What is the primary effect of parathyroid hormone (PTH)?

Parathyroid hormone (PTH) is a crucial regulator of calcium homeostasis in the body. Its primary effect is to increase blood calcium levels. PTH achieves this through several mechanisms. Firstly, it stimulates the release of calcium from bones into the bloodstream by promoting the activity of osteoclasts, which are cells that break down bone tissue. Secondly, PTH enhances the reabsorption of calcium in the kidneys, reducing the amount of calcium excreted in urine. Lastly, it stimulates the activation of vitamin D in the kidneys, which increases the absorption of calcium from the intestines.

The regulation of calcium levels is vital for various physiological processes, including nerve conduction, muscle contraction, and blood coagulation. An increase in blood calcium levels is essential when the body requires higher calcium availability, such as during periods of growth or recovery from certain medical conditions.

In contrast to this, decreasing blood calcium levels would counteract the effects of PTH. Promoting water retention and stimulating the production of thyroid hormones are not direct actions of PTH, as they pertain to other hormones such as antidiuretic hormone (ADH) and thyroid-stimulating hormone (TSH), respectively.