The blood calcium level is lowered by the deficiency of Parathormone.

Explanation:
Parathormone, also known as parathyroid hormone (PTH), is a hormone produced by the parathyroid glands. It plays a crucial role in regulating the blood calcium levels in the body.

Functions of Parathormone:
1. Increases Calcium Absorption: Parathormone acts on the bones, kidneys, and intestines to increase the absorption of calcium from these sources into the bloodstream. It stimulates the release of calcium from the bones, increases the reabsorption of calcium by the kidneys, and enhances the absorption of calcium from the intestines.

2. Stimulates Vitamin D Activation: Parathormone stimulates the activation of vitamin D in the kidneys. Vitamin D is necessary for the absorption of calcium from the intestines. Activated vitamin D increases the efficiency of calcium absorption from the diet.

3. Inhibits Phosphate Reabsorption: Parathormone inhibits the reabsorption of phosphate by the kidneys. This helps to decrease the phosphate levels in the blood. It is important to maintain an appropriate balance between calcium and phosphate levels in the body.

Deficiency of Parathormone:
When there is a deficiency of parathormone, it leads to a condition known as hypoparathyroidism. This condition can occur due to various factors such as surgical removal of the parathyroid glands, genetic disorders, autoimmune diseases, or damage to the parathyroid glands.

Effects of Parathormone Deficiency:
1. Low Blood Calcium Levels: Parathormone deficiency results in a decrease in blood calcium levels. This happens because parathormone is responsible for increasing calcium levels in the blood by stimulating its release from the bones, reabsorption in the kidneys, and absorption from the intestines.

2. Increased Phosphate Levels: In the absence of parathormone, the reabsorption of phosphate by the kidneys is not inhibited. This leads to an increase in phosphate levels in the blood.

3. Symptoms of Hypoparathyroidism: The low blood calcium levels resulting from parathormone deficiency can cause symptoms such as muscle cramps, spasms, tingling sensations, numbness, fatigue, and weakened bones.

In conclusion, the deficiency of parathormone leads to a decrease in blood calcium levels, highlighting the crucial role of parathormone in maintaining calcium homeostasis in the body.