Thyroid gland and iodothyronines Flashcards
Where is the thyroid?
top of trachea, bi-lobed
What is the isthmus?
tissue joining 2 lobes
What is the pyramid?
tissue on top of isthmus present in some people that could contain a tumour
What do follicles contain?
thyroid full of follicles
follicular cells line the follicle
colloid is the proteinaceous substance found inside the follicles
parafollicular cells are found between follicles and they also provide hormones
How do follicular cells produce iodothyronines generally?
thyrotrophin receptors on serosal/basal membrane
iodide ions pumped into cell from blood via IODIDE PUMPS
thyroid hormone produced on apical membrane
iodide pumped out into colloid by PENDRIN PUMPS
Steps of iodothyronine production?
- TSH binds to receptor to stimulate iodide pump to pump iodide into cell and pump out via pendrin pumps
- TSH in nucleus causes synthesis of THYROGLOBULIN (moved into colloid)
- TSH stimulates THYROID PEROXIDASE ENXYME (TPO) to convert iodide to reactive iodine (short lived) in presence of H2O2
- Reactive iodine iodinate thyroglobulin tyrosyl residues in 1 or 2 positions to form MIT/DIT
- TSH keeps stimulating TPO which allows coupling reactions to forms T3/T4
- iodothyronines stored in colloid
- TSH stimulates lysosomes to move to apical membrane so colloid uptaken
- colloid fuses with lysosome, protein broken down so T3 and T4 liberated and can move into blood
What is thyroglobulin?
long array of AAs with certain number of TYROSYL RESIDUES which can be iodinated by reactive iodine in 1 or 2 positions
Describe the structure of iodothyronines?
tyrosine converted to: (iodination at position 3/5) MIT DIT T3 = MIT + DIT T4 (thyroxine) = DIT + DIT
Describe the transport of iodothyronines?
by plasma proteins to prevent uptake by non-target cells
- globulins are specific to particular molecule
- thyroxine binding globulin (TBG) specific to T3/4 75% carried in this way
- albumin loosely binds T3/4 (not specific)
- prealbumin (transthyretin) binds some T4 little T3
bioactive form are free molecules not bound to plasma proteins
What is the latent period of T3 and T4?
T3 - 12hrs - MORE RAPID ACTING
T4 - 72hrs
What are the biological half lives of T3 and T4?
T3 - 2 days
T4 - 7-9 days
How is thyroxine deiodinated?
T4 main thyroid product but T3 most bioactive
T4 deiodinated in target tissues to T3
or in different position to form reverse T3 (rT3) that is biologically inactive - in situation where goal is reduced metabolism (e.g. starvation)
What are the main effects of iodothyronines?
increase BMR
- except in brain
- good for calorigenesis (2o heat production)/body temp regulation
increase protein/carb/fat metabolism
- anabolic and catabolic
- needed for normal growth and development
enhance effect of catecholamines (tachycardia, glycogenolysis, lipolysis)
interact with other endocrine systems (oestrogens)
affect CNS for brain development
increase Vit C synthesis from retinal (hypothyroidism leads to build up of retinal in blood leading to yellowish skin colouration)
How are iodothyronines involved in development?
important for foetal growth/development
lack can cause CRETINISM if untreated in first few months
thyroid hormone levels measured with heel prick
What are the mechanisms of action of iodothyronines?
GENOMIC ACTION
- Enter target cell - react readily with intracellular receptors
- Receptor-hormone-complex affects protein synthesis in nucleus/membrane transport in brain
- stimulates metabolic activity (affect mitochondria)
