Final answer:
Parathyroid hormone (PTH) is released in response to low blood calcium levels. It stimulates the conversion of 25-(OH) D3 to calcitriol in the kidneys.
Step-by-step explanation:
The release of parathyroid hormone (PTH) is stimulated by low blood calcium, which in turn stimulates the conversion of 25-(OH) D3 to calcitriol in the kidneys. PTH is produced by the parathyroid glands and is released in response to low blood calcium levels. It acts on the skeleton, kidneys, and intestines to increase blood calcium levels.