Master the Biology Bash: 2025 MCAT Biological Systems Practice Challenge!

Calcitonin is the hormone known for its role in inhibiting osteoclast activity. Osteoclasts are specialized cells responsible for the resorption of bone, a process that increases blood calcium levels by breaking down bone tissue. When calcitonin is secreted, particularly in response to elevated blood calcium levels, it acts to decrease the activity of osteoclasts, thereby reducing bone resorption. This action helps to lower blood calcium levels and promotes the deposition of calcium back into the bone.

While parathyroid hormone plays a critical role in increasing blood calcium levels by stimulating osteoclast activity, calcitriol (the active form of vitamin D) primarily increases intestinal absorption of calcium and phosphate, along with enhancing osteoclast activity indirectly. Hydroxyapatite is a mineral form of calcium apatite and is not a hormone; it is crucial for bone structure but does not have a regulatory effect on osteoclasts. Thus, calcitonin is specifically recognized for its inhibitory effect on osteoclasts and its role in maintaining calcium homeostasis in the body.