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Flashcards in Thyroid I Deck (17):

What are the C cells?

cells in the thyroid that produce calcitonin


How is iodine transported into the thyroid cell? What factors stimulate iodine transport into the thyroid?

NaI transporter or Na I symporter
located on the basolateral surface of the cell
stimulated by TSH and intracellular cAMP


What drugs act on the NaI symporter?

perchlorate and thiocyanate


How is iodine transported out of the cells? To where is it transported?

transported to the follicular lumen by Pendrin
this is on the apical membrane of follicular cells
also found in inner ear and kdiney
positively regulated by luminal thyroglobulin


What problems are caused by pendrin mutations?

deafness and goiter


How is thyroid hormone synthesized?

certain tyrosine of thyroglobulin are iodinated by TPO (thyroid peroxidase) and H2O2. This results in formation of MIT and DIT.
T3: MIT + DIT. coupled with H2O2 and TPO
T4: 2 DIT


What does TSH do to the thyroid

stimuates thyroglobulin synthesis, NIS and TPO


What is the Wolf-Chaikoff Effect?

high levels of ECF iodide increase iodide transport and organification up to a certain limit
then, there is a sudden decrease in synthesis: Wolf-Chaikoff effect
inhibition is brief- there is escape after a few days.
may be important, especially with contrast media or amiodarone


What is important to know about relative secretion and metabolism of T3 and T4? Half-lives?

The thyroid mostly secretes T4. If you need T3, you can make it in the periphery by metabolism of T4. T4 can also be metabolised to rT3 in the periphery, which is inactive.
T3 half life: 4-7 min
T 4 half-life: 7 days. Takes at least a month to reach a steady state


D1 deiodinase: location, role

primary peripheral deiodinase that converts T4 to T3
rapid kinetics- not easily saturated


D2 deiodinase location, role

primary deiodinase in the brain and pituitary that converts T4 to T3


D3 deiodinase location, role

deiodinase that inactivates T4 by conversion to rT3. helps keel brain T3 levels constant


thyroid hormone binding proteins: important actors and differences btw T4 and T3

An even greater percentage of T4 is bound than T3. both bound by thyroid binding globulin and albumin (and thyroid-binding pre-albumin), though in different percentages


Hypothalamic-pituitary axis and control of thyroid: stimulatory agents, other affects of stimulatory agents. What else is similar to TSH, structurally? What is the significance of that?

TRH released by hypothalamus and stimualtes TSH release. TRH also stimulates prolactin release
TSH is secreted by the pituitary. Has alpha and beta subunits. Alpha subunit is identical to alpha subunit of FSH, LH, and HCG, and there may be some cross-reactivity
TSH receptors on follicular cells stimulate thyroid hormone production


What feedback exists for the hypothalamic pituitary axis?

T3 attenuates the ability of thyrotrophs in the pituitary to respond to TRH
T3 directly blocks TRH secretion by the hypothalamus
T3 decreases transcription of TSH genes and decreases transcription of pro-TRH gene


What are some additional inhibitors of TSH secretion?

dopamine (esp. relavant in pts who are given dopamine as a pressor agent), somatostatin, and glucocorticoids


To review: how is thyroid hormone regulated?

iodine uptake into the thyroid gland
iodination of thyroglobulin by TPO
release of thyroid hormone
de-iodination of thyroid hormone
thyroid hormone binding to protein
hypothalamus/pituitary action