5 Thyroid endocrinology Flashcards
what do you find inside the thyroid follicles?
colloid AKA iodinated thyroglobulin
where do you find the parafollicular cells?
they are the clear cells that are scattered in between the follicles and they secrete calcitonin.
What is reverse T3?
the inactive form of T3
tyrosine + I2 equals what?
tyrosine + (2)I2 equals what?
1) Monoiodotyrosine (MIT)
2) Diiodotyrosine (DIT)
MIT + DIT equals what?
DIT + DIT equals what?
DIT + MIT equals what?
1) MIT + DIT = T3
2) DIT + DIT = T4
3) DIT + MIT = reverse T3
What is tetraiodothryronine?
Tetraiodothyronine=T4=thyroxine
on which side of the membrane is T3 released?
basolateral membrane
How does I2 get into the follicular cell?
Na+/I2 symport (NIS transporter)
how does the colloid get back into the follicular cells to release t3 and t4?
pinocytosis
What is the function of TPO in the follicular cell?
TPO= thyroid peroxidase
TPO converts iodide ions to iodine by oxidizing it.
Also binds together MIT/DIT with another MIT/DIT
Where is thyroglobulin made?
in the RER of the follicular cell.
What happens to the oxidized iodine?
it combines with the tyrosine residue on thyroglobulin to make MIT and DIT (colloid). It occurs on the apical membrane of the follicular cell.
Does T3 or T4 have the higher clearance rate? Half life?
T4 has the lower clearance rate and longer half life.
T3 has the higher clearance rate and shorter half life.
How is T3 and T4 transported in the blood?
1) Thyroxine binding globulin (TBG)
2) transthyretin (TTR delivers to CNS)
3) Free hormone (very little)
What is the major regulator for T3 and T4 release?
TSH
What does TSH do?
increases iodinated thyroglobulin breakdown
increased iodide pump
increased size/number of thyroid cells
increased T3 and T4 formation
where does TSH bind the follicular cells?
basolateral membrane
What happens to TRH expression with exposure to cold?
It goes up
what do high concentrations of blood iodine do to the thyroid gland?
decreases size and blood supply to gland
decrease iodide trapping
decrease formation of colloid
Thyroid hormones generally affect every cell in the body except?
1) brain
2) testis
Once released from the thyroid gland and near the target organ, what happens to the hormone?
it breaks apart from the TBG protein (carrier) and becomes T4. From there, within target tissue cytoplasm, T4 is deiodinated to form T3.
T/F T3 binds to the nuclear receptors with 10x higher affinity and activates or represses gene transcription?
true
The T3 receptor binds to DNA as a heterodimer/homodimer?
heterodimer unlike steroid hormones.
T3 binds to the thyroid hormone receptor and heterodimerizes with what to bind to the thyroid response element?
retinoid X receptor (RXR)
T/F T3 only binds to DNA?
False, it can also bind to ribosomes, mitochondria, plasma membrane to up the non genomic responses.
How does t3 and t4 enter the target cell?
facilitated diffusion
diffusion (standard)
is T3 associated with a HSP?
no
with increases in T3 you also see?
increased: (everything except muscle mass)
a) metabolic rate
b) oxygen consumption
Decreased:
a) muscle mass
T/F brain function increases with increased T3 levels?
True, but only with babies. Generally in adults, T3 does not affect the brain of testis.
T/F With T3 increases, you see increased anabolism and catabolism?
true
T3 and T4 in fetal development is required for what?
everything! especially:
1) promoting linear growth
2) promotes brain growth and function
3) tooth/skin/hair development
what do thyroid hormones do the free fatty acid levels in the blood?
elevates them
what do thyroid hormones do to the cholesterol levels in the blood?
it decreases them because more is being secreted into the bile.
what do you usually see with hypothyroidism?
increased cholesterol
T/F very high levels of thyroid hormones can cause weight loss?
true if high enough because it dramatically raises the BMR
what do thyroid hormones do to the heart?
1) increases heart rate
2) increases contractility
3) increases CO
increased levels of thyroid hormone can cause anxiety, and increased respiratory rates?
true
what do thyroid hormones do to the GI tract?
increases motility and secretions
Can Thyroid hormones cause osteoporosis?
Yes when in high concentrations
If you have constant lethargy and difficultly sleeping, what could this indicate about your thyroid levels?
thyroid hormone levels are high.
note hypothyroidism can cause lethargy as well but not generally sleeping problems
what are some sexual changes that happen with hyper/hypo excretion of thyroid hormone?
high=impotence in male, and oligomenorrhea (infrequent menstruation) in female.
Low= loss of libido in male and female, and excessive or frequent menstruation in females.
what generally causes hyperthyroidism?
thyroid adenoma or graves disease
would TSH levels be high or low in graves disease?
Low because of neg. feedback
when do you see exopthalmos?
only in graves disease and not with the adenoma
what are some common characteristics with hyperthyroidism?
intolerance to heat weight loss psychic disorders inability to sleep exothalmos (graves only)
what do thiouracil drugs do?
suppress T3 and T4 activity
What is thyroid storm?
a life threatening condition caused by untreated hyperthyroidism. Brought on by stress and can cause changes in alertness, tachycardia, heart failure, pulmonary edema.
what are some causes of hypothyroidism?
autoimmunity, endemic colloid goiter, non toxic colloid goiter.
what is myxedema?
bagginess under eyes and swelling of face. weight gain lethargy thin hair high cholesterol decreased BMR
what is myxedema coma?
a condition that usually occurs in older people as a result of not taking thyroid medications. Causes them to develop hypothermia and slip into a coma.
what is cretinism?
lack of thyroid hormone in infants causing stunted growth, retardation, and enlargement of soft tissues.
what is the most common type of endocrine neoplasm?
thyroid cancer
T/F Thyroid cancer affects more women than men and is more common in Asians?
true
what often happens with a MEN2 gene mutation?
multiple endocrine neoplasms including the thyroid.
how do you treat thyroid cancer?
1) radioactive iodine
2) thyroidectomy
