Final answer:
When hormone levels are deficient, the number of receptors on target cells typically increases, a process known as up-regulation, to enhance cell sensitivity to the hormone.
Step-by-step explanation:
When a hormone (or neurotransmitter) is deficient, the number of receptors may increase. This is part of a compensatory mechanism called up-regulation, which enhances the sensitivity of the target cells to the available hormone. Conversely, when hormone levels are high, target cells may reduce their number of receptors in a process known as down-regulation, making them less reactive to the hormone. Factors such as hormone insensitivity can result in ineffective receptor activity despite normal levels, as seen in conditions like Androgen Insensitivity Disorder.