Thyroid Hormones Flashcards
What are the two major thyroid hormones? What is a third thyroid hormone?
(1) T4- Thyroxine
(2) T3- 3.5.3 Triiodothyronine
(3) Calcitonin
What is calorigenesis?
Increase in heat production, basal metabolic rate (BMR)
What does T3 promote?
(1) Utilization of energy substrates
(2) O2 consumption
Where is T3 and T4 made?
Thyroid follicle; colloid core
What type of input stimulates thyroid gland?
Sympathetic nervous input
T/F Under normal conditions, the thyroid gland does expand and contract.
TRUE; the follicular epithelia is cuboid in resting state but when “stimulated” to make thyroid hormone, will become elongated
Thyroid Hormone Synthesis
Tyrosine +I2= MIT and DIT
T4 synthesis
DIT+DIT
T3,rT3 synthesis
MIT+DIT
Which hormones are calorigenic
T3 and T4, NOT rT3
Where do we get our sources of iodide? What is needed for “normal” function?
Bread, milk, iodized salt; 1mg per week (we get ~3mg per week)
Of the iodide ingested, what percentage goes to which tissues?
30% thyroid; 50-70% kidneys (excretion in urine); remainder to salivary glands, stomach
How much of the iodide that we ingest goes to the thyroid?
around 1/3
How does the thyroid gland actively accumulate iodide?
via a Sodium/Iodide co-transporter (NIS)
- the NIS allows iodide to be trapped within the thyroid gland
What is “iodide trapping” and when does it occur?
Sodium will go back out of the cell while iodide will stay in the cell regardless of the gradient; It occurs against concentration and electrochemical gradients
Basal levels of hormone synthesis is maintained by what? which NT?
Sympathetic Nervous System; Norepinephrine goes into the gland
What are the effects of TSH on Thyroid Hormone Synthesis?
(1) Increases iodide uptake (NIS)
(2) Activates Thyroid Peroxidase
(3) Increases Thyroglobulin Synthesis
(4) Increases Secretion of T3/T4, NOT storage
What are the steps of Thyroid hormone synthesis and storage?
- Thyroglobulin is made from RER and stored in colloid-
(1) uptake of iodide from NIS
(2) Iodide reaction with TPO to convert to iodine
(3) Iodine and TPO stimulate iodination of thyroglobulin/tyrosine molecule
(4) TPO stimulates coupling into T3/T4 that is stored
What are the steps to the secretion of Thyroid hormones?
(1) microvilli engulf portion of colloid (endocytosis)
(2) Meets with lysosome that engulfs it
(3) the TG molecules broken down by proteolytic enxymes to produce free T3/T4
(4) The T3 and T4 are released into the interstitial fluid that goes into capillaries
(5) MIT and DIT leftover are metabolized by intrathyroidal deiodinase (Iodide is recycled)
Relative levels of T3, T4 and rT3 in a normal, complete lack, and deficient body
(NORMAL) T4 90%, T3 10%, rT3 <0.1%
(LACK) No T4 or T3 production/secretion
(DEFICIENT) Total T4 and T3 production/secretion decreases; the proportion of T3 synthesized/secreted increases (e.g. 50% of each)
What is TSH? Does T3/T4 regulate TSH and TRH secretion?
Thyroid Stimulating Hormone; YES
T/F Thyroid hormones are usually not bound to proteins.
FALSE
What are the thyroid binding proteins? (4)
(1) 70% thyroxine binding globulin (TBO)
(2) 10-15% transthyretin (TTR)
(3) 15-20% bound to albumin
(4) 3% lipoproteins
What is the major function of the binding protein?
To take the T3/T4 around to the target cell
