Parathyroid hormone acts on blood calcium by

1. PTH secretion is itself regulated by circulating calcium levels. 2. Parathyroid hormone increases the calcium levels in the blood. 3. It stimulates bone resorption, releasing calcium from bone. 4. It also increases reabsorption of calcium by the renal tubules and absorption from digested food. _Source: syllabus Biology Ch19 'Chemical Coordination and Integration', section 19.2.5_