Thyroid gland and thyroxine

Thyroid gland consists of a pair of lobes, which lie one on either side
of larynx in the neck region. The paired lobes are joined by a narrow anterior
bridge of glandular tissue called Isthmus of the thyroid. The lobes of
thyroid in turn are divided into many lobules. The lobules consist of follicles.
The follicles are called acini (acinus – singular). Each acinus is lined with
glandular cubical epithelial cells. The cavity of acinus is filled with a
gelatinous material called colloid, which contains the thyroxine. The hormone
thyroxine is an iodinated hormone. It contains 65% iodine. The amino acid
residue in thyroxine is tyrosine.

Functions of thyroid (Thyroid gland)

Thyroxine stimulates normal growth and development, especially the
skeletal and nervous systems. It controls the rate of cellular oxidation and
increases the basal metabolic rate. The basal metabolic rate (BMR) is
defined as the amount of heat produced in the body in a given time, in
complete state of physical and mental rest at 20oC room temperature.

Actions of thyroxine (Thyroid gland)

(i) This hormone is very essential for the development of nervous
system particularly at the time of birth and during the first year, (ii) This
hormone increases the metabolism of all tissues except brain, gonads and
accessory sex organs, lymph nodes, spleen and lungs, (iii) The most
important function is to increase the absorption of glucose from the small
intestine. (iv) This hormone reduces serum cholesterol level, (v) It promotes
protein anabolism, and helps in growth, (vi) It increases heart beat rate, force
of contraction and pulse pressure, (vii) Presence of optimum level of
thyroxine in the blood maintains efficient muscle functions and (viii) The
optimum level of thyroxine in the blood is also necessary for normal gonadal
function.

Hypothyroidism (Thyroid gland)

The physiological effect due to deficiency of thyroid hormone is
referred to as hypothyroidism. It is manifested by iodine deficiency and simple
goiter, cretinism and myxoedema. If the dietary intake of iodine becomes
inadequate (below 10 micro grams per day) the synthesis of thyroxine is
impaired. As a result, the thyroxine level falls in circulation and secretion of
TSH increases, causing the hypertrophy of thyroid gland as a consequence.
The thyroid enlarges to enormous proportions. This is called simple goiter.
This condition is also called endemic goiter. It is caused due to lack of iodine
in the soils of different regions of the world.

Cretinism (Thyroid gland)

Cretinism is found in children who are deficient of thyroxine hormone
from the time of birth. The characteristics of cretinism are, retardation of
mental growth to extreme degree, dwarf stature, protruding tongue and abdomen, low basal metabolic rate, subnormal body temperature,
retardation in skeletal growth and arrest of pubertal sexual maturity etc.

Myxoedema (Thyroid gland)

Myxoedema in adults, is a syndrome with the following
characteristics viz., low BMR, dry, coarse, scaly skin, puffy and bloated
face, coarse and sparse hair, hoarse voice, slow speech, slow thought
processes, poor memory, etc. Other symptoms are muscular weakness and
fatigue, low blood pressure, anaemia with increased serum cholesterol, etc.

Hyperthyroidism or thyrotoxicosis (Grave’s disease or exophthalmic goiter) (Thyroid gland)

The hyper function of thyroid gland results in Grave’s disease. Grave’s
disease is characterized by increased BMR with increased pulmonary
ventilation, protrusion of eye balls from the sockets (exophthalmas), increased
heart beat rate, nervousness, emotional instability, weight loss, increased blood
glucose and decreased serum cholesterol, derangement of sexual function
etc.