THYROID AND PARATHYROID GLAND

THYROID GLAND:

·         The name thyroid was introduced by Thomas Wharton (1658).

·         It is derived from the Greek word “Thyreas” means a shield.

·         This is the largest endocrine gland of our body.

Origin:

It is endodermal in origin i.e. it originates from the endoderm of the embryo. The thyroid gland is bilobed, highly vascular organ.

Location:

 It is located in our neck region upon the ventral aspect of larynx (sound box or Adam’s apple) and a few anterior tracheal rings.

Structure:

 It is a dark brown and H- shaped bilobed gland. It consists of mass of rounded vesicles held together by connective tissue and interwoven with blood capillaries. Each vesicle is lined by an epithelial layer enclosing a colloidal substance. It is composed of two lobes; each of its lobes is located on either side of the trachea in the neck interconnected with each other through a thin flat connective tissue called Isthmus. It is composed of follicles (round in shape) held together by loose connective tissue called stromal tissue. Each thyroid follicle is composed of follicular cells, enclosing a cavity.

Hormones:

Follicular cells of thyroid gland synthesize following two hormones:

         i.            Tetra-iodothyronine or thyroxine (T4) hormone

       ii.            Tri-iodothyronine (T3) hormone

Both these hormones are iodinated form of amino acid (tyrosine). They are stored in the colloid that fills the follicles and are released into the blood when needed. Iodine (in diet) is essential for the synthesis of hormone at normal rate in thyroid.

We will discuss these hormones in detail:

        I.            Thyroxine: It is an iodine containing (6% iodine) amine hormone which is derived from the tyrosine amino acid. Chemically thyroxine is tetra-iodothyronine or it is also found in another form which is tri- iodothyronine. Secretion of thyroxine is inversely proportional to the blood level of thyroxine.

Thyroxine hormone performs following functions:

·         It regulates the growth of the cells of the body.

·         It regulates the rate of oxidation of food and helps in releasing energy during metabolism.

      II.            Thyrocalcitonin (TCT): It is long peptide hormone secreted by Para-follicular cells of the thyroid gland (C- cells). Its secretion is regulated by increased plasma level of calcium by feedback mechanism.

    III.            Calcitonin:  This proto- hormone reduces blood calcium level.  

Irregularities of Thyroid gland:

a)      Hypothyroidism: In this condition secretion of thyroxine hormone is decreased from the thyroid gland. Due to the deficiency of thyroxine hormone a disease may occur which is known as Cretinism.

b)      Hyperthyroidism or Thyrotoxicosis: When thyroxine hormone secretes in excess amount, there is a disease occur which is known as Exopthalmic Goitre in children. Enlarged thyroid gland in adults caused Goitre.

PARATHYROID HORMONE:

 These are four small rounded structure situated on the dorsal side of the thyroid gland.  PTH is also known as Collip’s hormone.

Location:

 These glands are located on the posterior side of the thyroid gland.

Origin:

 It is endodermal in origin.

Structure:

These are small flat and oval gland.  They are four in number. Each pair is situated in the two lobes of the thyroid gland on either side.

Hormone:

Parathyroid gland secretes a single hormone called Parathyroid Hormone (PTH) which is a protein. It increases the blood calcium level by metabolizing calcium. Hypo secretion of PTH lowers calcium concentration in blood and tissues due to excretion of calcium in urine. This increases the excitability of nerves and muscles abnormally and causing cramps and convulsions. This disorder is known as Parathyroid tetany.  The secretion of Parathyroid hormone or parathormone is regulated by the circulating level of calcium ions in the blood.

Excessive secretion of PTH leads to-

·         Fall in bone minerals leading to softening, bending and fracture of bones.

·         Increased level of blood calcium

·         Decreased urinary secretion of calcium which leads to calcification of soft tissues like blood vessels.

Thus, Parathyroid gland mainly influences calcium metabolism and it also secrete some amount of calcitonin hormone.

Function of Parathyroid Hormone:

·          It increases the level of calcium in the blood.

·         It stimulates the process of bone re-absorption i.e. dissolution or demineralization by acting on bones.

·         It also stimulates re-absorption of Ca+2 ions from the digestive tract. It is clear that PTH acts as a hyper-calcaemic hormone which increases the level of Ca⁺2in the blood.

·         PTH is under the feedback control of blood calcium level. A fail in calcium in blood stimulates them to secrete PTH. Thus, both the hormones (Thyrocalcitonin and Parathormone) play a significant role to control and regulate the concentration of calcium and phosphorous.

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