Flashcards in Thyroid Physiology Deck (46):
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
What do parafollicular cells generate
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
Iodide is present in most of our food
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?
THyroglobulin is made where?
In the Colloid
What is the surface of the colloid layer coated in>
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
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?
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?
What kinds of things increase the amount of TBG?
Pregnancy, heroin use
Which is more active, T3 or T4?
What must you have in the inner ring of thyroid hormone for it to be considered active?
The lack of iodine in the inner ring of what type of T3 molecule makes it inactive
What is the enzyme responsible for the conversion of T4 to T3
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?
Thyroid hormone is a peptide
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
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
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
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
Anti-thyroid antibodies that block and destroy thyroid function
Presentation: Painless goiter, edema, headache