Which hormone is primarily responsible for increasing serum calcium levels?

The hormone primarily responsible for increasing serum calcium levels is parathyroid hormone (PTH). PTH is secreted by the parathyroid glands and plays a critical role in maintaining calcium homeostasis in the body. When serum calcium levels drop, PTH is released into the bloodstream, where it acts on several target organs.

Firstly, PTH stimulates osteoclasts in the bones, which increases the release of calcium into the bloodstream by breaking down bone tissue. It also promotes renal tubular reabsorption of calcium, meaning that the kidneys retain more calcium and excrete less of it into the urine. Additionally, PTH enhances the conversion of vitamin D to its active form, calcitriol, in the kidneys. Calcitriol then increases intestinal absorption of calcium from the diet, further helping to raise serum calcium levels.

In contrast, calcitonin, produced by the thyroid gland, works to lower serum calcium levels. Insulin primarily regulates glucose metabolism, while growth hormone is involved in growth and metabolism but does not have a direct role in calcium homeostasis.