Lecture 67 Flashcards
_______ two hormones does the thyroid gland produce?
T3 and T4
Overall function of T3 and T4: Increase rate of basal oxygen use, _______, and rate of heat production
Basal metabolism
Overall function of T3 and T4: Modulate delivery of substrates and oxygen by
cardiovascular and respiratory systems to sustain metabolic rate
_______ does thyroid hormone levels change based on?
Energy need, calorie supply and environmental temperature
_______ do you need for hormone thyroid synthesis?
2 tyrosines and iodine
_______ is the major thyroid hormone produced?
3, 5, 3’, 5’ Tetraiodothyronine (thyroxine, T4)
Thyroxine functions as a circulating _______
prohormone
_______ is the thyroid hormone that is secreted in lesser quanitity
3,5,3’- triiodothyronine (T3)
_______ is the biologically active hormone?
T3
_______ is the T4 hormone?
3, 5, 3’, 5’ - Tetraiodothyronine
_______ is the T3 hormone?
3,5,3’- triiodothyronine
T3 provides all thyroid hormone activity in
target cells
_______ converts T4 to T3
5’-deiodinase enzyme
alternative product of T4 (inactive form of T3)
Reverse T3
when is reverse T3 produced?
when less thyroid hormone action needed
Thyroid gland structure: single layered circular follicles comprised of _______ surrounded by a basement membrane
Cuboidal endocrine cells
Thyroid gland structure: Lumen of follicle contains _______?
Colloid material
Colloid material is newly synthesized, stored hormones which are attached to _______?
thyroglobulin
under stimulation, colloidal thyroid hormones are absorbed into?
follicular cells
Parafollicular cells (C cells) secrete _______?
Calcitonin
how is iodine supplied for thyroid synthesis?
by the diet
Synthesis of hormones is partially
intracellular and extracellular
Where are hormones stored?
Extracellularly in follicular lumen until stimulated to secrete
T4 is _______ but T3 is biologically active
hormone secreted
Step 1 in thyroid synthesis: Need to synthesize protein molecule called?
Thyroglobulin
_______ is needed to synthesize thyroglobulin
tyrosine
where is thyroglobulin synthesized?
on rough ER and golgi of follicular cells and put into follicular lumen
Step 2 in synthesis of thyroid hormones: Iodide transported from blood into follicular epithelial cells via?
2 Na/ 1 I symporter
Iodide transported from blood into follicular epithelial cells via 2 Na/ 1 I symporter and the activity is regulated by?
iodine levels in diet ( Decrease in levels stimulate)
step 3 of thyroid synthesis: Iodide (I) transported across apical membrane into colloid follicle via _______ and oxidized to iodine
Pendrin
step 3 of thyroid synthesis: Iodide (I) transported across apical membrane into colloid follicle via pendrin and oxidized to iodine via?
thyroid peroxidase
step 4 of thyroid synthesis: Iodide incorporated into tyrosines at specific sites in
thyroglobulin via thyroid peroxidase
step 5 of thyroid synthesis: coupling reaction results in?
MIT (monoiodotyrosine) and DIT (diiodotyrosine)
step 5 of thyroid synthesis: MIT and DIT remain attached to thyroglobulin until?
stimulus for secretion
Two DIT molecules (T3/T4)
T4
is the synthesis of T3 or T4 faster?
T4 so most is T4
One DIT and one MIT (T3/T4)
T3 (slower)
ration of T4: T3 within gland is
10:1
Transient reduction in thyroid hormone levels caused by ingestion of a large amount of iodine- last about 10 days
Wolff- Chaikoff effect
Wolff- Chaikoff effect causes a large/small amount of iodine
LARGE
Wolff- Chaikoff effect: Excessively high levels of dietary iodide will suppress activity of ?
2 Na/ 1 I pump
_______ can you give short term to hyperthyroid cats?
Iodine short term
_______ inhibits formation of hormones and release
autoregulatory
wolff-chaikoff effect: Thryroid begins to resume normal function, iodine inside cells decrease and this stimulates synthesis to resume. _______ is this called?
Escape phenomenon
amount of hormone produced and release rises initially with
T iodide
amount of hormone produced and release rises initially with T iodide, but then intracellular iodide reaches a critical level which causes
hormone synthesis to stop
Thyroglobulin stored in _______ until stimulation for release of thyroid hormones
follicles as colloid
_______ is the stimulus for release of thyroid hormones?
TSH
TSH stimulates retrieval of thyroglobulin from follicle lumen into endocrine cells via?
Endocytosis
How does thyroglobulin get retrieved from follicle lumen by endocytosis?
cell membrane forms pseudopods and engulf pocket of colloid
_______ fuse with colloid droplet to release T3, T4, free MIT and free thyroglobulin
Lysosomes
T3 and T4 enter _______ after lysosomes fuse
blood
_______ happens to MIT and DIT when lysosomes fuse the colloid droplets?
rapidly deiodinated by deiodinase
why is MIT and DIT rapidly deiodinated by deiodinase?
to conserve iodide for recycling
T4 has a long circulating pool size but _______ about its half life?
LONG - 6 days
T3 has a small pool but _______ about the half life?
short - 1 day
do large or small breed dogs have more circulating T3 and t4?
Small breed
Circulating T3 and T4 increase or decreases with age?
Decreases
T3 and T4 are bound to _______ in circulation
protein
One of these can bind one T4 or one T3
Thyroxine- binding globulin (TBG)
Thyroxine- binding globulin (TBG) accounts for _______ of circulating T4 and T3 bound to it
70 percent
_______ has a lower affinity for T4 and readily gives up T4 to tissues?
Transthyretin
T3 and T4 can also bind to _______ to circulate
Albumin
Proteins help to buffer against _______ in thyroid gland function
acute changes
Increase or decrease in thyroid hormone output can be compensated for
binding or releasing from proteins
Binding proteins can vary in?
Species
A decrease in liver protein synthesis (hepatic failure) causes a decrease in?
TBG increasing circulating thyroid hormone and decrease hormone synthesis
Circulating levels of T3 and T4 abnormal but thyroid gland appears okay. _______ is this?
Sick euthyroid syndrome
T/F: Thyroid is functioning normally in sick euthyroid syndrome
T
Sick euthyroid syndrome is due to a dysregulation of ?
Thyrotropic functioning feedback control
T3 and T4 is (low/high) and rT3 is (low/high) in sick euthyroid syndrome
low, high
_______ profile is common in severe disease down regulation of 5’ deiodinase
low T3 and t4, high rT3
set point of thyroid _______ is changed in disease, illness and medication (alter binding protein)
homeostasis
_______ can cause sick euthyroid syndrome?
Fasting/ starvation, stress (cortisol), catabolic disease (cancer), hepatic and renal disease
Regulation of thyroid gland: Thyrotropin releasing hormone (TRH) is release from ?
hypothalamus
Regulation of thyroid gland: when TRH is released from hypothalamus this causes release of TSH from?
Anterior pituitary
regulation of thyroid gland: Once TSH is releases this causes release of T4 and t3 from?
Thyroid gland
regulation of thyroid gland: Once T4 and T3 is released the _______ of T4 and T3 to anterior pituitary and hypothalamus
Negative feedback
TRH is a _______ hormone
Tripeptide
TSH is a _______ hormone with 2 subunits
Glycoprotein
TSH has a _______ and _______ subunit
Alpha and beta
TSH a-subunit is
non specific (part of LH and FSH)
TSH B-subunit is
biologically active site
TSH stimulated iodine transport, endocytosis of colloid, release of T3 and t4 from?
Thyroid gland
a sustained exposure of TSH to the thyroid gland can cause
hyperplasia/ hypertrophy to follicular cells
Gland atrophies have _______
Absence of TSH
Stimulate iodine uptake increase thyroid hormone and blood flow
Trophic effects of TSH
