The thyroid gland: control of secretion, effects of the thyroid hormones Flashcards
Where does thyroid tissue arise from?
Midline at a point on the tongue - foramen caecum
In what week do epithelial cells sink downwards anterir to the hyoid and larynx?
Week 7
What structure connects the developing thyroid to the tongue?
Thyroglossal duct - atophies
What are remnants of the thyroglossal duct called?
Thyroglossal cysts
What are the epithial cells which surround the blood vessels in the thyroid called?
Follicular cells
What is the sac-like structure lies in the middle of the follicular cells called?
Colloid - contains many specialised proteins
What cell types lie between the follicles?
Parafollicular C cells
What do para-follicular C-cells release?
Calcitonin
What are the follicular epithelial cells involved in?
Thyroid hormone synthesis
Where do the small-bodied neurons synthesize and secrete TRH?
- Arcuate nucleus
- Median eminence
Where are the small-bodied axonal terminals located?
High up in the pituitary stalk almost in the median emminance itself
Where is TRH released?
Capillary network at pituitary stalk (high up) - taken by long portal veins to anterior pituitary where it diffuses on thyrotrophs
Explain the process of TRH on the thyrotrophs and how this results in TSH secretion? (lon flashcard)
- TRH binds to its receptor on cell surface of thyrotrophs
- TRH receptor is G-protein coupled
- G protein undergoes confirmational change which induces GDP GTP exchange on the alpha subunit.
- Alpha sub unit GTP complex falls away from the Beta and gamma sub units of the G protein and the alpha GTP complex activates phospholipase C (PLC)
- PLC is a membrane assiocated enzyme, hydrolises a phospholipid in the inner leaflet of the membrane
- Phospholipid is PIP2 - cleaves and yeilds IP3 leaving DAG in the membrane
- Both IP3 and DAG act as secondary messengers inside cells
- Ip3 binds to receptor on endoplasmic reticulum which is a calcium channel causes release of calcium from endoplasmic reticulum into cytosol
- Calcium levels increase above micromolar levels inside cell and activates proteins to exocytose TSH
- DAG and increased Calcium activates PKC whhich is brought to the membrane by DAG and Calcium
- PKC phsophorylates a number of proteins that are involved in exocytosis stimulating there activity
What can somatostatin inhibit the release of?
- GH (mainly)
- TSH
What can dopamine inhibit?
- Prolactin
- TSH release
Where does the TSH receptor lie on the follicular cell?
Basolateral surface (not facing the colloid)
Explain the process by which TSH stimulates the follicular cells to produce T4 and T3?
- Binds to TSH receptor (G-protein coupled receptor) - interacts with stimulatory G protein (GalphaS)
- Interacts with Adenylate cyclase which converts ATP into cAMP
- cAMP acts through acting on protein kinase A also cAMP diffuses into nucleus and activates proteins
- Slow response by stimulating transcription also
- Caelease of T3 and T4
Are T3 and T4 hydrophobic or philic?
Hydrophobic - able to pass through the lipid bilayer - (may have to be faciliated across membrane)
What are T3 and T4 able to inhibit?
Pituitary and hypothalamus (further release of TSH and TRH)
- Also decrease the number of receptors of TRH on thyrotrophs - fur
What are T3 and T4 based on the structure of?
Tyrosine
How much more active is T3 than T4?
5 - 10 x
What are T3 and T4 derived from what stored molecule?
Thyroglobulinm (found in colloid) (exists as a homodimer) (around 330 kilodaltons in size) (~ 120 tyrosines on the amino acid backbone)
What is reverse T3?
Inactive T3 - where the inner iodine has been removed
What is thyroglobulin broken down into?
Individual amino acids and T4 (and T3)
