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Insulin is not involved in the regulation of calcium levels. Its primary role is in glucose metabolism, where it facilitates the uptake of glucose into cells for energy and helps in the storage of excess glucose as glycogen in the liver and muscles. While insulin plays a critical role in various metabolic processes, it does not directly influence calcium homeostasis.

In contrast, parathyroid hormone (PTH) is crucial for increasing calcium levels in the blood by stimulating the release of calcium from bones, increasing intestinal absorption of calcium, and promoting renal reabsorption of calcium. Calcitonin serves to lower blood calcium levels by inhibiting bone resorption and promoting the deposition of calcium in bones. Vitamin D enhances calcium absorption from the gastrointestinal tract and works synergistically with PTH to maintain calcium levels in the bloodstream.