12th Zoology

Thyroid gland

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

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

Related Topics in Zoology:

Bio Zoology All Important Topics

  1. Human Physiology Introduction

  2. Nutrition

  3. Carbohydrates Poly hydroxyaldehydes (or) ketones

  4. Proteins (Polypeptides)

  5. Lipids

  6. Vitamins – Functions Of Vitamins

  7. Deficiency of Vitamin

  8. Minerals – Water – Role of water

  9. Balanced diet

  10. Obesity

  11. Digestive System

  12. Dental Caries (Tooth decay)

  13. Root Canal Treatment

  14. Peptic ulcer

  15. Hernia and Types

  16. Appendicitis (Appendix)

  17. Gall Stones

  18. Hepatitis

  19. Fractures – Types of fractures

  20. Mechanism of fracture

  21. Dislocation of joints

  22. Arthiritis

  23. Rickets and Osteomalacia – Orthopedics

  24. Muscles

  25. Mechanism of muscle contraction

  26. Types of muscle contraction

  27. Myasthenia Gravis

  28. Respiration – Process of pulmonary respiration

  29. Mechanism of Breathing

  30. Regulation of Respiration

  31. Pneumonia Tuberculosis Symptoms Treatment

  32. Bronchitis – Acute bronchitis, Chronic Bronchitis Causes

  33. Circulatory System – Functioning of Human heart

  34. Cardiac Cycle

  35. Coronary blood vessel and its significance

  36. Myocardial infarction

  37. Angina pectoris

  38. Angiogram – Angioplasty

  39. Atherosclerosis

  40. Heart block Echo cardiography Heart Valves

  41. Rheumatic Heart Disease (RHD), ICCU – (Intensive Coronary Care Unit)

  42. Blood Pressure

  43. Heart transplantation

  44. Pulse rate

  45. Cardio Pulmonary Resuscitation (CPR)

  46. Blood – Composition of plasma – Blood cells

  47. Clotting of Blood or Haemostasis

  48. Thrombosis

  49. Nervous system Co-ordination systems

  50. The Brain – Fore Brain, Midbrain, Hindbrain

  51. Memory

  52. Sleep – Types of sleep

  53. Stroke – Brain haemorrhage

  54. Alzheimer – Meningitis (Brain fever)

  55. Conditioned reflex

  56. Electroencephalography EEG

  57. Right and Left brain concept

  58. Spinal cord functioning

  59. Chemical co-ordination – Functions of Endocrine glands

  60. Hypothalamus

  61. Pituitary gland – hormone

  62. Hormones of Neurohypophysis – vasopressin

  63. Thyroid gland

  64. Parathyroid Gland

  65. Pancreas

  66. Adrenal gland

  67. Gonads

  68. Receptor Organs – Eye

  69. Photochemistry of Retinal visual Pigments

  70. Errors of refraction

  71. Optometry – Retinopathy

  72. Cataract – Lens Replacement – Glaucoma – Nyctalopia

  73. Eye Infections and Eye Care

  74. Ear

  75. Mechanism of hearing

  76. Defects of the ear

  77. Hearing Aid – Noise pollution

  78. Skin and functions of skin

  79. Melanin functions

  80. Effects of solar radiation / UV radiation – Skin grafting

  81. Dermatitis

  82. Tongue – Mechanism of Stimulation

  83. Excretion Ureotelism Nephron

  84. Mechanism of urine formation

  85. Renal Failure, Dialysis, Kidney Machines

  86. Kidney stone – Kidney transplantation

  87. Diabetes mellitus

  88. Functioning of male reproductive system

  89. Functioning of female reproductive system

  90. Ovulation and fate of the ovum – Menstrual cycle

  91. Fertilization

  92. Birth control

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