Why do only some cells respond to a circulating hormone even though the hormone is present throughout the bloodstream?

Only cells that express the specific receptor for that hormone can bind it and transduce the signal, so other cells are unresponsive despite exposure.

All cells respond but only target cells show visible changes because they have more abundant mitochondria to carry out the hormone's action.

Hormones are rapidly degraded in all tissues except their target cells, which is why only target cells are exposed to functional hormone concentrations.

The blood-brain barrier prevents hormones from reaching any peripheral tissue except the hypothalamus, so peripheral responses are mediated by neurotransmitters only.