Thyroid gland
Morphology of thyroid gland :
This is the largest endocrine gland in the body weighing 25 to 30 gms and measures about 5 cms in length and 3 cms in width. It is located in the anterior region of the neck just below the larynx, ventrolateral to the trachea. Thyroid is derived from the endoderm of the embryo. It varies in size as per age, sex and diet. It is reddish brown colour. It is bilobed and highly vascular. The two lobes right and left joined by connective tissue called isthmus which is at 2 to 4 tracheal cartilage. The structural and functional units of thyroid gland are the thyroid follicles.
Externally the gland is covered by thin connective tissue capsule. From the capsule arises number of septa called trabeculae which divide the interior of gland into the lobules. Total number of thyroid follicles are about three million.
Hormones of Thyroid gland :
The thyroid gland synthesize, store and discharge the thyroid hormones which are thyroxine (T4) also called tetraiodothyronine, triiodothyronine (T3) and thyrocalcitonin. T3 and T4 are iodinated derivatives of amino acid tyrosine. Both are stored in follicular cavity before secretion and supposed to have same function. The secretion of T3 and T4 are regulated by Thyroid stimulating harmone (TSH) or thyrotropin of pituitary gland in negative feedback manner. T3 is more active and T4 is more potent harmone.
Functions of Thyroxine :
Thyroxine is the main metabolic hormone in the body. It maintains basal metabolic rate (BMR) by increasing glucose oxidation and energy production known as calorigenic effect. It controls normal protein synthesis. It also controls the physical growth, development of gonads and development of mental faculties. It controls tissue differentiation and metamorphosis particularly in amphibia. It controls body weight, respiration rate, heart rate, blood pressure, temperature, digestion etc.
Deficiency of thyroxine in childhood causes cretinism. The symptoms are retardation of physical and metal growth.
Deficiency of thyroxine in adults causes myxoedema. Symptoms are thickening and puffiness of the skin and subcutaneous tissue particularly of face and extremities. Patients are with low BMR, It also causes mental dullness, loss of memory and slow action. Simple goiter is caused due to deficiency of iodine in diet or drinking water. It causes enlargement of thyroid gland. Use of iodised table salt prevents the disorder. It is common in hilly regions.
Excessive secretion of thyroxine causes
Exopthalmic goiter (Grave's disease). These is slight enlargement of thyroid gland. It increases BMR, heart rate, pulse rate and BP. Deposition of fats in eye sockets, muscular weakness and weight loss occurs.
Thyrocalcitonin (TCT) is secreted by parafollicular cells. It regulates blood calcuim level. This hormone stimulates bones to take up (Ca++) from the blood for deposition of calcium phosphates in the bones; thereby decreasing blood (Ca++) level. Increased calcium level of blood stimulates 'C' cells to secrete Thyrocalcitonin and vice versa.