THYROID DISEASES-I

THYROID-A VIEW

The thyroid gland which is looking like a butter fly with  spreaded wings is situated in the anterior(front side)of the neck or throat just below the laryngeal prominence (Adam's Apple).It contains two lobes on either side spreaded like the wings of a butterfly.

Thyroid gland is one of the largest ductless gland (endocrine) in our body and it controls many important body functions such as rate of use of energy sources.protein synthesis and controls the body's responses to other hormones.

The thyroid gland synthesizes,stores and secretes hormones which are controlling and participating the processes like growth and protein anabolism.These hormones are as follows:-

1.Tetraiodo thyronine or thyroxine (T-4)-Less active-More stable-and Major secretion of the gland

2.Triiodo thyronine (T-3)-More active-Less stable-Minor secretion of the gland.

A portion of the thyroid gland is secreting a hormone known as calcitonin which is controlling calcium metabolism.

The sequences of thyroid secretion of hormones are as follows,

In our brain system there is a gland like a peanut known as thalamus.Below the thalamus a minute part of the brain is known as hypothalamus which is controlling all our endocrine systems including thyroid.

Hypothalamus secrets a hormone known as Thyrotropin Releasing Hormone(TRH) which inturn stimulates anterior pitutary gland to release Thyrotropic Hormone or Thyroid Stimulating Hormone(TSH) which inturn stimulate thyroid gland to secrete its hormones T-4 and T-3 which on free circulation will send the negative feed back signals to the anterior pitutary to stop to secret the TH.The process will continue like a cycle.Thus the amount of the free circulating hormones are maintained in a constant level.

Synthesis Thyroid hormones are synthesised by thyroid gland by combining tyrosine with the available iodine.

Iodine containg foods like sea foods are essential for the synthesis of thyroid hormones.The food release in the plasma iodide in an inorganic form.

Before synthesis starts a process of manipulating the iodide ion by a process known as the organification which involves the iodide oxidised by peroxidase and bound to tyrosyl residues within the thyroglobulin molecule as follows

1.The inorganic iodide from the plasma is trapped by the gland and the process is known as iodide pump

2.The inorganic iodide has to undergo the organification after thatTyrosine combines with iodide to form Mono Iodo Tyrosine(MIT)

3.MIT binds with another iodide ion to form Di Iodo Tyrosine(DIT)

4.Two DIT couples to form the stable T-4,the Thyroxin

5.One MIT and one DIT couples to form the unstable T-3 which is highly active

Hormone Transport
1.After the gland get stimulated by TSH (Thyrotropic Hormone or Thyroid Stimulating Hormone) from the anterior pituitary gland the T-3 and T-4 are cleaved from the thyroglobulin and released into the blood circulation.
2.In the blood the hormones are bound up with plasma proteins like thyroxine binding globulin (TBG),Prealbumin and Albumin in order to protect them from inactivation by early metabolic processes.These protein carriers help the hormones to reach the site of action very easily.
Metabolism Mostly occur at pituitary,liver,and kidney by conversion of 80% of T-4 into T-3.
The active T-3 is responsible for 99% of the thyroid activity in our body and its abnormal elevation and fall are the complete representations of thyroid dysfunctions.
T-3 is further degraded step by step by a process of deiodization finally converted to thyronine and excreted in feces and urine.
Functions of thyroid hormones
1.Growth
2.Delelopment of our body
3.Energy production by increasing the rae of basal metabolism.
4.Tissue sensitivity to hormones like adrenalin and there by affecting cardio vascular system like increasing heart rate,blood flow,cardiac out put,metabolic rates
5.Increasing the activity of nervous system.
6.Causing tremour.
7.Thyroid over activity may cause sleepiness and tired
8.Thyroid underactivity may cause somnambulance.
9.Affecting lipid meabolism.
Thyroid tests-A.Serum Total T-4 Test.
1.Radio immuno assays these tests more sensitive and rapid to assay total thyroxine T-4.which will indicate the total availability of hormone to the tissues.
Unless otherwice not affected by other factors like pregnancy kidney failure,or liver damage an elevated Total T-4(TT-4) may indicates Hyperthyroidism and a dippressed T-4 may indicate Hypothyroidism
Serum total T-3 Test
This sensitive and most specific test indicates measure of TT-3.
This test is more accurate and help early detection in hyperthyroidsm as the elevation of TT-3 is immediate and more quicker than TT-4.Many of the symptoms of hyperthyroidsm is due to the elevation of TT-3.
But on the contrary in hypothyroidism this test is not reliable as even though there is a fall in TT-3 but will stay at the normal level.In rare case the TT-3 may be low.These test may also mislead by those factors which affect the test result of TT-4.
Resin T-3 uptake(RT,U)
This test will clarify whether the abnormal T-4 levels are due to thyroid disorder or a disorder by protein uptake because it evaluate the binding capacity of TBG
This test read more accurate changes of the abnormal levels of the hormone in the blood.
Serum Thyrotropin(TSH) and Sensitive TSH Assays
a)Serum Assay This assay results are more reliable and sensitive as the hypothalamus - pituitary axis is more sensitive in responding even a minute variations of the circulating blood thyroid hormones to release or stop the TSH.This axis is very sensitive to indicate hypothyroidsm as a small dip in the blood hormone will receive a quick compensatory response from the hypothalamus - pituitary axis.
The TSH levels may be elevated even much before the TT levels are dipped and very useful and easy to detect Hypothyroidsm in advance.
This test is not reliable to asses Hyperthyroidsm as the hypothalamus-pituitary axis is varyingly responding to the elevated TT in plasma
Sesitive Assay This is an advanced technology using immunoradiometric methods instead of the old radioimmune assay and is very expensive.This is the most occurate method of detecting thyroid defects.
This is a very useful test to monitor paients receiving hormone replacement therapy to control hormone over exposures,since over exposure to thyroid hormones may precipitate unwanted elevation of liver enzymes,bone demineralizations and ECG effects.