Topic 80 - The thyroid hormones

Question 1

Words to include

Answer 1

• Placenta
• Normal development
• Metabolism
• Homeothermy
• Thyroid gland
• Bloodstream
• Iodine
• Lipid soluble hormones
• Hormones
  
  • Thyroxine (T4)
  • Triiodothyronine (T3)
  • Reverse triiodothyronine (rT3)
  • Calcitonine
    
    • Bone metabolism
• ​Synthesis
  
  • Thyroglobulin (TG)
  • Follicular cells
  • Iodine
  • Active pump
  • Lysosomal peroxidase
  • Atomic iodine
  • Monoiodotyrosine (MIT)
  • Diiodotyrosine (DIT)
  • Epithelial cells
  • Colloid TG
  • Endocytosis
  • Secreted
  • Passive diffusion
  • Follicle
  • Plasma side
  • Basal side
  • Microvilli
  • TSH (Thyroid stimulating hormone)
• Regulation
  
  • Feedback regulation
    
    • Long feedback (Peripheral gland → hypothalamus)
    • Short feedback (Peripheral gland → pituitary gland)
  • TRH synthesis
  • TSH synthesis
  • Exogenous factors
    
    • Photoperiod
    • Feeding
    • Temperature
    • Stress
    • Pharmacons (drugs)
  • Endogenous factors
    
    • Genetic determination
    • Physiological state
    • Hormone receptors
    • Peripheral cells
• Effects
  
  • Metamorphism
  • Thermoregulation
  • Intermediary metabolism
  • Carbohydrate metabolism
  • Lipid metabolism
  • Protein metabolism
  • Neuronal effect
  • Cardiovascular effect
• Hypothyroidism
  
  • Dwarfism
  • Slow sexual development
  • Hair loss
  • Slow reflexes
  • Reproductive abilities
• Hyperthyroidism
  
  • Basal metabolic rate
  • O₂ consumption
  • Weight loss
  • Irritabillity
• Gotier
  
  • Struma

Question 2

Topics to include in the essay

Answer 2

1. The thyroid gland
   
   • Development
   • Role
2. Thyroid hormones
3. Synthesis
4. Regulation
   
   • Feedback
   • Exogenous factors
   • Endogenous factors
5. Effects
6. Conditions
   
   • Hypothyroidsm
   • Hyperthyroidism
   • Gotier

Question 3

Development of the thyroid gland

Answer 3

• The thyroid develops during early embryonic life
  
  • This is because the thyroid hormones cannot get through the placenta
• The thyroid produces metabolites into the bloodstream and iodine containing hormones

Question 4

Give the thyroid hormones

Answer 4

1. Thyroxine (T4)
   
   • The most important hormone
2. Triiodothyronine (T3)
   
   • 2^nd most important hormone
3. Reverse triiodothyronine (rT3)
4. Calcitonin
   
   • Has an effect on bone metabolism

• The thyroid hormones are lipid soluble

Question 5

Synthesis

Answer 5

• Thyroglobulin (TG) will be degraded by the follicular cells to hormones (T4, T3 and rT3), and are now ready for entering the circulation. Every step but the release is facilitated by the hypophyseal TSH hormone

1. Iodine enters the gland by the active pumps
2. By the help of lysosomal peroxidase enzyme the iodine will be converted into atomic iodine
3. The atomic iodine makes either monoiodotyrosine (MIT) consisting of 1 iodine or diiodotyrosine (DIT) consisting of 2 iodine
   
   • The MIT or DIT will then bind to the TG (thyroglobulin)
   • 2DIT will make one T4, while 1DIT + 1MIT will make either one T3 or rT3.
4. The epithelial cells synthesize the colloid TG, for the purpose of storing the hormones
5. Endocytosis of TG will be the first step of hormone release
6. Only the T4 and T3 get into the bloodstream, while the other proteins will be degraded intracellularly
7. T4 and T3 will be secreted at the basal side of the cells by the help of passive diffusion

Question 6

Regulation

Answer 6

• Controlled by feedback regulation:
  
  • Long feedback
    
    • Under the effect of the TRH synthesis
    • Peripheral gland → hypothalamus
  • Short feedback
    
    • Under the effect of the TSH synthesis
    • Peripheral gland → pituitary gland
• Both feedback types are inhibited by T3
• T3 is mostly derived from T4 (T4 becomes T3 in the tissue)
• T4 is regulated both by exogenous and endogenous factors
• Exogenous factors
  
  • Photoperiod
  • Feeding
  • Temperature
  • Stress
  • Pharmacons (drugs)
• Endogenous factors
  
  • Genetic determination
  • Physiological state
  • Changes of hormone receptors and the activation ability of the peripheral cells

Question 7

Effect of thyroid hormones

Answer 7

• The hormones have an effect on the metamorphosis (normal growth and development), thermogenesis and regulation of intermediary metabolism
• The hormones will:
  
  • Facilitate the carbohydrate metabolism
    
    • Facilitation of glucose absorption and insulin-dependent glucose intake
  • Increase lipid metabolism
    
    • Lipolysis
    • LDL receptors and cholesterol intake increases
  • Stimulate the protein metabolism
    
    • Anabolism and catabolism increases
• The hormones will have a neuronal and a cardiovascular effect
  
  • Bea receptor synergism
• Hypothyroidism:
  
  • ​Deficiency of thyroid hormones
  • Young:
    
    • Dwarfism
    • Slow sexual development
  • Adult:
    
    • Loss of hair/fur
    • Slow reflexes
    • None or low reporductive abilities
• Hyperthyroidism:
  
  • ​Overproduction of thyroid hormones
  • Symptoms:
    
    • Basal metabolic rate ↑
    • O₂ consumption ↑
    • Weight loss
    • Increased irritabillity
• Gotier:
  
  • ​Enlargement of the thyroid gland
  • Also called struma
  • Occurs at the same time sa hyperthyroidism or hypothyroidism