Endocrine Physiology
The Parathyroid Gland The four parathyroid glands are embedded in the thyroid gland. Each is oval in shape and approximately 6 mm. long. Microscopically the glands consist of columns of cells with vascular channels between the columns.
The hormone produced is the parathyroid hormone or parathormone The hormone produced is the parathyroid hormone or parathormone. It is a protein having a molecular weight of 15,000. Since it is a protein it is inactive by mouth. Parathormone plays an important part in the maintenance of the plasma calcium level at 10 mg%.
It is usual to consider the plasma calcium level rather than the blood calcium level because there is practically no calcium in the red cells. The blood calcium level will be lower at 5.5% since: Blood calcium level = 100- H\100 x plasma calcium level. (where H is the haematocrit)
Unlike the thyroid gland, the parathyroid glands are not controlled by the pituitary gland, but probably by the plasma calcium level. In order to study the actions of the parathyroid gland, it is necessary to consider the composition of bone.
The Bone Bone consists of a matrix of protein (mainly collagen) embedded in a mucopolysaccharide ground substance. The matrix is made hard and rigid by the deposition of mineral salts, principally The double salt of calcium phosphate. Calcium hydroxide termed calcium hydroxyapatite 3Ca3(PO4.Ca(OH)2.
The majority of bone is preceded by cartilage but some, such as the roof and sides of the skull, is preceded by membrane. The bone is formed by osteoblast cells which lay down calcifiable collagen fibrils. As ossification proceeds the original cartilage cells degenerate and disappear.
The ossification of bone is a complex process involving a local increase in the concentration of calcium and phosphate ions so that their solubility product is exceeded.
The collagen in bone seems to differ from collagen in other tissues in that its molecular arrangement favours the deposition of calcium hydrogen phosphate, CaHP04.2H20, which is then converted into crystals of calcium hydroxyapatite.
Bone is in a state of dynamic equilibrium and is being continually remoulded by the combined action of: Osteoblast cells which are forming bone. Large multi-nucleated osteoclast cells which are destroying it.
Actions of Parathyroid Hormone The actions of parathormone (PTH) may be summarized as follows: It has a rapid action on the kidney tubules causing an increased excretion of phosphate which leads to a mobilization of calcium and phosphate from bone.
It has a delayed effect on bone causing: A Proliferation of the osteoclasts. A decrease in osteoblastic activity leading to a decrease in bone matrix formation.
It has a delayed action on the digestive tract, reducing the calcium in the digestive secretions, and hence reducing the calcium lost in the faeces and increasing the calcium retained by the body.
Deficiency of Parathyroid Hormone (Hypoparathyroidism) A deficiency of the circulating parathyroid hormone due to underactivity or removal of the parathyroid glands leads to: A decrease in the phosphate excretion by the kidneys. An increase in the phosphate level in the blood. At the same time the plasma calcium level falls (hypocalcaemia).
There is an inverse relationship between the plasma calcium and inorganic phosphate levels. Thus, when the plasma phosphate level rises from 6 mg.% to 10 mg.%, the plasma calcium level tends to fall from 10 mg.% to 6 mg.%.
If the plasma calcium level falls to 6mg.%, tetany occurs, where: There is an increased excitability of nerves and neuromuscular junctions which leads to muscular spasms particularly of the hands and feet which is known as carpopedal spasm. An increased excitability of the nervous system may lead to convulsions.
Before the onset of tetany, the increased excitability of nerves (latent tetany) may be demonstrated in different ways: Tapping the facial nerve (VII) as it crosses the angle of the jaw has no effect in a normal person, but in latent tetany the muscles on that side of the face will twitch and may go into spasm (Chvostek's sign of latent tetany).
A tourniquet or blood pressure cuff around the upper arm in latent tetany will stimulate the underlying nerves and cause the onset of carpo-spasm (Trousseau's sign). The plasma calcium level depends upon An adequate absorption of calcium from the intestines, as well as; The level of parathormone.
Thus tetany may occur in rickets and osteomalacia due to a vitamin D deficiency, since this vitamin is essential for adequate absorption of calcium from the intestines.
Tetany may also occur, unrelated to the plasma calcium and parathyroid levels, in alkalosis resulting from the ingestion of alkalis and following over ventilation. In this case the increased excitability of nerves is brought about by the fall in the hydrogen ion concentration.
Excess of Parathyroid Hormone (Hyperparathyroidism) An excess of parathormone due to: Overactivity of the glands or; An injection of the hormone raises the plasma calcium level to 20 mg.% or more (hypercalcaemia). The phosphate reabsorption by the kidney is inhibited and the phosphate is lost in the urine.
Calcium and phosphate are mobilized from the bone leading to: Rarefaction. Cyst formation. Spontaneous fractures. There is an increased calcium excretion in the urine so that the 'skeleton is passed out through the urethra'.
However, the calcium is excreted less rapidly than the phosphate and the blood calcium level rises. The rarefaction of bone in hyper-parathyroidism is confirmed by comparing the X-ray appearance of the bone of a limb with that of a normal limb taken simultaneously as a control.