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Thyroid gland is the largest gland in a normal adult. The gland weighs 15- 25g.
It is a highly vascular organ. The gland is bilobed and connected by a bridge of
tissue called isthmus.
The function of the gland is controlled by thyroid stimulating hormone, TSH
from anterior pituitary which is regulated by thyrotropin releasing hormone,
TRH from hypothalamus .Thyroid hormones, T3 and T4 in circulation, provide
negative feedback on secretion of TSH and TRH on anterior pituitary and
REGULATION OF TSH SECRETION:
Hypothalamus TRH Anterior Pituitary TSH Thyroid Gland. T3,
Factors increasing TSH secretion:
3. Fall in free T3 ,T4
There exists inverse relationship between T3 T4 and TSH.
Thyroid hormones regulate lipid and carbohydrate metabolism.
They are important for normal body growth. Thyroid gland produces two main
hormones -T3 and T4.
FORMATION OF THYROID HORMONE
1. Iodine from diet is converted to Iodide which is reabsorbed into thyroid cells
by Iodide pump.
2. Iodide pump is mediated by Na+- K+ dependent ATPase system.
3. Iodide is transported into colloid and gets oxidized by thyroid peroxidase into
4. I2 then gets bound to 3 position of tyrosine forming mono- iodo- tyrosine,
MIT and di-iodo-tyrosine, DIT.
5. Thyroid peroxidase and coupling enzymes form T3 and T4.
6. Iodinated T3 and T4 are deiodinated by Iodotyrosine dehalogenase and
secreted into circulation.
DIT + DIT—-----thyroid peroxidase------ Thyroxine T4+ alanine
MIT +DIT ------------ T3
T4 - I_ ----deiodination--------- T3
DIT + MIT ---------------reverse T3
TRANSPORT OF THYROID HORMONES:
Thyroid hormones are transported in blood by:
1. Albumin: Highest capacity to bind thyroid hormones.
2. Thyroid binding globulin (TBG): Least capacity to bind thyroid
Free thyroid hormones are in equilibrium with protein bound thyroid hormones
in plasma. Free T4 and T3 are physiologically active form. When concentration
of free thyroid hormones is increased in plasma, the rate of entry of thyroid
hormones in tissues is increased thereby maintaining the euthyroid state.
1. Normal plasma levels: 3-8 microgms%
2. 99.9% bound to TBG.
3. Longer duration of action, but onset of action is slow
4. Free plasma levels of T4 are less than T3.
TRI IODO THYRONINE (T3).
1. Normal plasma levels: 0.15 microgm%
2. 99.8% bound to TBG and albumin.
3. Duration of action is short but onset of action is rapid.
4. Free plasma levels are higher than T4.
ACTIONS OF THYROID HORMONES:
1. Energy Metabolism: Thyroid hormones stimulate heat production in the
body, due to stimulation of O2 consumption that increases the BMR. T3
and T4 increase the O2 consumption of all tissues except brain, testes,
spleen, lymph node, ovary, uterus and anterior pituitary.
2. Protein Metabolism:
In physiological dose: T4 is anabolic and increases protein synthesis.
IN PHARMACOLOGICAL DOSES, T4 has catabolic effect on body
3. Carbohydrate metabolism:
Thyroid hormone in physiologic dose,
Increase peripheral utilization of glucose, can cause
Increase glucose absorption from intestine
Decrease breakdown of insulin.