Thyroid Physiology Flashcards
Thyroid means
shield
What are two clinical properties that distinguish the thyroid gland from the other endocrine glands?
- Can be seen and palpated during routine physical.
- Has a trace element, iodine
Microscopic stucture of thyroid gland
Follicular cells surrounding Colloid in circular patterns. In between layers of follicular cells you have parafollicular cells.
What does Colloid contain
Thyroglobulin
What do parafollicular cells generate
calcitonin
The six steps of thyroid hormone synthesis
1) Iodide uptake by Na/I symporter
2) Thyroblobulin synthesis and release into the follicular lumen
3) Iodination of tyrosine residues in thyroglobulin
4) Endocytosis of iodinated thyroglobulin into follicular cells
5) Generatin and secretion of T3 and T4
6) Iodide recycling
Iodide is present in most of our food
true
Iodide travels from the gut into the bloodstream, through the bloodstream to the thyroid gland and comes into the follicular cells of the thyroid gland through what?
Na-Iodide symporter on the basolateral side of the cell (the side facing the blood vessel)
What can inhibit the uptake of iodide?
CIO4, TcO, SCN….I dont know what any of that is
What carries iodide from the apical side of the follicular cell into the colloid lumen?
Pendrin
THyroglobulin is made where?
In the Colloid
What is the surface of the colloid layer coated in>
Thyroid Peroxidase
What is the purpose of thyroid peroxidase
oxidizes iodide to iodine
Iodine then does what to thyroglobulin
reacts with the tyrosine residues to “iodinate them”
Internal rearrangement of two iodinate tyrosyl residues results in
Iosothyronine. This is catalyzed by thyroid peroxidase
As long as T3 and T4 are attached to the thyroglobulin backbone,
they are inactive
At the appropriate time, a droplet of colloid is endocytosed into the follicular cell
ok
This endocytosed colloid fuses with what
a native lysosome forming a lysoendosome
What happens inside the lysoendosome
Enzymes inside the lyso-endosome hydrolyze the tyrosine residues and you get active T3 and T4….THIS IS BIG
T3 and T4 get released into the blood
Iodide is recycled?
Yes
90% of what is released into the bloodstream is
T4, 10% T3….KNOW THIS
Once in the bloodstream, the thyroid hormones are immediately bound by
Plasma proteins like albumin or thyroid binding globulin (most attached to this)
What kinds of things decrease the amount of Thyroid Binding Hormone TBG?
Steroid Use
What kinds of things increase the amount of TBG?
Pregnancy, heroin use
Which is more active, T3 or T4?
T3
What must you have in the inner ring of thyroid hormone for it to be considered active?
Two iodines
The lack of iodine in the inner ring of what type of T3 molecule makes it inactive
Reverse T3
What is the enzyme responsible for the conversion of T4 to T3
5/3- Deiodinase
What are the two types of 5/3-Deiodinase?
Type 1 and type 2
Type 1 deiodinase is found where?
Liver, Kidney, Thyroid
Type 2 deiodinase is found where?
Pituitary, CNS, Placenta
Which Deiodinase is mainly responsible for the amount of free T3 found in the blood?
Type 1
Thyroid hormone is a peptide
KNOW
All peptide hormones have what type of Receptor? EXCEPT THYROID
ALL have a plasma membrane receptor except thyroid
Thyroid hormone has what type of receptor
nuclear
Thyroid hormone receptor (THR) forms a dimer with Retinoid X Receptor (RXR) and does what
Acts directly on the DNA to regulate transcription
Why does hyperthyroidism (physiologically) increase metabolism
Directly increases oxygen consumption and when oxygen consumption goes up, the amount of heat generated by the cells goes up
Increases in TRH lead to increases in TSH
truth
TRH acts on what cells where
Thyrotrophs in the anterior pituitary…generates the secretion and release of TSH
How are TRH and TSH levels controlled
Feedback mechanism involving T4 and T3. As free T4 and T3 levels rise, TRH goes down via direct and indirect mechanisms
WHat is the indirect method of TRH control
High T4 and T3 levels cause the downregulation of TRH receptors on Thyrotrophs
What is the direct method of regulation
Low amounts of TSH released from Thyrotrophs
Know the slide about TSH regulation of thyroid function
ok
What is Cretinism
Dwarfism caused by endemic thyroid deficiency during development.
- Mental retardation
- short stature
- delayed motor development
Treatment within a few days of birth restores normal development
Treatment after this time restores physical development bu tnot mental development
Hashimoto thyroiditis
Anti-thyroid antibodies that block and destroy thyroid function
Presentation: Painless goiter, edema, headache
Graves Disease
Hyperthyroidism. Caused by Ig Antibodies that mimic Thyroid Stimulating hormone.
Antibodies continually bind to TSH receptors on the thyroid and continualy drive thyroid hormone production.
Wt loss, sweating, diarrhea, fatigue
Eyeball protrusion, dry epithelial layers, ulceration of the eye
