Nutrients are required for chemical reactions (substrates or cofactors) whose rate is dependent on their presence. As they are the building blocks of the organisms, their concentration is usually high (m Molar)
Signals regulate the reaction rate giving (how???) some information (which one ???) concerning the availability of nutrients (T3/T4 = oxygen, insulin = serum glucose, etc.). As they directly don't take part in the organism growth and metabolism their concentration is usually 1000/10.000 times lower (micro or nano M).
As a matter of fact, the whole metabolism is always and only regulated by its own substrates either directly as Nutrients or through a Signal