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Calcitonin is the hormone responsible for decreasing blood calcium levels. It is produced by the parafollicular cells (C cells) of the thyroid gland and plays a crucial role in calcium homeostasis. When blood calcium levels rise, calcitonin is released into the bloodstream, where it acts primarily to inhibit osteoclast activity in the bones. Osteoclasts are the cells responsible for bone resorption, so when their activity is reduced, less calcium is released into the bloodstream from the bones. Additionally, calcitonin promotes the excretion of calcium in the kidneys, further contributing to a decrease in blood calcium levels.

In contrast, parathyroid hormone increases blood calcium levels by promoting bone resorption, enhancing intestinal absorption of calcium, and reducing calcium excretion in the kidneys. Glucagon and insulin are hormones that primarily regulate glucose metabolism and do not directly influence calcium levels in the body. Thus, among the options provided, calcitonin is uniquely positioned to lower blood calcium levels effectively.