Thyroid gland and parathyroid glands their importance in metabolism

The thyroid gland is one of the endocrine glands of the body and is situated in the neck in front of the wind pipe.

Its shape somewhat resembles a butterfly. It has right and left lobes connected across the mid-line by a narrow portion called isthmus.

It weighs about 30grams and is slightly heavier in females. In females it slightly enlarges during pregnancy and in both sexes it enlarges during adolescence.

It produces two hormones-Thyroxine (T4) and triiodothyronine (T3) which control metabolism. T4 is produced in greater amounts than T3. But T3 is the more active form. T4 is converted by tissues into T3. The thyroid hormone is essential for normal growth and development of a child, and then for normal metabolism of cells.

It regulates the speed of metabolism of every cell. In the blood T3 and T4 are partly bound to a protein called Thyroxine Binding Globulin. A small fraction of the hormone is free and is biologically active.

Iodine, one of the trace minerals is necessary for the synthesis of thyroid hormones. An amino acid called tyrosine is also needed for the synthesis of thyroid hormones. If diet is deficient in iodine the synthesis of hormones decreases. This leads to increased production of TSH and enlargement of thyroid known as goitre.

The function of the thyroid gland is controlled by the pituitary gland which produces a hormone called Thyroid stimulating hormone (TSH). Its level goes up when the level of thyroid hormones comes down and vice versa.

Deficiency of thyroid hormone from birth results is a condition called congenital hypothyroidism. If the condition is not diagnosed and treated early the child will suffer from irreversible damage to the brain and mental retardation.

The thyroid gland produces another hormone called calcitonin which regulates the level of calcium in the blood. This hormone causes calcium to move into bones.

The parathyroid glands are situated just behind the thyroid gland. They are much smaller than the thyroid gland. There are four parathyroid glands-two superior and two inferior.

They secrete a hormone called parathormone, which plays an important role in calcium and phosphorous metabolism. Parathormone mobilizes calcium out of the bones and increases the level of calcium in the blood and hence its action is exactly opposite to that of calcitonin.

It acts on the kidneys and increases the re-absorption of calcium in the tubules. It also increases the absorption of calcium from the intestine by increasing the production of active Vitamin D. The net effect of all these actions is increase in the calcium level in the blood.

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