SPR L1 Thyroid Hormone Flashcards Preview

• Outline the steps in the manufacture of thyroid hormones
• Describe the structural and functional differences between T3 and T4
• Outline the cellular mechanisms of action of T3 and T4
• Explain how the hypothalamus and pituitary regulate circulating levels of thyroid hormones
• Describe how thyroid hormones are transported in the plasma
• Describe the metabolic effects of thyroid hormones
• Describe the systemic effects of thyroid hormones
• Relate the normal effects of thyroid hormones to the abnormalities seen with deficient and excess production

Inferior to the larynx and superior to the trachea

Isthmus joins the two lobes

Brown because of the iodine necessary for thyroid hormone

Lumpy because of the lobes

Comprised of lobules of spherical follicles that are filled with colloid

Lined by cubboidal epithelial cells

C cell / parafollicular cell - these produce calcitonin

Thyroglobulin

2-3 month supply of thyroid hormone within the colloid within follicle within lobule within lobe

T3 and T4

Tyrosine residues incorporated – means that these are AMINE hormones

tyrosine & iodine

(can't make iodine in the body - can get it from shellfish etc)

Salt is ionised to make sure thyroid hormone can be produced

I  +  tyrosine => monoiodotyrosine (MIT)

MIT  +  I => diiodotyrosine (DIT)

DIT  +  DIT => thryoxine(T4)

DIT  +  MIT => triiodothyronine (T3)

1. Iodine is cotransported with Na+ from the capillary
2. Diffusion across the follicle cell
3. Iodine is oxidised and attached to rings of tyrosines in thyroglobulin (in the lumen of the follicle - colloid) Iodising agent HYDROGEN PEROXIDE. Enzyme catalysing oxidation of iodide to iodine = thyroid peroxidase. 
4. The iodinated ring of one MIT or DIT is added to a DIT at another spot (in the lumen of the follicle- colloid)
5. Endocytosis of thyroglobulin contains T3 and T4
6. Lysosomal enzymes release T3 and T4 from TG
7. T3 and T4 moves back out into the interstitial fluid

T3

90%

Liver and Kidney

Almost 100% is bound - 0.02% isn't, but can still carry out a lot of biological effects

Thyroid Binding Globulin TBG

Thyroid Binding Prealbumin TBPA

Thyroid Binding Albumin TBA

1. TBG
2. TBA

Increases the number of mitochondria and their efficiency 

Then also binds to nuclear receptors. Protein synthesis, enzyme activity altered. (increased sodium potassium ATPases). Acts like steroid hormones through Hormone Receptor Complex. Can exert genomic effects and non-genomic effects.

• Increase O2 consumption - increased mitochondria
• Increase BMR
• Calorigenic effect
• Carbohydrate metabolism:
  • increased absorption of glucose from GIT, increased insulin secretion, increased gluconeogenesis
  • glucose oxidation, glycogen degradation (hypersecretion)
• Lipid metabolism:
  • lipolysis - increases circulating FFA levels, accelerates FFA oxidation
  • decrease cholesterol, triglycerides & phospholipids in plasma (hypersecretion)
• Protein metabolism:
  • protein synthesis
  • breakdown (hypersecretion)

Na K ATPase is increased

1. increase HR, CO, SBP and decrease DBP

   Increased blood flow to skin 

2. increase ventilation rate 

3. Increases appetite

   Increases secretion digestive juices

   Increases GIT motility

4. Essential for normal reproduction and lactation

5. promotes normal body growth and maturation of skeleton

   promotes normal function and development of muscles

6. promotes normal neuronal development in feotus and infant (developmental)

   promotes normal neuronal function in adult - increases synaptic activity

   enhances effects of sympathetic nervous system – sympathomimetic: upregulates B1 adrenergic R in heartR

The Hypothalamus and Pituitary

1. Inputs
   1. Stress - less TH in blood to reduce stress on heart
   2. Cold
2. Upon the hypothalamus
3. TRH
4. TSH
5. T3 and T4
6. Increase BMR, protein synthesis, and sympathetic tone
7.  Negative feedback loop