Thyroid Flashcards
(25 cards)
What are the active forms of TH (thyroid hormone)?
T3 and T4
Where is TH stored?
In follicles of the thyroid gland
TH stored in follicles are in colloid with
thyroglobulin
Where does iodide used for TH come from?
eating!
What AA is necessary for making TH?
tyrosine
The steps of synth and storage of TH is?
1) In the endoplasmic reticulum, thyroglobulin (TG) molecules are produced, packaged in vesicles by the Golgi, and exocytosed into the lumen of the follicle. 2) Iodide (I-, from the diet) enters the thyrocyte via basolateral Na+/I- cotransporters (aka the I-trap). The iodide exits the cell on the apical side into the lumen via I-/Cl- antiporters. 3) In the follicular lumen, I- is oxidized to iodine by thyroid peroxidase and substituted for H+ on the benzene ring of tyrosine residues of thyroglobulin. 4) Binding of one iodine will form monoiodotyrosine (MIT), and binding of two iodine moieties will form diiodotyrosine (DIT). This reaction is termed organification. Thyroid peroxidase also catalyzes the coupling of DIT to another DIT, forming T4. Some DIT will also couple to an MIT, forming T3. These products remain attached to TG. 5) The mature TG, containing MIT, DIT, T4, and T3 (in order of greater to lesser abundance), is endocytosed back into the follicle cell and can be stored as colloid until secreted. 6) Colloid proteolysis is stimulated by TSH and constituent molecules released. MIT and DIT reenter the synthetic pool; T3 and T4 exit the basolateral membrane into the blood.
Addition of one I- to tyrosine makes
MIT
Addition of one I- to MIT makes
DIT
What is organification wrt TH?
Addition of I- to tyrosine and MIT
What couples two DITs together? What is the product?
Thyroid peroxidase, this forms T4
What is it called with a DIT and MIT combine
T3
The main TH secreted into blood is
T4
Is circulating TH free or bound? If bound, to what?
Largely bound, to TBG, transthyretin, and albumin
How does peripheral T4 become T3 or rT3?
Via the action of peripheral deiodinases
What is the effect of TH on the lungs?
Increased ventilation
What is the effect of TH on the heart?
Increased CO
What is the effect of TH on the kidneys?
Increased renal function and urea output
Congenital TH deficiency can cause
both growth and mental retardation
How do you treat congenital TH deficiency
T4
What is the effect of TH on metabolism?
Increased glu absorption, increased glycogenolysis, gluconeogenesis, lipolysis, protein synth AND deg (net is protein breakdown)
Match: wt loss/wt gain with hypothryoid/hyperthryoid. Why?
Wt loss in HYPERthyroidism because TH increases BMR. Wt gain in HYPOthyroidism because low TH would lower BMR
Outline TH feedback
hypothal–>TRH–>ant pit–>TSH–>thyroid–T3,T4. T3 and T4 neg feedback on ant pit.
What blood test do you use to eval for hypo/hyperthyroidism?
TSH! This is the most sensitive
If TSH is elevated this is primary hypo/hyperthyroidism?
hypothyroid
