is situated in the lower part of the neck and when enlarged it forms
goitre. Two hormones secreted by the thyroid gland are :
(i) Thyroxine : It is the principal hormone secreted by the thyroid gland
and its main role is to increase the metabolic rate of he organs and
tissues of the whole body. 60% of thyroxine consists of iodine, an
element which is essential for the gland to enable it to synthesize its
hormone. The basal metabolic rate (B.M.R.) is increased in
hyperthyroidism and reduced in hypothyroidism.
(ii) Calcitonin : This hormone lowers the calcium level in the ways :
(A) By inhibiting renal tubular calcium reabsorption.
(B) By inhibiting bone calcium reabsorption.- Hypothyroidism : This results from lack or deficiency of thyroid
hormone secretion. It is manifested differently in children as
compared with adults. Cretinism affects children and is due to
congenital defect of either absence or defect of the gland. In this
disease growth is stunded, the features are coarse, frequently the
child has a protruding tongue and an enlarged abdomen; the
mentality of the child is low and retarded. Myxoedema is the
condition caused by thyroid deficiency in adults. It affects woman
more frequently than men. It is characterized by puffy face, thick
skin, dry cough, cold and loss of hair. There is a deposition of mucin
and fluid retention in extracelluar spaces. B MR is lowered Iodine
deficiency causes simple goitre. - Hyperthyroidism : This results from excessive secretion and over
action of thyroid hormones, As excessive amount of thyrozine is
poured into the blood and the metabolism of the body is speeded up.
The person starts loosing weight, has an increased pulse rate, suffers
from nerves excitement and there is protrusion of eye balls. These
toxis sings and symptoms are responsible for the condition being
known as toxic goitre. Other names are thyrotoxicosis,
exophathalamic goiltre and grave’s disease.