That belief assumes that TSH perfectly reflects the
levels of the biologically active
thyroid hormones, Free T3 and Free T4. In actuality, TSH cannot be shown to correlate well with either Free T3 or Free T4, much less with symptoms, which are most important. The best correlation of TSH to Free T3 and Free T4 that I have yet to find is shown in Fig. 2 of this link. As you can see and read, the correlation is poor.
http://nahypothyroidism.