Thyroid gland lies in the neck in front of the trachea. It consists of two lobes connected by a narrow band called isthmus.
Functions of Thyroid Gland
The thyroid gland secretes two hormones, thyroxine and triiodothyronine. They greatly increase the metabolic rate of the body. They promote the growth of the body tissues. They stimulate tissue differentiation, as seen in promotion of metamorphosis of tadpoles into adult frogs. Thyroid also secretes Calcitonin.
If the thyroid gland fails to secrete the required amounts of thyroxine during childhood, the child remains physically stunted and mentally retarded. This condition is called cretinism. In the case of adults, if the thyroid hormones are not secreted in adequate quantities, it produces Myxedema (lake of alertness), puffy appearance, decreased body temperature, lethargy, etc. Also, the patient suffers from slow heart rate and low metabolic rate.
The thyroid enlargement known as goiter is due to deficiency of iodine, it is accompanied by bulging of the eye ball. The over secretion of thyroid results in high metabolic rate, rapid heart rate, nervousness, restlessness, irritability, etc and this is called Grave’s disease or Hyperthyroidism.
These are four small glands situated close to the lobes of the thyroid gland. The hormone secreted by the parathyroid glands is called Parathormone.
Functions of Parathyroid Glands
This hormone maintains the blood-calcium level. Whenever there is a fall in the calcium-ions in the blood plasma, it mobilizes more calcium from the bone to the plasma. It increases phosphate elimination in the urine and thereby lowers the phosphate concentration in the blood plasma. Thus, parathormone regulates calcium level by increasing its deposition in the bone. Thus, parathormone and calcitonin have antagonistic action and regulate blood-calcium level.
Over secretion of parathormone leads to Hyperthyroidism. It increases the mobilization of bone minerals which leads to softening of bones, rise in calcium concentration in blood plasma, etc. On the other hand, the hypothyroidism (under-secretion of parathormone) leads to fall of calcium concentration in blood, leading to increased excitability of nerves and sustained contraction of muscles causing tetanus.