synergism

Synergism is when two hormones acting together produce an effect greater than the sum of what each would produce alone — two plus two making five. Each hormone genuinely contributes to the same outcome, and their combined push is more than additive. It is teamwork in which the partners reinforce one another.

A clear example is the control of blood glucose during stress or fasting: glucagon, epinephrine, cortisol, and growth hormone all raise blood sugar, and given together their effect on glucose output far exceeds the sum of their individual contributions. Another is the cooperation of FSH and estrogen in driving follicle growth, and of several hormones together in preparing the breast for milk production.

Synergism differs from a permissive effect: in synergism both hormones are actively producing the effect, whereas in permissiveness one merely enables the other. Recognizing synergy matters clinically because it explains why a combination of counter-regulatory hormones can drive blood glucose dangerously high, and why blocking just one member of a synergistic group may only partly blunt the response.