Permissiveness (biology)

In biology, permissiveness refers to a certain relationship between hormones and the target cell. It can be applied to describe situations in which the presence of one hormone, at a certain concentration, is required in order to allow a second hormone to fully affect the target cell. For example, thyroid hormone increases the number of receptors available for epinephrine at the latter's target cell, thereby increasing epinephrine's effect at that cell. Without the thyroid hormone, epinephrine would only have a weak effect.