Hypothalamic-pituitary-thyoid Axis Flashcards
What is the HPT axis concerned with
Production of thyroid hormone (T3 and T4)
In the HPT axis what does the hypothalamus release
TRH
In the HPT axis, what does the anterior pituitary gland release
TSH
In the HPT axis, what does the thyroid release
T4 and T3
Thyroxine
T4
What happens to T4 in target tissues
Converted to T3
Which is more potent, T3 or T4
T3
Which is easier to make in the thyroid, T3 or T4
T4
That’s why T4 gets released from thyroid and then it gets converted to T3
Feedback of the HPT axis
Both short and long loop
- TSH inhibits TRH
- T3-T4 inhibit TSH and TRH
What are the master metabolic hormones
T3/T4
What are the metabolic functions of T3 and T4
- signals through steroid pathway (but its amine)
- bone growth and maturation
- CNS maturation
- increases BMR and heat production
- increases all body metabolism
- increases cardiac output
A consequence of hypothyroidism would be
Cold intolerance
What does the thyroid contain
Follicular epithelial cells
What do the follicular epithelial cells of the thyroid do
Make thyroid hormone
Where does the basal membrane of the follicular epithelial cells of the thyroid face
Blood
Where do the apical membrane of the cells of the follicular epithelium of the thyroid face
Colloid
Mixture of forming thyroid hormone attached to thyroglobulin
Colloid
Protein that binds and helps form thyroid hormone
Thyroglobulin
What does thyroid hormone synthesis require
Dietary iodine
-iodized salt and other sources
How Is thyroid hormone synthesized
By stepwise addition of iodine to tyrosine
What thyroid hormones have biological functions
T3 and T4 only
How are thyroid hormones synthesized
- iodide pumped into the cell from blood (forms iodine when entering the colloid)
- tyrosine on thyroglobulin are iodinated (iodine is added to tyrosine, makes MIT and DIT)
- iodinated thyroglobulins combine
- thyroglobulin with T3, T4, DIT, and MIT stay in colloid
What are the combinations of iodinated thyroglobulins that make up thyroid hormones
- two DITs make T4
- one DIT and one MIT make T3
- remainder of other MIT and DIT remained attached
What makes MIT and DIT
Tyrosine on thyroglobulins are iodinated
When does iodide turn to iodine?
When entering the colloid
How are T3 and T4 secreted into circulation
- upon stimulation thyroglobulin and the attached thyroid hormones is brought into the cell
- T3 and T4 are removed from thyroglobulin and secreted into circulation
- MIT and DIT on thyroglobulin are deiodinated and recycled to repeat the process
What is the average intake of iodine a day
500ug, only need 150
What happens to most of the iodine
Taken up by the thyroid to make T3 and T4
Degradation of T3 and T4
Liver degrades T3 and T4 and releases iodine back into blood
Where is most iodine secreted
By the kidney
T3/4 in blood
- not easily dissolved in blood
- is carried bound to proteins
- albumin and specific thyroxine binding proteins
What kind of T3 and T4 can be sensed and regulated
Only free T3 and T4
What kind of T3 and T4 can be degraded
Only free T3/T4
Yo are researching a new drug and discover it rapidly increases albumins affinity for T3/T4. What affect would this have on TSH levels?
Increase
Why kind of steroid is T3/T4
Amine, but acts like a steroid
How are T3 and T4 carried
Binding proteins
What do T3 and T4 activate
Intracellular receptors
What does T3 and T4 do once it binds intracellular receptors
Directly initiates transcriptional changes
What converts T4 to T3?
**
Iodinase in tissues
**
Thyroid hormone action
- primarily a metabolic regulator
- causes bone formation and growth
- allows for maturation of CNS
- increases basal metabolic rate
- increases metabolism
- increases cardiac output
Thyroid hormone and bone growth
- causes bone formation and growth
- required for growth to adult stature
- act with growth hormones to regulate growth of bones
Thyroid hormone and maturation of CNS
- allows for maturation of CNS
- required in the prenatal period for full CNS development
- in adults altered T3/T4 levels cause central defects
How does thyroid hormone increase BMR?
- increase Na/K ATPase
- increase O2 consumption (except brain, gonads, and spleen)
- increase heat production
- increase BMR
Thyroid hormone and metabolism
Increases availability and uptake of glucose, proteins, and fats
Thyroid hormone and cardiac output
- higher O2 consumption requires more CO to meet demand
- also activates SNS
Hyperthyroidism
Too much T3/T4
What are some examples of disorders that cause hyperthyroidism
- Graves’ disease
- thyroid tumor
- TSH/TRH secreting tumor
What us Graves’ disease
Autoimmune disease where Ab activate the receptors on the thyroid causing it to release too much T3/T4
What are some symptoms of hyperthyroidism
- weight loss
- heath intolerance
- sweating
- increased cardiac output
- exopthalamos
- goiter
Hypothyroidism
Not enough T3/T4
What are some disorders that can cause hypothyroidism
- Hashimotos
- iodine deficiency
- congenital (cretinism)
What is Hashimotos
Destruction of thyroid gland by immune system
What are some symptoms of hypothyroidism?
- weight gain
- cold intolerance
- low cardiac output
- mental slowness
- lack of energy
- myxedema (swollen, puffy, skin)
Treatment for hypothyroidism
Give them exogenous T3/T4 or iodine
What is the treatment for hyperthyroidism
Remove the thyroid/ tumor
Congenital hypothyroidism
- cretinism
- due to necessity of T3/T4 for normal development prenatal hypothyroidism causes both physical and metal developmental abnormalities
- learning disabilities
- growth is inhibited
Enlarged thyroid gland
Goiter
When can you get a goiter
Can occur in either hyper or hypothyroidism
Why is the thyroid enlarged when you have a goiter
High TSH levels
What kind of hypothyroidism would you not have a goiter
Hashimotos
What kind of hypothyroidism could give you a goiter
Iodine deficiency
What kind of hyperthyroidism could give you a goiter
Graves
Thyroid tumor
TSH/TRH secreting tumor
