This is (so far in preprint form so not generally available) a paper which shows how our cells are controlled by deiodinase enzymes. The answer is quite clear: The deiodinase called D2 is stimulated by T3 using one activator, whereas deiodinase D3 is inhibited by T3 using a deactivator. The latter is the one that makes rT3 from T4. So the expression of the right amount of T3 in each cell depends on the balance between D2 and D3 action, which pull opposite ways. This accounts not only for the unique expression in a particular tissue or organ, but the balance between each action can vary from moment to moment according to individual tissue need. So the way individual tissues respond to T4 (competing T3 or rT3 production) explains completely how individual tissues and organs act both in the long term and the short.
A very important paper. I don't know the journal yet as it was sent to me by certain channels . When it is finally accepted I'll send on to TUK or show the doi number if possible to download free
DEIODINASES AND THE METABOLIC CODE FOR THYROID HORMONE ACTION
Samuel C. Russo , Federico Salas-Lucia , and Antonio C. Bianco