**3.1 Effect of PTH on the kidneys**

PTH causes an increase in the calcium reabsorption of the glomerular filtrate. This is mainly due to some factors. First, it acts in the loop of Henle, increasing the voltage gradient to allow the passive transport of ionized calcium. Then, it acts on the granular portion of the distal contorted tubules, inducing translocation of preformed calcium channels on the cell surface, increasing the entry of calcium into the vascular lumen. Finally, it also acts on the collecting tubules by changing the activation of the Na+ /Ca2+ pump [11–13]. In addition, PTH has other effects on kidney cells, such as an increase in phosphate excretion, with increased phosphaturia and decreased serum phosphate; increased bicarbonate clearance with urine

**Figure 2.** *Mechanisms of action of parathyroid hormone.*

alkalinization, resulting from decreased bicarbonate reabsorption in the proximal renal tubules; inhibition of sodium reabsorption in the proximal tubules, with increased clearance of free water and greater urinary flow; and increased activity of vitamin D1 alpha-hydroxylase, with greater production of the active form of vitamin D (calcitriol) [12, 14, 15].

### **3.2 Effect of PTH on bones**

PTH produces both anabolic and catabolic effects, depending on the phases of action. In the early phase, there is a mobilization of calcium from the bones, entering equilibrium with the extracellular fluids. In the late phase, there is an increase in the synthesis of bone enzymes, such as lysosomal enzymes, which promotes bone resorption and remodeling. PTH also inhibits osteoclasts and stimulates bone resorption, leading to an increase in serum alkaline phosphatase and urinary hydroxyproline (markers of osteolysis) [11–13].

#### **3.3 Effect of PTH on the intestine**

PTH has no direct action on the intestine. Its effect is due to an increase in the synthesis of calcitriol (activate form of vitamin D) in the kidneys, through the stimulation of the enzyme α1-hydroxylase. Calcitriol has an effect on increasing serum calcium from the resorption of bowel lumen [11].
