Thyroid Gland - CB Flashcards
What are the biologically active forms of thyroid hormone (TH)?
T3 (triiodothyronine) and T4 (Thyoxine)
Fact: the thyroid is highly vascularized
It also has a large store of TH
What stimulates the synthesis and release of TH?
TSH (Thyroid stimulating hormone) from ant. pituitary
What cells synthesize and release TH?
follicular epithelial cells
What cells synthesize/release calcitonin?
parafollicular cells
What is the amino acid precursor of TH, T3, T4? What else is needed for syn?
tyrosine bound to TG (thyroglobulin)
iodide (only hormone that need an essential trace element)
Is reverse T3 bioloically active?
no, just regular T3
Is T4 or T3 preferentially synthesized/released?
T4
Where is iodide stored? how is it obtained?
thyroid gland associated with Thyroglobulin, from Diet, a 2Na+/I cotransporter (symporter)
Fact: TSH is under negative feedback by TH
I should have been an engineer…
Describe the synthesis and storage of T3/T4?
- TG is made in ER, packed into vessicles in the golgi and exocytosed into the lumen of the folice
- Iodide enteres thyrocyte via 2Na+/I cotransporter and exits into the lumen via I/Cl antiporter
- Iodide is oxidized in lumen by thyroid peroxidase and added to a tyrosine on TG
- Organification: the binding of Iodine to tyrosine one TG
- MIT: monoiodotyrosine (1 Iodine)
- DIT: diiodotyrosine (2 Iodines)
Thyroid peroxidase couples MIT and DIT forming T3 and 2 DITs forming T4 - The mature TG complex is endocytosed back into the folliculocyte and stored as colloid (Mature TG has MIT>DIT>T4>T3)
- TSH stimulates Colloid proteolysis relseasing T3 and T4 from TG and it exits into the blood via the basolateral membrane. The MIT and DIT reenter the synthetic pool
What is circulating TH bound to?
thyroid-binding protein mainly (Not TG!)
also transthyretin and albumin
small amount it free (bioavailable) ( it can enter target cell nuclei and bind)
What is T4 metabolized into?
T3 and reverse T3 (rT3)
What from of TH is most biologically active?
T3, T4 is called a prohormone
What is TH broken down into?
Thyronine
What is used to treat hypothyroidism? Why?
T4, longer half life, metabolized into the more active T3
What form of TH can enter the cell? Where are its targets?
Unbound, in the nucleus, the receptor binds T3 then DNA
What tissues have TH receptors?
ALL
What does TH do?
regulates (increases) the metabolism of carbs, proteins and lipids
also acts with Growth Hormone and somatomedins to promote bone growth, ossification, bone maturation
CNS development
How are cytoplasmic T3 and T4 ratio maintained?
5’/3’ monodeiodinase converts T4 to T3 so they are equal
Describe the kinetics of TH? (Onset speed and duration)
Slow onset, long lasting (weeks)
Fact: in hypothyroidism, bone age is less than actual age
excessive treatment of hypothyroidism with T4 can cause osteoporosis (post menopausal women)
TH deficiency in infants can have what effect?
Growth and Mental retardation
What are some system effects of TH?
growth, CNS maturation, Increased BMR, Increased metabolism, increased cardiovascular output
