12 - Thyroid Gland Physiology Flashcards
The thyroid gland produces the prohormone __________ and the active hormone __________.
Tetraiodothyronine (T4 or Thyroxine)
Triiodothyronine (T3)
The thyroid ________ is the functional unit of the gland. It is surrounded by a single-layer of epithelial cells.
Follicle
The thyroid follicular lumen itself is filled with ________.
Colloid
T/F. The size of the epithelial cells and amount of colloid in the thyroid follicle stays the same.
False. The size of the epithelial cells and the amount of colloid changes with activity.
The thyroid gland also contains _________ cells, which secrete ________.
Parafollicular (C cells)
Calcitonin
T/F. The thyroid gland receives a rich blood supply.
True
The thyroid hormones are synthesized by what?
Follicular epithelial cells
Colloid is composed of newly synthesized thyroid hormones attached to _________.
Thyroglobulin
The thyroid follicle epithelium sits on a…
Basal lamina (outermost part of the follicle)
The thyroid follicle cell secretes 60 ug of new hormone/day. Enough hormone is stored as ________ ________ in the follicular colloid to last the body 2-3 months.
Iodinated Thyroglobulin
***Iodine is stored iodinated as tyrosine of thyroglobulin
When iodinated TG is stored in the follicular colloid, 8,000 ug total is stored and of this 600 ug is _____ and _____.
T4
T3
From the ECF, 120 ug of _________ is trapped in the thyroid gland.
Iodide
From the thyroid gland, 60 ug of ________ is leaked into the ECF.
Iodide
Iodine and Tyrosine react together to form what?
MIT (Monoiodotyrosine) or DIT (Diiodotyrosine)
DIT + DIT forms…
T4 (Thyroxine)
***This is a fast reaction! 10x more T4 is produced than T3!
DIT + MIT forms…
T3 (Triiodothyronine)
Thyroid hormones contain large amounts of iodine, and the major secretory product is _______.
T4
Where does synthesis of the hormones occur?
Part intracellularly and part extracellularly
______ percent of T3 production is produced by peripheral conversion from T4. ______ percent of circulating T3 comes from direct secretion from the thyroid gland. Provides circulating T3 for uptake by other tissues in which T3 supply is too low.
80-90
10-20
T4 is converted to T3 via _________.
Deiodinase
What types of deiodinase create active T3?
Type 1 and 2
***Outer ring deiodination
What types of deiodinase create inactive T3 (reverse T3)?
Type 3
***Inner ring deiodination
There are some clinical states associated with a reduction in the conversion of T4 into T3, which are…
Fasting
Medical and surgical stress
Catabolic diseases
Step 1 of thyroid hormone production – Multiple tyrosines create _________, which exits the follicular epithelial cell into the follicular lumen via the rough ER and golgi.
Thyroglobulin
Step 2 of thyroid hormone production – ATP produced from the Na/K pump allows for the secondary active transport of Iodide as a symporter with 2 Na+. This is called the…
Na+ Iodide Symporter
***This occurs at basolateral membrane
Step 3 of thyroid hormone production – Iodide that was trapped from the symporter is then transferred from the follicular epithelial cell to the follicular lumen. This transport is done by a ________ pump, and _________ converted the Iodide to I2.
Pendrin
Peroxidase
Step 4 of thyroid hormone production – In the follicular lumen, the Thyroglobulin and I2 with peroxidase creates what?
MIT and DIT attached to Thyroglobulin
Step 5 of thyroid hormone production – In the follicular lumen, the MIT/DIT and Thyroglobulin complex along with peroxidase creates what?
T4, T3, MIT, and DIT all attached to Thyroglobulin — This is stored as Colloid
Step 6 of thyroid hormone production – ______ stimulation causes the pinocytosis of Colloid into endosomes within the follicular epithelial cell.
TSH
Step 7 of thyroid hormone production – Proteases lyse the Colloid within the endosomes, which allows the release of ______ and ______ into circulation.
T4
T3
Step 8 of thyroid hormone production – Proteases lyse the Colloid within the endosomes, which allows the release of ______ and ______ within the follicular epithelial cell. These are then reacted with intrathyroidal deiodinase and converted back to Tyrosine and Iodide to start the process over again.
MIT
DIT
When there is a deficiency of deiodinase, what does it mimic clinically?
Dietary Iodide deficiency
When the availability of Iodide is restricted, the formation of (T3/T4) is favored.
T3
This is a chloride/iodide pump that is located in the apical membrane of thyroid follicular cells.
Pendrin
Mutation in the Pendrin gene (SLC26A4, also called PDS) causes defects in the transport across the ________ _______.
Apical membrane
Mutation in the Pendrin gene also affects the ________, where it results in a sensorineural hearing loss.
Cochlea
Patients with this syndrome usually have hypothyroidism with goiter.
Pendred Syndrome
This occurs due to high levels of Iodide inhibiting organification and synthesis of thyroid hormones.
Wolff-Chaikoff Effect
This inhibits Iodide and Peroxidase from their normal interactions from Thyroglobulin to prevent formation of T3 and T4. It is an effective treatment for hyperthyroidism.
PTU (Propylthiouracil)
***Good for Graves’ Disease
These can inhibit the Na+ Iodide Transporter and prevent the trapping of Iodide into follicular epithelial cells.
Perchlorate, Thiocynate
Review the slide on thyroid gland activity by radioactive iodine uptake (Slide 14).
Review 5 minutes
Thyroid hormones circulate in the bloodstream either bound to ______ ______ (99 percent) or free (1 percent). There is an equilibrium between bound and free circulating T3 and T4 in the bloodstream.
Plasma proteins
What are the main binding proteins for thyroid hormones?
Thyroxine-binding globulin (TBG) – 70 percent
Transthyretin (TTR) – 10-15 percent
Albumin – 15-20 percent
This thyroid hormone binding protein is synthesized in the liver and has a higher affinity for T4 than T3.
Thyroxine-binding globulin (TBG)
Most circulating thyroid hormone is ______.
T4
What is the 1/2 life for T4?
6 days
What is the 1/2 life for T3?
1 day
What happens to free T4 in the circulation?
It relocates to the tissues where it is converted to active T3 and inactive T3 (reverse T3).