The thyroid gland is situated in the neck just in front of larynx and trachea, it is a highly vascular gland of about 25g and is covered or surrounded by a fibrous capsule. It has a butterfly shape and consist of two lobes present on the either side of the thyroid cartilage. The two lobes are held together a narrow isthumus , present in the front of trachea. The lobes are cone shaped and are about 5 cm long. The blood supply to the gland is through superior and inferior thyroid arteries and the venous return is done through thyroid veins. The gland is composed of large follicles which are spherical in shape are made of cuboidal epithelium. These follicles secrete and store colloid, a thick sticky material.
Thyroid gland secrete 3 hormones which are-
- Thyroxine ,
- Tri-iodothyronine and
Thyroxine and tri-iodothyronine:-
Iodine is very essential for the formation of thyroid hormone, thyroxine(T4) and tri-iodothyronine(T3), these are named as 3 and 4 as these contain 3-4 atoms of elemental iodine. The thyroid gland selectively takes iodine from the blood and this process is known as iodine trapping.
Thyroid hormones are synthesized by large molecules, called thyroglobulin which is found from colloid and the T3 and T4 hormones are stimulated in the blood by thyroid stimulating hormone(TSH) of anterior pituitary. Again the secretion of TSH is stimulated by thyrotrophin releasing hormone(TRH) which is in turn secreted by exercise, stress, malnutrition, low glucose levels etc.
Secretion of TSH depends upon the concentration of T3 and T4, which is also responsible for the secretion of TRH from the anterior pituitary, increased level of T3 and T4 decreases the stimulation of TSH. In case of iodine deficiency, the TSH secretion increases which causes proliferation of thyroid gland cells and enlargement of the gland known as goiter. T4 hormone is more abundant the t3 but the t3 hormone is more potent and is physiologically more important that’s why most of the t4 hormone is converted to t3 hormone inside the target cells.
Functions or importance of thyroid hormone includes:-
regulation of gene expression, that is they increase or decrease protein synthesis; they also enhance the effect of epinephrine and non epinephrine ; increases the heat production and also regulates the carbohydrate , protein and fat metabolism. essential for the growth and development of the skeleton.
This hormone is secreted by the parafollicular cell or the C-cells present in the thyroid gland. The main function of this hormone is to lower down the high blood Ca levels, it does it by
• Acting on the bone cells and promotes their storage capacity for calcium
• Acting on the kidney tubules and inhibits the reabsorption of calcium ions.
Release of calcitonin is stimulated by high blood calcium level, also this hormone is important during childhood when bones undergoes changes in shape and size.
In our body there are four parathyroid gland of about 50g each and which are situated in pairs in the posterior surface of each of the two lobes of thyroid gland. They are surrounded by connective tissue capsule which contain cells which are arranged in columns.
Functions of parathyroid glands include
These glands secrete parathyroid hormone, their secretion is stimulated by low blood calcium level; that means that this hormone works in opposite effect to that of calcitonin.
The main function of parathyroid hormone is to increase the blood calcium level and this is done by increasing the Ca absorption from small intestine and also by increasing the reabsorption from renal tubule. PTH also if necessary stimulates osteoclasts(bone destroying cells)and calcium is released from cells.
Blood calcium level is maintained by both PTH and calcitonin which is essential for
• Muscle contraction
• Transmission of nerve impulses
• Blood clotting
• Regulates the action of many enzymes.
Multiple choice questions(MCQs)
1. Where is the thyroid gland situated?
A. In the neck B. in the brain
C. in heart D. in pelvic cavity
2. Where are the parathyroid glands situated?
A. in the anterior surface of each lobe of thyroid gland
B. in the posterior surface of each lobe of thyroid gland
C. in the inferior surface of each lobe of thyroid gland
D. in the superior surface of each lobe of thyroid gland
3. The superior thyroid artery is branch of which artery?
A. internal carotid artery B. subclavian artery
c. external carotid artery D. none of the above
4. The process through which thyroid gland collects iodine from blood is known as?
A. iodine collection B. iodine trapping
C. iodine isolation D. selective collection of iodine
5. Match the following-
I. T3 and T4 a. synthesized by thyroglobulin
II. TSH b. stimulated by TSH
III. TRH C. Stimulated by TRH
IV. Thyroid hormone d. stimulated by exercise, stress etc
6. The parafollicular or C-cells stimulates the secretion of which hormone?
A. calcitonin B. parathyroid hormone
C. T3 D. T4
7. Increased level of T3 and T4 in the body is known as?
A. hypothyroidism B. hyperthyroidism
C. goitre D. benign tumuors
8. which of the following are the symptoms of hypothyroidism?
A. diarrhea B. weight loss
C. increased basal metabolic rate D. none of the above
9. Which of the following statement is NOT true?
a. secretion of parathyroid hormone is regulated by blood Ca level.
b. enlargement of thyroid gland is known as goitre
c. the effect of calcitonin is opposite to that of T3 hormone.
d. T3 and T4 regulates metabolism of carbohydrates protein and fats.
10. which of the following is the function performed by PTH and calcitonin collectively?
A. muscle contraction B. blood clotting
C. action of many enzymes D. all of the above
1. in the neck
2. in the posterior surface of each lobe of thyroid gland
3. external carotid artery
4. iodine trapping
5. 1 – b 2 – c 3 – d 4 – a
8. none of the above
9. the effect of calcitonin is opposite to that of T3 hormone
10. all of the above
REFRENCE: 1. Ross and Wilson-Anatomy and physiology in health and illness; 12th edition; page no.-: 222-224.