What major physiological function does insulin serve?

Insulin primarily functions to facilitate cellular glucose uptake, which is crucial for regulating blood glucose levels in the body. When food is consumed, glucose levels in the bloodstream rise. In response, the pancreas secretes insulin, enabling cells throughout the body—especially in muscle and fat tissue—to absorb glucose for energy or storage. This process not only helps to lower blood sugar concentrations but also ensures that cells receive an adequate energy supply to function properly.

In contrast, the other options pertain to different physiological processes. Increasing metabolic rate relates to hormones such as thyroid hormones, which help to regulate the body's energy expenditure and use of calories. Regulating calcium levels involves hormones like parathyroid hormone and calcitonin but is not a function of insulin. Stimulating thyroid hormone production is associated with thyroid-stimulating hormone (TSH), rather than insulin, which does not play a role in thyroid function. Therefore, the primary and most significant role of insulin remains its ability to facilitate cellular glucose uptake.