Lecture 61-Thyroid Hormones 1 Flashcards
What are the two hormones the thyroid produces?
Thyroxine (T4)
Triiodothyronine (T3)
What do thyroid hormone levels change based on?
-energy need
-calorie supply
-environmental temperature
What are the overall functions of T3 and T4?
-increase rate of basal oxygen use, basal metabolism, and rate of head production
-modulate deliver of substrates and oxygen by cardiovascular and respiratory systems to sustain metabolic rate
What do you need for thyroid hormone synthesis?
2 tyrosines
Iodine
What is the MAJOR thyroid hormone produced?
3,5,3’,5’-tetraiodothyronine
Thyroxine , T4
Which thyroid hormone is produced in lesser quantity?
3, 5, 3’ triiodothyronine (T3)
Biologically active hormone
Which hormone is produced when less thyroid hormone action is needed? What is this an alternative product of?
Referees T3
this is an alternative product of T4 (inactive form of T3)
What is the thyroid gland structure?
-Single-layered circular follicles
-lumen of follicles = newly synthesized, stored hormones are attached to thyroglobulin
Under stimulation what happens to colloidal thyroid hormone?
Absorbed into follicular cells
What do parafollicular cells (C cells) secrete?
Calcitonin
____ is major hormone secreted, but _____ is biologically active thyroid hormone.
T4
T3
Where are hormones stored?
Stored extracellularly in follicular lumen until stimulated to secrete
Synthesis of hormones is partially _______ and partially _______.
Intracellulär and extracellular
For thyroid hormone synthesis to occur you need large amounts of?
Iodine
What are the steps in synthesis of thyroid hormones?
- Need to synthesize protein—Thyroglobulin
- Iodine transported from blood into follicular epithelial cells via 2 Na+/1 I- symporter
- Iodine transported across apical membrane into colloid of follicle via pendrin and oxidized to iodine
- Iodine incorporated into tyrosines at specific sites in thyroglobin via thyroid peroxidase
- Coupling reaction—results in MIT (monoiodotryosine) and DIT (diiodotryosine)
- Two DIT molecules = T4
- One DIT + one MIT = T3
What is the ratio of T4:T3 within the thyroid gland?
10:1
Where is thyroglobulin synthesized?
Rough ER and golgi
What regulated the 2 Na/1 I- symporter?
Dietary iodide
**low levels stimulate more activity!
What enzyme oxidizes iodide to iodine?
Thyroid peroxidase
One DIT + one MIT = T3
Faster or slower than …
Two DIT molecules = T4
T4 is the fastest reaction
What are T3 and T4 catalyzed by?
Thyroid paroxidase
(TRUE/FALSE) Not all MIT and DIT make hormones—leftover stays attached to thyroglobulin
True
What will high levels of dietary iodine suppress activity of?
2 Na/1 I- pump
What is the Wolff-Chaikoff effect?
Amount of hormone produced and released increases intimacy with increase iodide in diet -> intracellular iodide reaches a critical level -> hormone synthesis stops
What is the escape phenomenon?
The thyroid begins to resume to normal function when the iodine levels decrease inside cells to down regulation of transporter, and resume synthesis
Where is thyroglobulin stored?
Stored in follicles as colloid until stimulation for release of thyroid hormones
How is thyroglobulin retrieved from the follicle lumen into the endocrine cell?
Endocytosis
Engulf a pocket of colloid, pinched off cell membrane
When lysosomes fuse with colloid droplets, what do they release?
T3, T4 and free MIT and free DIT
When a stimulus is received for the release of thyroid hormones, what happens to MIT and DIT?
Gets released rapidly and deodinated by deiondinase enzyme to recycle iodine
____ circulating pool size of T4, and _____ half-life
____ circulating pool size of T3, and _____ half-life
Large, long
Small, short
What are the (4) circulate bound to proteins?
- Thyroxine-binding globulin (TBG) -> THIS ONE IS IMPORTANT BC 70% OF HORMONE IS BOUND TO THIS
- Transthyretin
- Albumin
- Proteins help to buffer against acute changes in thyroid gland function
What effect does hepatic failure have on circulating thyroid hormone?
Decrease hepatic protein synthesis -> less TGB -> transient increase in circulating hormones -> transient decrease in hormone synthesis
What is sick euthyroid syndrome?
circulating levels of T3 and T4 abnormal, but thyroid glands doesn’t appear abnormal
In sick euthyroid syndrome why is the T3 and T4 low, but rT3 high?
downregulation of 5’ deiodinase
Where is the thyroid-stimulating hormone (TSH) released from?
Anterior pituitary
Where is the thyroptin-releasing hormone (TRH) released from?
Hypothalamus
Where is T3 and T4 released from ?
Thyroid gland
Where does the negative feedback of T4 and T3 go to?
Anterior pituitary and hypothalamus
What are the four steps of thyroid gland regulation?
- TRH is released from hypothalamus
- Causes release of TSH from anterior pituitary gland
- Causes release of T4 and T3 from thyroid gland
- Negative feedback of T4 and T3 to anterior pituitary and hypothalamus
What type of hormone is TRH? What does it stimulate?
TRH is a tripeptide hormone
Stimulate transcription of TSH
What are the two subunits of TSH?
Alpha subunit - non specific part of LH and FSH as well
Beta subunit - biologically active site
What are the trophies effects of TSH?
Increase synthesis and secretion of thyroid hormone
Increase blood flow to glands
What happens if there is an absence of TSH
The gland atrophies
What occurs if there is sustained exposure of thyroid gland to TSH?
Hyperplasia and hypertrophy of follicular cells
Increase growth, increase size, increase of everything !
