Thyroid, Parathyroid and Thymus Gland | Biology for JAMB PDF Download

Thyroid Gland

It is the largest endocrine gland in the body. This is situated at the latero- ventral side of the joint of trachea and larynx in the neck region of man. The shape of this gland is likeletter H. It is bilobed in birds and mammals, but it is single lobed in reptiles. Both of its lobes are connected by non-glandular band, formed of a connective tissue. This band is called isthmus.

• In human weight of this gland about 25 gm to 35 gm approximately. (This is some what larger in women as compared to men).
• Each lobe of thyroid gland is made up of connective tissue. There are present so many follicles made up of glandular cells in connective tissue.
• These follicles are scattered in loose connective tissue, the stroma. A layer of cuboidal glandular cells is found in the wall of follicles. An iodised colloidal substance Thyroglobulin is filled in the cavity of these follicles.

Thyroglobulin is glycoprotein in nature. Parafollicular cells are occasionally found in between basement membrane of the follicle & these cells secrete calcitonin Thyroid is the only endocrine gland in the body which stores its hormone in its inactive state.

Production of Thyroxine :– Thyroxine contains two phenyl rings linked up by an ether bridge.

Biosynthesis : 

(1) Iodide trapping – when KI taken up by follicular cells.

(2) Oxidation of Iodide – KI → I₂.

The oxidation of iodine is promoted by the enzyme peroxidase.

(3) Organification – The binding of iodine with the thyroglobulin molecule is called organification of the thyroglobulin     Tyrosine + I₂ – MIT & DIT.

(4) Coupling – MIT + DIT → T₃.

DIT + DIT → T₄

All of these step of thyroxine biosynthesis are stimulated by TSH hormone.

T₁ = Mono Iodo tyrosine

T₂ = Di Iodo tyrosine

T₃ = Tri Iodo thyronine (20%)

T₄ = Tetra Iodo thyronine (80%)

• Secretion of T₄ is comparatively more than T₃, and T₃ hormone is four times more effective than T₄ hormone.
• T₄ changes into T₃ on reaching in the tissues.
• T₄ is called thyroxine in this T- chain of hormones.
• Thyroxine or Tetra- Iodo- Thyronin is a derivative of aminoacid.
• Each thyroglobulin molecule contain an average of T₃ molecule for every 14 molecule of thyroxine (1 : 14 – T3 : T4).
• Thyroid hormones in the form of thyroglobulin are stored in the follicles in an amount sufficient to supply the body with its normal requirements of thyroid hormone for 3 months.
• E.C. Kendal first of all crystallised this thyroxine hormone.
• Harrington and Barger studied the molecular structure of thyroxine.
• Thyroid gland requires iodine "120 mg" every day for the production of thyroxine.
• Normal recommended minimum intake 200 mgm/day, if iodine in take < 50 mgm/day, then thyroid disorder develops.
• If there is deficiency of iodine in food then thyroid try to absorbs more and more iodine from blood and increases its size it is called simple goitre. 
• Goitre is found more abundantly in the persons those live on mountain slopes, because iodine (at that place) flows along with water. When most of the people show the symptoms of this disease then it  is called endemic goitre.
• Persons who take sea foods, never show the symptoms of goitre. 1. Growth, Development and Metamorphosis :–

• Thyroxine stimulates the metamorphosis of tadpole larva in amphibians.
• Due to its deficiency, larva does not show metamorphosis to be an adult.
• Some amphibian larvae e.g. Ambystoma and Necturus (Water dog) do not undergo metamorphosis to be an adult, and these larval stages start reproducing without gaining adulthood. This process is called as Paedogenesis.

2. METABOLISM :–

General : 

(i) Thyroxine regulates the Basal metabolic rate (BMR) in the body.

BMR : The rate of calorie consumption that is necessary for life is called basal metabolic rate of  body.

BMR increases→ Body Temp. increase→ loses weight.

Over production of heat called as calorigenesis.

The hormone enhances the oxidative metabolism of body cells as a result of it energy production is also increased in the form of calories so this hormone is also called calorigenic hormone.

(ii) Increase activity of Na+ – K+ ATPase. It increases the number of mitochondria in all the cells of body i.e. it increases metabolic rate of life. It increases the consumption of oxygen by the cells of body.

Fat Metabolism :– 

• Enhances enzyme activity both synthesis & predominantly catabolism of cholesterol.
• Carbohydrate action →
• Blood sugar increases, act as a diabetogenic hormone.
• Protein→ both catabolism & anabolism but at optimum concentration of thyroxine, anabolism is dominant.

3. ON INDIVIDUAL SYSTEM : Heart

• Thyroxine increases – Heart beat increases (Tachycardia) – Increases Cardiac output.
• Maintain the contraction of myocardium.
• It regulates the heart- beat because it acts directly on SA-Node.
• CNS – optimum conc. of thyroxine is required for development of the nerve fibre & their myelination.
• GIT – thyroxine increase motility of GIT causes diarrhoea. Appetite also increase.
• On blood – Thyroxine stimulate RBC formation .

Regulation of Thyroid Hormone Secretion :–

• The hypothalamus and anterior pituitary gland control rate of thyroid secretion.
• Anterior pituitary gland secretes TSH .
• The most important effect of TSH is proteolysis of thyroglobulin which cause release of thyroxine into the blood.
• Anterior pituitary secretion of TSH is controlled by a hypothalamic hormone thyrotropin releasing hormone (TRH).
• Increase thyroid hormone in the body fluid decreases the secretion of TSH by anterior pituitary.

IRREGULARITIES OF THYROID GLAND AND ITS DISEASES :–

1. Hyposecretion of thyroid or Hypothyroidism :– 

• It may be a genetic disorder or due to deficiency of iodine in food or due to excess iodine excretion in urine.
• In childhood, hypothyroidism causes Cretinism, these children are called cretin, they may show such symptoms like :- Thick lips, protruding tongue, pot belly, ill developed sex organs & retarded physical & mental growth. The children remain dwarf and become ugly. Their BMR, rate of heart beat and body temperature decreased. They are sterile.
• Hypothyrodism may cause abnormal skin, deaf - mutism.
• In adult women, hypothyroidism may cause irregular menstrual cycle.

• In adults, hypothyroidism causes Myxoedema. (Gull's disease) The symptoms of this disease are falling of hair, loose and swollen skin, deposition of adipose fat and mucous beneath the skin so body as a whole becomes obese, BMR and blood pressure are reduced. Patient becomes sensitive to cold and shows loss of sexual power.

Mental slowing, bradycardia, weight gain occurs.

• Simple goitre/Endemic goitre :- It is due to deficiency of iodine in food, and also known as Colloid goitre.

Thyroid gland enlarges due to swelling. It is not a genetic disorder. The neck also swells up and looks like a collar. It is cured by extra intake of iodine in food or intake of sea food.

• Hashimoto's disease :- In this disease, there is acute deficiency of thyroxine. Then the medicines given for the treatment of disease or even hormone thyroxine itself acts as poison or antigen. In its reaction, body produces antibodies, which destroy the thyroid gland itself. It is known also as suicide of thyroid or Autoimmune thyroiditis.

2. Hypersecretion of thyroid or Hyperthyroidism :– The gland shows enlargement due to some microbial infections or genetic disorders, and this enlarged gland secretes thyroxine in excess amount. As a result of it, BMR, heart beat rate, blood pressure, absorption of glucose in intestine and consumption of oxygen increase. Too much energy is produced in mitochondria, that it is not stored in the form of ATP but it is released in the form of  heat in the body. Thus in place of growth, unnecessary irritation, exhaustion are observed in the body of patient. Due to excess heat/calorie/energy formation, patient feels extreme hot.

Hyperthyroidism may cause following disorders in the body :-Exophthalmic Goitre or Grave’s disease or Basedow’s disease or thyrotoxicosis In this disease, deposition of mucous beneath the eyeball takes place. As a result of that eyes look enlarged and buldging or protruding out side the eye socket, giving the patient a fearful and staring look. In this disease, whole gland shows enlargement in the neck region.

Plummer's disease :– In this disease, thyroid gland does not show even growth but there become small tumours all over the thyroid just like buds. It is also called Toxic adenoma.

Thyroid gland shows enlargement (swelling) at the time of hypersecretion.

Para follicular cells or C–cells:– These cells are found in the stroma of thyroid gland & basal part of follicle. These cells are of endocrine nature.

They are the remains of ultimobranchial bodies made up of fifth  branchial pouches of embryo, i.e. para follicular cells are endodermal in origin.

These cells secrete thyrocalcitonin (Calcitonin) hormone which lacks iodine. It is a protein.

Thyrocalcitonin reduces the destruction of bones and increases the rate of excretion of Ca⁺⁺ in urine, thus reduces the number of Ca++ in extra cellular fluid.

• It enhances the deposition of Ca⁺⁺ in bones thus making bones solid and strong.
• This hormone is antagonistic to Collip hormone or parathormone.

Parathyroid Gland

• These glands remain embedded in the dorsal surface of thyroid gland .They are two pairs in number . They remain embedded in each lobe of thyroid completely or partially.
• It was discovered by Raynard and its detailed description was given by Sandrom.
• Total weight about 140 mg and its size is 6 × 3 × 2 mm.
• These glands are made by epithelium of third and fourth branchial pouches or pharyngeal pouches slits of embryo i.e. these are also endodermal in origin.
• These glands secrete only one hormone - parathormone. It is also known as Collip's hormone or PTH. Its was obtained by Collip in its pure form.
• This hormone is proteinaceous in nature/Polypeptide hormone.
• Parathormone is essential for survival because it significantly contributes to "homeostatis" by regulating the amount of calcium and phosphate ion in ECF.
• Calcium is key element in many physiological functions like proper permeability of cell membranes, muscular activities, nerve impulse conduction, heart beat, blood coagulation, bone formation, fertilization of ova.
• Calcium is most abundant of all minerals found in the body and about 99% of calcium and phosphorus are contained in the bones. (1% Ca+2 found in ECF).
• Maintenance of proper calcium level under "homeostasis" is in fact, a combined function of parathormone, thyrocalcitonin and vitamin D3 (cholecalciferol).
• Parathormone promotes absorption of calcium from food in the intestine and its reabsorption from nephron in the kidneys.
• Simultaneously, it accelerates elimination of phosphate in urine (phosphaturic action). Thus, calcium level tend to rise in the ECF due to effect of parathormone and phosphate level is decreased in ECF.
• This calcium is then utilized by bone-forming cells, (Osteoblast) in bone formation under the influence of vitamin D₃.
• Parathormone stimulates the osteoclast cells to feed upon bones, these cells remove unnecessary parts of bones by melting, thus change asymmetrical bone into symmetrical bone. The remoulding of bone is done by these cells life long. As a result of this, amount of Ca remains constant in blood in normal conditions. Each 100 ml of blood contains 12 mg of Ca⁺⁺. About 1 kg of calcium is found in an adult man.
• Parathormone maintains the activity of muscles.
• Just opposite to it, thyrocalcitonin hormone works antagonistically to oppose the parathormone and Vitamin D. Thyrocalcitonin reduces the amount of Ca++ in blood by increasing the excretion of Ca++ in urine and by reducing destruction of bone.

1. Hyposecretion

• Due to the deficiency of Ca++ in blood, muscles and nerves get unnecessarily irritated and start convulsion and cramping. Sometimes voluntary muscles remain contracted for a long time, it is known as tetany disease.
• If this tetany happens in intercostal muscles and diaphragm, then animal dies due to Asphyxia.
• Removal of parathyroid gland causes quick death. It is the most fatal state/ lethal condition.

2. Hypersecretion 

As a result of this, bones become brittle and weak. This condition is called as Osteoporosis.

• When quantity of Ca++ is increases in ECF and level of PO4--- is reduced, this condition is known as hypercalcaemia and hypophosphatemia respectively.
• Due to excess deposition of Ca++ in kidneys and gall bladder, stones are formed.

Thymus Gland

• This is situated in the anterior part of heart called mediastinal space. It is a bilobed gland. It is originated by third branchial pouch of embryo, i.e. it is endodermal in origin.
• Thymus gland is well developed in a new born child, but it reduces continuously from adulthood to old age. It remains in the form of a string at last.
• Its structure is just like a lymph gland. It is covered by connective tissue coat capsule and internally both the lobes are redividing in to small lobules.
• Each lobule has a dense, darkly staining peripheral cortex and a looser lightly staining central medulla.
• The cortex consists of densely packed lymphocytes.
• The medulla consists of reticular epithelial cell, a few lymphocytes and the "Corpuscles of Hassalls" or thymic corpuscles.
• Thymus gland is related to immune system of body.

 Hormones and Functions of Thymus Gland 

• Thymus gland secretes thymosin or thymin hormone, that is proteinaceous in nature/Polypeptide.

• This hormone helps in immune system of the body.
• Hassall's corpuscles are found in thymus gland, these are also called thymic corpuscles or epithelial cell or reticular cell, they act as phagocytes.
• After the birth, T - Cells or T - lymphocytes are matured in thymus gland, then these lymphocytes are releases by thymus gland, reach to lymphatic organs like spleen, payer's patches and lymph nodes & deposited in it.
• Thymosin hormone stimulates the maturation of lymphocytes to destroy the antigens produced by bacteria or pathogen.
• According to one of the theories of Ageing  the decline an disappearance of Thymus gland by middle age is the primary cause of ageing.
• Thymosin helps in the development of sex gland but inhibits sexual maturity in early young age.
• Thymus provide cell mediated immunity so thymus is also called "Throne of immunity" or "Training school of T-lymphocytes".
• Thymosin also premote production of antibody to provide humoral immunity. 
• Secretion of thymin decreases the neuromuscular transmission, so hyper secretion of thymine may cause myasthenia gravis. It provides the antibody against receptor & block the NM junction.