THYROID HORMONES SYNTHESIS| Transport| Actions of Thyroid hormones| Feed-back Regulation

THYROID HORMONES

• The thyroid gland is essential to the endocrine system.
• It is located in front of the neck and is responsible for the production of thyroid hormones.
• The thyroid gland releases triiodothyronine (T3) and thyroxine (T4).These hormones play an important role in regulation of your weight, energy levels, internal temperature, skin, hair, nail growth.
• These hormones are regulated by hypothalamus and pituitary gland.
  

SYNTHESIS OF THYROID HORMONES

1. Thyroglobulin synthesis 

• Thyroglobulin is a glycoprotein, synthesize by follicular cells. 
• Thyroglobulin contains many tyrosyl residues and serves as a matrix for he synthesis of T3 and T4.  

 2. Uptake and concentration of iodide 

• Iodide(I-) along with Na+ from plasma is actively transported by sodium-iodine symporter(NIS) situated in the basolateral membrane of thyrocyte. Na+ again back to plasma through Na+/K+ ATPase. At the apical membrane, pendrine, protein involved in anion transport and apical iodine transporter mediates iodide efflux in to lumen area.

3. oxidation of iodide

• In lumen, iodide get oxide by thyroid pereoxidase, release by Dual oxidases(DUOX), and converted in to iodine(I+) 

4. Iodination of thyroglobin and coupling

• Iodine incorporate into tyrosyl residues on thyroglobulin and form mono-iodotyrosine (MIT) and di-iodotyrosine(DIT). MIT and DIT combine to form tridothyrine(T3), DIT and DIT combined to form thyroxine(T4).

5. Secretion 

•  MIT, DIT,T3 and T4 get package up via pinocytosis into follicular cell where MIT and DIT deiodination. Remaining T3 and T4 secrete into plasma.

Transport of T3 & T4 in blood

T3 & T4 are highly bound to plasma proteins namely

• Thyroxine Binding Globulin
• Thyroxine Binding Pre albumin (transthyretin)
• Thyroxine Binding Albumin.

Action on target cell

• T3 exerts its effects through interaction with nuclear Thyroid receptor (TR) that has high affinity and specificity forT3.  They combine with retinoid X receptors (RXRs). TR/RXR complex binds with target response element in promoter region of target gene. The complex of T3 TR/RXR recruit other coactivators, - leading to increased mRNA and protein production.

Actions:

1. Growth & development:

• Essential for physical & mental growth.
• Myelination of sensory & motor tracts.

2. On metabolism:

• Overall increase in metabolism by enzyme induction.
• increase BMR,  increase O2 consumption,  increase heat production.
• increaseLipolysis & decrease body fat.
• increase Carbohydrate absorption from GIT & their utilization.
• Increased gluconeogenesis
• Increased glycogenolysis

 

3. CVS

• Increase heart rate
• increase myocardial contractility
• increase cardiac output
• increase systolic pressure but decrease diastolic pressure

4. Respiratory Effects

• stimulate oxygen utilization
• Increases resting respiratory rate, 
• minute ventilation and the ventilatory responses to hypercapnia and hypoxia
• Increase hematocrit secondary to EPO secretion
• Increase dissociation of O2 from hemoglobin by increasing red cell 2,3 diphosphoglycerate (DPG).

Feed-back Regulation

Share this note SYNTHESIS OF THYROID HORMONES to your colleagues. If you have any inquiry comment below. Thank you