Thyroid Physiology Flashcards
(41 cards)
The lumen of the thyroid gland is filled with ______.
Thyroglobulin
NOTE: Thyroglobulin gene is transcribed and translated within the follicular cell. After post-translational modification and packaging, it is exocytosed into the lumen.
Where are iodide molecules concentrated?
Thyroid gland
Salivary glands
Gastric glands
Lacrimal glands
Mammary glands
Choroid plexus
NOTE: Iodiine uptake by the sodium/iodide symporter (NIS) on the basal membrane of the thyroid gland can raise I- within the cell as much as 250 times that of the plasma.
I- is transported to the lumen by ______.
Pendrin
Circulating thyroglobulin is minimal in healthy individuals. In what instances is thyroglobulin seen in circulation?
Thyroiditis
Nodular goiter
Cancerous thyroid tissue
Mechanism by which Tyrosine is converted to T4 and T4 is converted to T3
NOTE: The catalystic enzyme is TPO
Pieces of colloid are endocytosed back into follicle and fuse with _________.
Lysosomes
Moving toward the basal membrane, thyroglobulin is broken into free amino acids. What components are released?
T4, T3, MIT, and DIT
Where are thyroglobulin and peroxidase synthesized?
Rough ER
Golgi
What are the inhibitors of the Na+-I+ trnasport NIS symporter?
Perchlorate
Thiocynate
What enzyme catalyzes the oxidation of I- to I2?
PTU, Methimazole (used to treat an overactive thyroid)
Which steps of thyroid hormone synthesis are inhibited by PTU and methimazole?
- Oxidation of I- to I2
- Organification of I2 into MIT and DIT
- Coupling of MIT and DIT
Which hormone is most frequently secreted from the thyroid?
T4 (90%)
T3 (10%)
RT3 (less than 1%)
Which thyroid hormone has the greater affinity for nuclear receptors TRa and TRb?
T3
NOTE: TRa transduces thyroid hormone action on the heart
In the extracellular fluid thyroid hormones circulate bound. What hormones do the hormones bind to ?
Thyroxine-binding globulin (TBG)- 70%
Transthyretin (TTR) - 10-15%
- TTR provides thyroid hormones to the CNS.
What immediate affects does TSH have on the thyroid?
- Endocytosis of colloid
- Iodide uptake
- Increase TPO activity
- Stimulates generation of peroxidase
- Stimulates proteolysis of thyroglobulin
Which enzymes induce the formation of T3 and RT3 from T4?
T3- Deiodinases 1 and 2
RT3- Deiodinase 3
NOTE: The brain maintains constant intracellular levels of T3
What intermediate (hrs to days) affect does TSH have on the thyroid?
- Stimulation of protein synthesis and expression f genes including NIS, thyroglobulin, TPO
What long-term (sustained TSH secretion) affect does TSH have on the thyroid?
Hypertrophy and hyperplasia of follicular cells
In what organs is T4 converted to T3?
Liver kidneys
TSH has an alpha and beta subunit, but the _ subunit is what gives us specificity.
Beta
What is the body’s response to a deficient iodine intake?
- The thyroid gland makes more T3 and less T4. The decrease in T4 stimulates TSH secretion
- It increases iodine trapping, maintains T3 secretion and causes formation of a goiter
REMEMBER: T4 is responsible to negative feedback and feedback is mainly on the pituatary and to a lesser extent on the hypothalamus
What effect would excessive intake of iodide have?
Supression of NIS and TPO
*This is known as the Wolff-Chaikoff effect
Excess calories (mainly carbohydrates) increase production and amount of circulating ______.
T3
NOTE: Fasting has the opposite effect.
What are the actions of T3
-
Growth
- Growth formation
- Bone formation
-
CNS
- Maturation of CNS
-
Basal metabolic rate
- Increases O2 consumption
- Increased heat production
- Increased BMR
- Increased Na-K ATPase
-
Cardiovascular
- Increased cardiac output
- Increased heart rate
- Increased sweating
- Increased ventilation
-
Metabolism
- Increased glucose absorption
- Increased glycogenesis
- Increased glycolysis
- Increased glucogenesis
- Increased Lipolysis; lipogenesis
- Increased protein synthesis; degradation
