Biosynthesis of thyroid hormones.
It helps to understand how the thyroid hormones are synthesized from our body form tyrosine and iodine. What are the steps involved in the biosynthesis?
2. Introduction
• Thyroid hormone by thyroid gland [ butterfly
shaped]
• Secretory unit is thyroid follicle
• Outer layer of epithelial cells
• Colloid [ amorphous material Thyroglobulin
and thyroalbumin]
• Also has parafollicular or ‘C’ cells calcitonin
3.
4. Biochemistry of TH
• TWO hormones
• Thyroxine [T4][ primary secretory product]
• Triiodothyronine[T3]
• In addition , small amount of
• Reverse triiodothyronine [ rT3]
• Monoiodotyrosine[MIT]
• Diiodotyrosine[DIT]
• rT3 is biologically inactive
7. Further info..
• Half life of T4 [ 4-7 days] and T3 is about 1 day.
Out of total T4 secreted
↓
Deiodinase enz.[se] in peripheral tissue
↓
40%T3 and 45%rT3
• T3: peripheral deiodination of outer ring of T4.
• rT3: peripheral deiodination of inner ring of T4.
8. • 85% of daily T3 production is due to peripheral
deiodination of T4 rather than direct secretion
from TH. gland.
• T3 is 4-5 times more potent than T4 in biological
system
• Unbound i.e., free T3 and free T4 biologically
active. concept of Free vs bound
9. concept of Free vs bound
• Both T3 & T4 are bound reversibly and almost
completely to carrier protein
• Thyroxine binding globulin [TBG] [ Predominant]
• Thyroxine binding prealbumin[TBPA]
• Albumin
• In collectively, 99.97% of T4 & 99.7% of T3 is bound
• So, fT4 [0.03%] & FT3 [0.3%] free thyroid
hormone actual biological function.
10. Biological action
Many important biological function.
• ↑ es O2 consumption in almost all metabolically
active tissue.
• ↑ the activity of Na+-K+ pump. Na+-K+ ATPase
activity is directly correlated to TH hormone.
• Basal metabolic rate (BMR) is ↑ ed.
• ↑ thermogenesis
11. • ↑es mitochondrial metabolism[ mito. Respiration
and ox. Phosphorylation]
• ↑ es the sensitivity of adrenergic receptors to
catecholamines.
• Stimulate adrenergic activity with ↑ ed heart rate
and myocardial contractility
• Stimulation of RNA synthesis and consequent
protein synthesis acts like steroid hormone
12. • Stimulate neural development and normal growth
• Promote sexual maturation
• Promote intestinal absorption and utilization of glucose.
↑ es gluconeogenesis and glycogenolysis
• Fatty acid metabolism is ↑ed . Lipid turnover and
utilization is stimulated. ↓ TH ↑ cholesterol.
• ↑ es the Ca++ and Po4
– metabolism.
• Also contribute to peristalsis movement.
15. Steps of synthesis
• Normally iodide [I-] is trapped from
circulating plasma.
• Follicular cell concentrate iodide to 30-40
times plasma conc by iodine pump[Na/I
sym].
• Rate limiting step: Iodide transport to follicle
is 1st and R.L step.
16. 1.Uptake of iodine
• Thyroid gland takes up and concentrates
iodine by Na/I symporter
• Energy requiring process [ Na+-k+ ATPase
pump]
• Anti thyroid agent such as thiocyanate
perchlorate and pertechnetate inhibit iodine
transport inhibit iodide transport
17.
18. 2.Oxidation of Iodine [ Active iodine]
• The thyroid is the only organ which can
perform this oxidation step.
• by the enzyme thyroperoxidase [ TPO]
19. 3.Iodination of Tg [ Tyrosine residue]
• Tg[ 660Kda] contains tyrosine residue.
• Active Iodine will bind to tyrosine residue of Tg
organification MIT and DIT
• by thyroperoxidase [ TPO]
20. 4. Coupling
• Coupling or condensation is also done by TPO.
• DIT+DIT T4
• DIT+MITT3
• Occurs at follicular cell-colloid interface and also
with in colloid.
• Formed TH hormones are stored in colloid [
approx. 3 months]
21.
22. 5.Release
TSH
Globules [colloid] endocytosed by follicular cells
Lysosomal protease breaks down peptide bond
of Tg
T3 and T4 circulation
MIT&DIT deiodinated by dehalogenase
Free iodides are then reutilized
↓
↓
↓
↓
24. TSH almost effects each steps
• ↑es Iodine pump activity.
• ↑es thyroglobulin synthesis
• ↑es organification
• ↑es colloidal uptake by follicular cells
• ↑es rate of proteolysis
• Also, ↑es in the size and no. of TH follicular cells.
25. Metabolic fate of T3&T4
• Conjugates with glucuronic acid or sulphate in liver
• Excrete through bile
• Also, deamination to produce
• tetraiodothyroacetic acid[T4]
• triiodothyroacetic acid[T3]
• Undergo conjugation and excretion.
rT3 is biologically inactive [ acute or chronic stress /illness]
Form 5’ position of the outer ring An iodine is lost .
Deiodination mainly in liver
10-15 % is only direct T3 from TH gland
Because 1/3 of all T4 is converted to T3 during course of metabolism T4 is considered as prohormone.
The thyroxine binding globulin (TBG) (54 kD) carries about 80% of T4 and 60% of T3.
Thyroid hormone attaches with specific nuclear receptor receptor hormone complex binds to DNA.
One mg of T4 will produce an excess of 1000 kcal.
Rate of O2 consumption and energy expenditure under resting condition.
On transcriptional level. So anabolic hormone growth and development but in hyperthyroidism protein catabolism, neg nitrogen balance and loss of body weight.
Incraeses sympathetic acitivity.
Hypoth causes constipation
Nitrates also
antithyroid drugs such as thiourea, thiouracil and methimazole
Electron acceptor is the hydrogen peroxides.
140 in satyanarayan
115 in Vasudevan
35 can be iodinetaed
After complete, it has 6-8 mol. Of T4 and 10:1 ratio with T3. The thyroglobulin contains about 6-8 T4 residues per molecule [ T3:T4 around 1:10]
During condensation alanine is released