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Parathyroid Gland
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The blood calcium level is lowered by the deficiency of:Parathormone (parathyroid hormone, PTH) is secreted by the parathyroid glands and increases the $Ca^{2+}$ level in the blood; it is a hypercalcemic hormone.
It acts on bones and stimulates bone resorption (demineralisation), stimulates the reabsorption of $Ca^{2+}$ by the renal tubules and increases $Ca^{2+}$ absorption from the digested food.
So when parathormone is deficient, the blood calcium level falls (hypocalcaemia), which can lead to tetany.
Calcitonin (thyrocalcitonin) from the thyroid gland lowers the blood calcium level; its deficiency would raise blood calcium rather than lower it.
Thyroxine regulates the basal metabolic rate and has no direct role in lowering or raising blood calcium.
So the blood calcium level is lowered by the deficiency of parathormone.