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• Regulation of thyroid function

  1. TSH: stimulate thyroid function

  2. TRH: increase TSH secretion

  3. T4 & T3: -ve feedback inhibition of pituitary cells (Thyrotropes)

  4. T3 production in pituitary cells enhance T4 –ve feedback inhibition

  
• Removal of pituitary


  Depress thyroid function and cause gland atrophy

  
• TSH injection


  Stimulate thyroid function

  
• Effects of TSH within few minutes

  1. Increase iodine binding

  2. Increase T3 , T4 , MIT, DIT synthesis

  3. Increase thyroglobulin secretion into colloid and colloid endocytosis

  
• Effects of TSH within a few hours

  1. Increase blood flow

  2. Increase gland weight (hypertrophy)

  3. Prolong TSH stimulation: cause goiter (enlarged gland)

  
• Widespread effects of thyroid hormones

  • Stimulation of O2 consumption (calorigenic action)
  • Increase growth and development
  • Regulate lipid metabolism
  • Increase carbohydrate absorption from intestine
  • Increase dissociation of O2 from Hb by increasing RBC 2,3-DPG

  
• Thyroid receptors (TR)


  Found in cytoplasm

  
• T3


  Binds avidly to TR
• T4


  Not avidly bind to TR
• Mechanism of action of thyroid hormones

  1. T3-TR complex then binds to DNA via zinc fingering and cause increase expression of different genes that coded for proteins that regulate cell function

  2. Nuclear thyroid receptors are members of hormone-sensitive nuclear transcription factor superfamily