What Does TSH Actually Do?

[Catecholamine]

I'm not referring to the TSH test (which most of us are probably aware is rarely sufficient and often misinterpreted), but the hormone itself.

It seems that when T4 and T3 levels are low, TSH goes up, and vice versa. There are exceptions, like with pituitary tumors and such, but I'm referring to the commonly seen model of high TSH/low thyroid (and vice versa). But if it actually stimulated the thyroid, wouldn't enough TSH solve the problem? Or even cause hyperthyroidism? And in theory, could it be viable to take TSH for hypothyroidism instead of thyroid hormone?

I'm not saying to actually do that, but would it theoretically be viable? And, more broadly, if high TSH doesn't actually cause the thyroid to make more of the good stuff, then, uh... why is TSH?

 

[sele]

TSH is like a person ringing the doorbell, waiting for someone (T4/T3) to come out.
But what happens when no one is at home? TSH rises and rings the bell harder.
This is just the way I see it.
TSH is a pituitary hormone that can only summon T4/T3 but cannot increase them if there is a problem in production or if stress hormones or PUFA are blocking the pathway.