Lecture 7 - Thyroid Phys (Also a Pharm lecture) Flashcards
How much iodine do you need from your diet per day?
150 ug/d
this is needed for T4 production
The production of T3 and T4 is dependent on…
Iodine
Selenium
What are factors that can inhibit deiodination?
drugs selenium deficiency burns trauma disease diet
What is the effect of starvation on T3 levels?
goes down
Most circulating TH is bound to….
plasma proteins
>99% bound
mainly thyroxin binding globulin
- 03% free T4
- 3% free T3
Thyroxine
T4
What is the mechanism by which iodine is accumulated into the follicle cell?
NIS
sodium - iodine symporter
What is the most abundant form of thyroid hormone released by the thyroid?
T4
What influences the production of T4?
TRH levels released from the hypothalamus
What factors alter the production of TRH released from the hypothalamus?
cold increases it
acute psychosis increases it
circadian and pulsatile rhythms increase it
severe stress decreases it
TRH
Thyroid releasing hormone
released from the hypothalamus to activate the pituitary gland
TSH
thyroid stimulating hormone released by the pituitary in response to TRH from hypothalamus
If you remove the pituitary gland, what will happen to the thyroid?
hypothyroidism because you don’t have TSH stimulating the thyroid
What happens if you administer synthetic TSH post removal of pituitary?
you will have thyroid function restored
What might happen in the long term if you administer TSH?
goiter
Wolff - Chaikoff Effect
thyroid gland autoregulation
when there is really high iodine levels it actually suppress the production of thyroglobulin
What does thyroid escape mean in regards to autoregulation?
If a pt is exposed to high levels of iodine for a long period of time (roughly 10 days) they will eventually down regulate the NIS so that they can return to normal levels of synthesis (escape)
What are mechanisms by which thyroid activity is regulated?
hypothalamus
pituitary
autoregulation
What happens when the thyroid hormone reaches its target organ?
it binds to the thyroid hormone receptor (TR) and disassociates from protein used to transport and protect hormone in plasma. Then the hormone gets into the cell where it goes into the nucleus.
T 4 is converted to T3 in the nucleus by D1
What has the highest affinity to the thyroid hormone receptor?
T3
Where is T3 and T4 degraded and where is it excreted?
degraded in liver
excreted in bile
What are the sxs of hyperthyroidism?
nervousness weight loss hyperphagia heat intolerance warm, soft skin high BMR
Subclinical hypothyroidism
normal free T4 and T3 and increase TSH
asymptomatic
Levothyroxine ONLY if they have hyperlipidemia
