What is meant by "catalytic amount of a hormone"? This textbook says:&source=bl&ots=Dq7keZKMNm&sig=0BPP9GXSEPqDfpxudWANzfA8aiI&hl=en&sa=X&ved=0ahUKEwiO-5uzqMjSAhWBRo8KHTiDDgcQ6AEIGzAA#v=onepage&q=In%20the%20classic%20definition%2C%20hormones%20are%20secretory%20products%20of%20the%20ductless%20glands%2C%20which%20are%20released%20in%20catalytic%20amounts%20into%20the%20blood%20stream%20and%20transported%20to%20specific%20target%20cells%20\(or%20organs\)&f=false) > In the classic definition, hormones are secretory products of the ductless glands, which are released in **catalytic amounts** into the blood stream and transported to specific target cells (or organs), where they elicit physiologic, morphologic and biochemical responses. What is meant by catalytic amount? Does it mean that amount required for catalysis? I know only renin which is catalytic and act as an enzyme, and many other hormones are non enzymatic so why the term catalytic is used in general sense?

The writer obviously means **“small amount”** , and has chosen his words badly as hormones do not themselves catalyse reactions.

The idea he is probably trying to convey is that _a few molecules_ of hormone binding to their receptor can cause changes to _many molecules_ within the target cell. This often (generally?) involves catalysis within the cell produced by the receptor activating an enzyme, so one can understand the line of thought leading to the misuse of this term.