The blood calcium level is lowered by the deficiency of 

• parathormone

• thyroxine

• calcitonin

• Both (a) and (c)

A.

parathormone

The chief cells of the parathyroids secrete parathormone. Its deficiency causes the lowering of blood calcium level. This increases the excitability of nerves and muscles causing cramps and convulsions. This caused parathyroid tetany characterised by sustained contractions of the muscles of larynx, face, hands and feet.

Calcitonin is secreted when calcium level is high in blood it has an opposite action to that of parathyroid hormone and lowers the calcium level by suppressing the release of calcium ions from the bones.
Thyroxine is secreted from the thyroid gland. It regulates the metabolic rate of the body and thus, maintain basal metabolic rate, stimulate protein synthesis and therefore, promote the growth of the body tissue.