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

Parathyroid hormone plays a crucial role in regulating calcium levels in the blood. It is secreted by the parathyroid glands and is essential for maintaining calcium homeostasis. When calcium levels in the bloodstream drop below normal, parathyroid hormone is released into circulation. This hormone acts on several target organs to increase blood calcium levels in multiple ways:

1. It stimulates the release of calcium from bones by promoting the activity of osteoclasts, which are the cells that break down bone tissue and release calcium into the bloodstream.

2. It enhances the reabsorption of calcium in the kidneys, reducing the amount of calcium excreted in urine.

3. Parathyroid hormone promotes the conversion of vitamin D into its active form, calcitriol, in the kidneys. Calcitriol, in turn, increases intestinal absorption of calcium from the food we consume.

These combined actions of parathyroid hormone effectively raise blood calcium levels, making it the primary regulator of calcium in the body.

In contrast, calcitonin is involved in lowering calcium levels but is not the primary regulator; it acts to inhibit osteoclast activity and promote bone formation. Thyroid hormone does not directly regulate calcium levels, and aldosterone primarily controls sodium and potassium balance