Answer: Kidneys, intestine and skeletal system
Step-by-step explanation:
Parathyroid hormone is the hormone that is responsible for regulation of the calcium levels in the extracellular fluid in the body.
It exerts influence over the skeletal system to reabsorb the calcium by the oesteoclasts. It causes the reabsorption of the calcium by the small intestine. It also prevents the excretion of calcium through urine by reabsorption by renal tubules.