• Thyroid gland is located in neck, anterior to trachea. It consists of two lobe-one on each side of junction between laryngotrachea. The lobes are connected by a thyroid tissue called Isthmus.
• The gland is composed of two types of cells; follicular cell (secrete thyroid hormone) and para-follicular cell (secrete and store calcitonin hormone).
• Thyroid glands consists of hundreds of thousands of thyroid follicules which stores thyroid hormones.
• Follicles are made up of single layer of follicular cells.

Follicular cell:

• these cells are most prevalent cell.
• They secretes thyroid hormones (T3 and T4)

Parafollicular cell:

• cluster of parafollicular cells are found between follicluar cells.
• they are larger than follicular cell.
• parafollicular cells synthesize and secretes calcitonin.

*Thyroid is the only endocrine gland that is able to store its secretion outside the principle cells.

Location: in neck region on either side of trachea

Size: Each lobe is 4-6 cm long, and 1.5 cm thick

Hormones of Thyroid gland

1. Thyroxine (Tri-Iodothyronine and tetra-Iodothyronine)

• Follicular cells secretes thyroxine which contains 4 atoms of Iodine to form tetra-iodothyroxine (T4).  Follicular cells also secretes tri-Iodothyroxine (T3) which contains only 3 atoms of Iodine.
• T3 and T4 collectively known as Thyroid hormone.

Functions of thyroxine hormone:

• Increase basal metabolism rate by increasing O2 comsumption in most tissues except lungs, brain, testis and retina
• Essential for growth of skeletal in children. Thyroxine is regulated by TSH from Pituitary gland.
• Increase carbohydrate metabolism, promote gluconeogenesis
• Effect on lipid metabolism
• Stimulate NA-K ATPase.
• Influence water and electrolyte balance.
• It is regulated by TSH
• At first T4 is synthesized which is converted to T3. T3 is five times more potent than T4.
• Deficiency of thyroxine: Goiter, Hypothyroidism
• Excess of thyroxine: thyrotoxicosis, hyperthyroidism.

2. Calcitonin

• Parafollicular cells secretes calcitonin. Calccitonin is a polypeptide hormone.
• Secretion of calcitonin is regulated by concentration of calcium in blood but not by feed back mechanism of Pituitary gland.

Functions of calcitonin:

• It lowers calcium level in blood ( antagonistic to parathyroid hormone)
• It acts directly on Osteoclast to reduce the remodelingof bone and increase reabsorption of calcium.
• It increases the movement of Ca++ from blood to bone.

 Disorder of thyroid gland

1. Cretinism:

• deficiency of thyroxine in children
• pot belly, pigeon chest in children with physical and mental retardness
• dwarfism (cretins)

2. Myxedema:

• deficiency of thyroxine in adults
• puffiness of skin
• mental retardness
• low body temperature, BMR and blood pressure

3. Simple goitre:

• deficiency of iodine in diet and hence deficiency of thyroxine
• swelling of thyroid gland, visible in neck region
• low BMR. low Blood pressure and heart rate
• fatigue and sluggishness

4. Exopthalmic goitre (Grave’s disease):

• over stimulation thyroid gland
• goitre
• bulging of eye ball
• increased BMR, blood pressure and heart rate
• restlessness and nervousness

Thyroid gland: Hormones and functions

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