Thyroid Gland

Thyroid Gland

Discovered by Wharton (1908) it is the largest endocrine gland (3 - 7 cm in diameter and 15 -20 gm in weight).

Situated below thyroid cartilage of larynx it is bilobed structure, connected by isthmus.

It’s the only endodermal gland homologous to endostyle of protochordates and develops as a ventral outgrowth from pharyngeal wall.

Fig. A. Thyroid gland, B. Follicles suspended in stroma of a lobule

Consists of many follicles formed by follicular cells as epithelial part and interfollicular stroma of areolar connective tissue with para follicular cells.

Follicles contain colloid where synthesis of thyroxine hormone takes place.

Growth, maintenance and secretion of this gland is controlled by TSH.

(i) Thyroxine

It is a derivative of amino acid tyrosine. Its molecular weight is about 60,000.

E.C. Kendall (1918) first isolated its active component and named it thyroxine.

For its synthesis it requires iodine, which is obtained from food. Out of 100 to 150 µg of daily intake about 80 µg is absorbed by follicular cells

from the blood (in iodide form) and released into follicular cavity. Here, the tyrosine monomer of thyroglobulin combines with iodine to form

tetraiodothyronine (T4 or thyroxine) and triiodothyronine (T3). Maximum amount of iodine is present in thyroid gland.

Harrington and Barger (1927) synthesized it in laboratory.

Functions

It is the main hormone to influence all the metabolic processes in body.

Increases the intake of O2 resulting in increased cellular respiration and energy production.

Controls the basal metabolic rate (BMR) and respiratory quotient (R.Q.)

Promotes growth and differentiation of tissues and development of mental faculties.

Reduces the formation of ketone in blood and influences the blood pressure and heat production. (calorigenic effect)

Influences the growth and activity of nervous system and sex organs hence influencing the sexual maturity.

Controls embryonic development, Gudernatsch (1912) discovered its effect on metamorphosis of frog. In iodine deficient water the tadpole

fails to metamorphose into adult, and this is why newts and salamander show neoteny.

In fishes, amphibians and reptiles it also has role in osmoregulation and control of moulting.

Hyposecretion in childhood causes cretinism, with following symptoms:

Decreased metabolic rate, retarded body growth (dwarfism); retarded growth of sex organs.

Improper mental development with defective memory, low intelligence and no skills. These features resemble the traits of Down’s syndrome (Mongoloid idiocy).

Hyposecretion (in adult) causes various defects like

Myxoedema (Gull’s disease) with following symptoms:

Mentally dull individuals with low blood pressure, puffy appearance, slow heart rate and low body temperature, anaemia, loss of eyesight, poor

memory, thinking, speech expression, husky and low voice, muscle cramps, insomnia, etc.

Failure of reproductive capacity, menstrual abnormalities (in females).

The inadequate level of thyroxine secretion stimulates the release of higher quantity of TSH that causes simple goiter, extra enlargement of

thyroid gland appearing as swelling of the neck region.

The most common reason is iodine deficiency in the food/water in some areas. It is also caused due to genetic defect i.e., sporadic goitre.

Hashimoto disease

This is autoimmune disease.

Sometimes thyroxine secreted in very less amount is mistaken as antigen by the body’s immune system hence antibodies are formed against it,

this attacks the thyroid gland which is further harmed.

Hypersecretion causes exophthalamia (or toxic goiter), with following symptoms:

Also called as Grave’s disease (or Basedow’s disease or thyrotoxicosis).

Increased heart beat, nervous activity, and muscle movements, increased BMR with excess heat generation and sweating.

Enlargement of rectus muscles of eyes and excess fat deposition behind eyeball causing its outbulging.

It is also caused due to tumorous growth of one or more nodules called as Plummer’s disease or toxic adenoma.

(ii) Calcitonin (or thyrocalcitonin)

Secreted from para-follicular cells (C-cells), it is a polypeptide of 32 amino acids, mole weight 4,500.

Decreases Ca++ level of plasma, hence it is antagonistic to parathromone.

Prevents bone dissolution and enhances the loss of Ca++ in urine.