Which hormone is primarily responsible for the regulation of calcium levels in the blood?

Parathyroid hormone (PTH) plays a critical role in regulating calcium levels in the blood. It is secreted by the parathyroid glands in response to low blood calcium levels. When PTH is released, it increases blood calcium concentration through several mechanisms: it promotes the release of calcium from bones by stimulating osteoclast activity, enhances renal tubular reabsorption of calcium, and stimulates the production of active vitamin D (calcitriol) in the kidneys. Active vitamin D increases intestinal absorption of calcium from the diet, further contributing to elevated calcium levels in the blood.

This function of PTH is essential for maintaining homeostasis, and any disruption in its secretion can lead to significant disturbances in calcium balance, affecting muscle function, nerve transmission, and bone health. In contrast, calcitonin has a more minor role in calcium regulation as it lowers blood calcium levels primarily by inhibiting osteoclast activity, and it is not the primary regulator. Thyroxine (T4) is crucial for metabolic regulation but does not directly regulate calcium levels. Aldosterone is involved in sodium and water balance but does not influence calcium regulation directly. Hence, the primary responsibility for calcium regulation rests with parathyroid hormone.