Parathyroid hormone is the major regulator of the plasma concentration of ions

Calcium

Explanation:

Parathyroid hormone (PTH) is a hormone that is secreted by the parathyroid gland. It is the major regulator of the plasma concentration of calcium ions. It regulates calcium ions in the kidney, intestine and bones.

In the kidney, is involved in the re-absorption of calcium in distal ducts and the renal collecting duct, it is also involved in activating the conversion of 25-hydroxy vitamin D into 1,25-dihydroxy vitamin D (calcitriol).

In the intestine,  PTH is involved in the absorption of calcium, by increasing vitamin D production.

In the bone, it helps in regulating calcium levels; when serum calcium level is low, PTH stimulates the activity of the osteoclast, to produce more calcium.