Final answer:
Altered ion concentrations promote hormone release in response to ion concentrations in the blood.
Step-by-step explanation:
The stimulus that promotes hormone release in response to ion concentrations in the blood is altered ion concentrations. When ion concentrations in the blood change, it can trigger the release of certain hormones. For example, when the concentration of calcium ions decreases in the blood, the parathyroid gland releases parathyroid hormone to increase calcium levels.