advanced imaging techniques and testing fetal hormone levels. 
Hmmmm interesting. Funny you mention it because I was taking gaba not too long ago and it was in a mixture with some other stuff. Anyways, I think it was causing me to wake up kind of uncoordinated and feeling just kind of weird. Dissociative I guess you could say, which makes sense because it’s related to the NMDA receptors somehow I think. It supposedly “can’t” cross the blood brain barrier, at least this is repeated despite the fact that it can and does, it’s just an unreliable mechanism in which it does. Low levels of gaba were insinuated to absorb more in the brain also. I didn’t have this issue as if I took the full 750 mg I felt very strange consistently and none of the other supplements in the mixture would seem to do this.