Thyroid gland and Parathyroid gland

Thyroid Gland and Parathyroid Gland

THYROID GLAND

It is larget butterfly shaped endocrine organ situated in the neck close to the trachea.

The gland consists of two elongated oval lobes joined by a narrow band called isthmus.

It is a highly vascular organ and contains many spherical or oval sac-like follicles.

Cells lining the thyroid follicle; secrete, two thyroid hormones, thyroxine or tetraiodothyronine (T₄) and triiodothyronine (T₃).

Both are iodinated forms of an amino-acid called tyrosine and remain stored in the jelly like semifluid material (colloid) in the lumen of follicles.

T₃ is more active and several times more potent than T₄.

From the thyroid mainly T₄ is secreted and converted to T₃ in peripheral tissues e.g., Liver.

Thyroid is the only endocrine gland which stores its secretory product in large quantity also involve in iodine metabolism.

When necessary, the hormones are released from the colloid to the blood.

In between thyroid follicle are present C-cell or parafollicular cells which release calcitonin (TCT) or parafollicular hormone.

It is involved in calcium homeostasis.

It is hypocalcemic and hypophosphatemic.

Both parathormone and calcitonin help in the maintenance of calcium ions in blood plasma.

Functions:

1.Thyroid hormones greatly increase the metabolic rate of the body and consequently, enhance heat production (Calorigenic effect) and maintains BMR (basal metabolic rate).

2.Thyroid hormones also promote growth of body tissues-both physical growth and development of mental faculties are stimulated.

3.They stimulate tissue differentiation. Because of this action, they promote metamorphosis of tadpoles into adult frogs.

4.To enhance some actions of neurotransmitters : adrenaline and nor-adrenaline.

5.Support process of red blood cells formation.

6.Maintenance of water and electrolyte balance is also influenced by thyroid hormones.

Disorders Due to Thyroid Hormone Imbalances

Hypothyroidism

It can develop due to inactivity of thyroid itself or hyposecretion of TRH or TSH or insufficient dietary intake of iodine.

1.Cretinism:

Failure of thyroid secretion from infancy or childhood slows body and mental development and reduces metabolic rate markedly.

The child remains physically stunted and mentally retarded low intelligence quotient, abnormal skin deaf mutism.

The body temperature, heart rate and blood pressure lower than normal.

The patient is pot-bellied and pigeon chested and has a protruding tongue.

Other features include neonatal jaundice, lethargy, respiratory problems and constipation.

This disease is called cretinism.

Normal growth and development may be restored in certain cases by an early administration of thyroid hormones.

2.Myxedema (Gull's diseases):

Deficiency of thyroid hormones produces myxedema in adults.

The patient has a puffy appearance and lacks alertness, intelligence and initiative.

The patient also suffers from low metabolic rate, slow heart rate, low body temperature and reproductive failure.

Administration of thyroid hormones cures the symptoms. it occurs more commonly in females.

3.Simple Goitre:

In mountainous regions, the dietary deficiency of iodine frequently produces thyroid enlargement (Iodine Deficiency Goitre), accompanied by cretinism or myxedema.

Hyperthyroidism

Exophthalmic Goitre:

In Graye's disease or Exophthalmic goitre, a thyroid enlargement (goitre) is accompanied by a bulging of eyeballs (exophthalmos).

The enlarged thyroid is overactive and secretes excessive amount of thyroid hormones.

So, the goitre is associated with symptoms of thyroid overactivity such as high metabolic rate, rapid heart rate, rise in body temperature, emaciation, nervousness, irritability, tremor and restlessness.

It also occurs more often in females.

It is an autoimmune disorder in which the person produces antibodies that mimic the action of TSH, but are not regulated by normal negative feed back control.

PARATHYROIDS GLAND

These are four small pea sized glands situated very close to the thyroid.

They secrete a hormone called parathormone (Collip's hormone).

They are under the feedback control of blood calcium level.

A fall in blood calcium stimulates them to secrete parathormone, a rise in blood calcium inhibits parathormone secretion from them.

Parathormone increases the concentration of calcium ions in the blood plasma, because it mobilises more calcium from the bones to the plasma and reduces urinary elimination of calcium.

It is secreted whenever the plasma Ca²⁺ concentration falls and restores the Ca²⁺ concentration to normal in the plasma.

On the other hand, it increases phosphate elimination in the urine and consequently lower the phosphate concentration in the plasma.

Thus, parathormone regulates the metabolism of calcium and phosphorus.

Calcium Homeostasis:

(i) A higher than normal level of calcium ions (Ca²⁺) in blood stimulates parafollicular cells of the thyroid gland.

(ii) They release more calcitonin as blood Ca²⁺ level rises.

(iii) Calcitonin promotes deposition of blood Ca²⁺ into the matrix of bone tissue. This decreases blood Ca²⁺ level.

(iv) A lower than normal level of Ca²⁺ in blood stimulates principal cells of the parathyroid gland.

(v) They release more parathyroid hormone (PTH) as blood Ca²⁺ level falls.

(vi) PTH promotes release of Ca²⁺ from bone matrix into the blood and retards loss of Ca²⁺ in the urine. These actions help raise the blood level of Ca²⁺.

(vii) PTH also stimulates the kidneys to release another hormone called calcitriol.

(viii) Calcitriol stimulates increased absorption of Ca²⁺ from foods in the gastrointestinal tract, which helps increase the blood level of Ca²⁺.

DISORDER OF PARATHYROID

Hypoparathyroidism

Parathyroid Tetany:

If the parathyroids fail to secrete sufficient amount of parathormone, the concentration of calcium ions falls abnormally in the plasma.

This increases the excitability of nerves and muscles due to deficiency of Ca²⁺ which causes depolarisation without usual stimulus.

Consequently, sustained contractions (tetany) of the muscles of larynx, face, hands and feet are produced.

This disease is called parathyroid tetany.

It can also develop due to accidental damage to the parathyroid or their blood supply during thyroidectomy surgery.

Hyperparathyroidism

The parathyroid tumours secrete excessive amount of parathormone, which causes increased mobilisation of bone minerals into the blood, softening of bones, rise in the concentration of calcium iens in the plasma, and deposition of calcium in kidney tubules and other soft tissues.

It may cause Osteitis Fibrosa Cystica.