Parathyroid gland

In man the parathyroid gland are small oval yellow-brown bodies
found attached to the posterior surface of the thyroid gland. The parathyroid
gland secrete two hormones namely, 1. Parathormone and 2. Calcitonin.

Parathormone

Purified parathormone is a simple polypeptide chain. It has short
duration of biological activity. The half-life of the hormone is of about 20-30
minutes only.

Physiological effects of Parathormone

Parathormone manifests its regulatory effects at three different loci in
the body namely the skeleton, kidneys and the gastro intestinal tract. In
skeleton, the hormone directly acts upon the bone tissue to stimulate the
activity of osteoclast cells (bone destroying cells). Under the influence of this
hormone calcium is released from the bone matrix into the circulation. As a
result plasma calcium level increases. Thus it helps in the skeletal
remodelling. (parathyroid gland)

In kidney, parathormone induces a marked increase in phosphate
excretion. In the gastro intestinal tract, parathormone stimulates the
absorption of calcium and phosphate from the gut by enhancing the
vitamin D synthesis. As a result, more phosphate and calcium are
transported into the blood stream. Its other physiological effects include its
inhibitory action over the osteoblasts or bone forming cells, bicarbonate
reabsorption and reduction of urine pH, etc.

Calcitonin

It is a calcium-lowering hormone secreted by the parafollicular cells
of the parathyroid gland. It is a protein. Its physiological effects are antagonistic
to that of parathormone. It inhibits bone resorption. In kidney, it inhibits the
reabsorption of calcium, phosphorus, sodium, potassium, magnesium and
other ions. It decreases gastric HCl secretion. It also decreases the insulin
and glucagon secretion. (parathyroid gland)

Hyperparathyroidism

It is a condition where there is an increased amount of parathyroid
hormone in circulation. Excess secretion of parathormone brings about
demineralization of the bones.

The protein matrix of the bone is also absorbed. These changes result in bone cyst and the elevation of the calcium level in the blood. The latter causes calcification of kidneys, arteries, stomach and lungs. (parathyroid gland)

Hypoparathyroidism

Removal of parathyroids causes the blood calcium levels to fall and
results in tetany. Tetany is characterized by low serum calcium
(Hypocalcemea), reduced urinary excretion of calcium and phosphate,
neuromuscular hyperexcitability, spasms of muscles and cramps etc. (parathyroid gland)