Thyroid Flashcards
Where does the thyroid sit?
Lies in front of the trachea and larynx
Below thyroid cartilage and cricoid cartilage
What joins the two lobes?
Isthmus
What is the histology of thyroid tissue?
Follicular cells arranged in spheres called thyroid follicles
Follicles filled with colloid (extracellular)
Parafollicular cells around follicles
What is colloid?
A deposit of thyroglobulin
What do follicular cells produce?
Thyroid hormone
What do parafollicular cells produce?
Calcitonin
What are the thyroid hormones?
T3 - triiodothyronine
T4 - tetraiodothyronine (thyroxine)
What is the structure of thyroid hormones?
2 tyrosines linked together with iodine at 3 or 4 positions on the aromatic rings
What is the function of thyroglobulin?
Acts as a scaffold for formation of thyroid hormones
What is thyroid peroxidase?
Membrane bound enzyme that regulates reactions involving iodine
Which reactions does thyroid peroxidase regulate?
- Oxidation of iodide to iodine (requires h2o2)
- Addition of iodine tyrosine acceptor residues on thyroglobulin
- Coupling of MIT or DIT to generate thyroid hormones
How is dietary iodine absorbed?
Reduced to iodide before absorption in small intestine
Iodide taken up from blood by thyroid epithelial cells
- sodium-iodide symporter
How is T4 converted to T3?
Thyroid secretes T4
Converted to T3 in liver and kindeys
How are T4/T3 transported in blood?
Thyroxine-binding globulin
What is TSH?
Thyroid stimulating hormone
Glycoprotein hormone
What is the effect of TSH?
Trigger for thryoid hormone release
Stimulates
- iodide uptake
- iodide oxidation
- thyroglobulin synthesis
- thyroglobulin iodination
What are the general actions of thyroid hormone?
Increase in basal metabolic rate
- increase number + size of mitochondria
- stimulate synthesis of enzymes in respiratory chain
Stimulation of metabolic pathways
- lipid metabolism - stimulate lipolysis + β-oxidation of fatty acids
- carb metabolism - increase gluconeogenesis + glycogenolysis
Increase cell response to catecholamines
What are the cardiovascular actions of thyroid hormone?
Increases heart’s responsiveness to catecholamines
- increase cardiac output
- increase heart rate
- increase force of contraction
- increase peripheral vasodilation
What are the nervous system actions of thyroid hormone?
Increase myelination of nerves
Increase development of neurons
What is goitre?
Enlargement of thyroid gland
What causes hypothyroidism?
Failure of thyroid TSH/TRH deficiency Inadequate dietary iodine Autoimmunity Post-surgery
What are the hormone levels in hypothyroidism?
Low T3
Low T4
High TSH
What are the general symptoms of hypothyroidism?
Obesity Lethargy Intolerance to cold Bradycardia Constipation
What happens in infants with hypothyroidism?
Cretinism
- dwarfed stature
- mental deficiency
- poor bone development
- GI disturbances
