Question

This a mechanism by which organisms maintain homeostasis when the level of one substance influences the level of another substance or the activity of an organ

Answer 1

The answer would be a feedback mechanism.

Most of the homeostasis is based on a negative feedback mechanism. One of the examples would be thyroid hormone. To produce the hormone, hypothalamus needs to secrete TRH which in turn cause secretion of TSH. TSH will bind to thyroid, causing it to secrete the active thyroid hormone T3/T4 that will decrease the TRH.
One of the positive feedback mechanism would be found on giving labour