Thyroid Gland Physiology Flashcards
what is the functional unit of the thyroid gland?
the thyroid follicle
what is the thyroid follicle surrounded by?
a single layer of epithelial cells
what is the follicular lumen filled with?
Colloid
what is colloid composed of?
newly synthesized thyroid hormones that are attached to thyroglobulin
tyrosine + I2 (iodine) yields what?
either MIT or DIT
combination of 2 DIT molecules produces what?
T4
Coupling of 1 DIT molecule and 1 MIT molecule produces what?
T3
what molecule is the major secretory molecule of the thyroid gland?
T4
what process ensures that t4 is converted into the active form T3?
peripheral conversion
what is the enzyme that is responsible for converting T4 into its active form of t3?
deiodinase type 1 and 2*
what is the enzyme that is responsible for converting T4 into its inactive form of T3?
deiodinase type 3
there are some clinical states associated with a reduction in the conversion of T4 to T3. What are those states?
fasting, medical & surgical stress, and catabolic diseases
what would fasting, medical & surgical stress, and catabolic disease lead to?
the conversion of T4 into its inactive form using deiodinase type 3
what is the role of thyroglobulin?
it is the backbone that provides all the tyrosine residues that can be iodinated
how does iodide get into the cell?
using the Na/I synporter
how does iodide (I-) get converted into iodine(I2)
a peroxidase oxidizes it
how does iodine get out into the lumen?
the enzyme pendrin
all of the processes that are occurring in the lumen/apical site are mediated through what?
PTO
What is the TG-T4/T3 stored as until TSH stimulation?
as colloid
what happens if the availability of iodide is restricted?
the formation of T3 is favored
what exactly is pendrin?
it is a chloride/iodide pump
what does a mutation in the pendrin gene (PDS) cause?
defects in the transport across the apical membrane as well as a defect in the cochlea
patients who present with a pendrin syndrome, usually have what symptoms?
hypothyroidism with a goiter
whenever you have way too much iodide avaialbe because the Na I synporter is super active, what needs to happen?
we have to have a mechanism to limit the amount of iodide that is being used to synthesize thyroid hormone
how do you limit the amount of iodide that is being used to synthesize thyroid hormones?
we suppress the PTO (peroxidase) with PTU or we could also block the Na+ I synporter with perchlorate or thiocynate
what is the wolff-chaikoff effect?
a reduction in thyroid hormone synthesis caused by a large consumption of iodide
what disease shows a very very active thyroid gland and the uptake of radioactive iodine is really really quick?
grave’s disease
what is the main binding protein for T4?
thyroxine-binding globulin (TBG)
where is TBG synthesized?
the liver
what is the 1/2 of t4?
6 days
what test will assess the circulating levels of TBG?
resin uptake test
hyperthyroidism will have what levels of T4 and resin uptake?
increased T4 and increased T3 resin uptake
hypothyroidism will have what levels of T4 and resin uptake?
Decreased levels of T4 and decreased T3 resin uptake
pregnant females will have what levels of TBG?
increased levels
high levels of TBG will have what type of levels of T4 and resin uptake?
increased T4 levels and decreased T3 resin uptake
hepatic failure will have what levels of TBG?
decreased levels of TBG
decreased levels of TBG will have what levels of T4 and T3 resin uptake?
Decreased T4 and increased T3 resin uptake
TSH has two effects on the thyroid gland. What are they?
it increases the synthesis and secretion of thyroid hormones and it has a trophic effect on the thyroid gland (makes it grow)
What is TSH regulated by?
TRH and Free T3
Are increased levels of TBG stimulatory or inhibitory of thyroid secretion?
stimulatory
the thyroid hormones stimulates the synthesis of what very important protein?
Na+ K+ ATPase
how do thyroid hormones affect cardiac muscle cells?
the myocardium will have an increased number of B1 receptors and so it will be more sensitive to stimulation by sympathetic nervous system
what effect do the thyroid hormones have on growth?
growth formation/ bone maturation
what effect do the thyroid hormones have on the CNS?
maturation of CNS
What effect do the thyroid hormones have on BMR?
increase Na+ K+ ATPase increases O2 consumption, which increases heat production, which causes an increase in BMR
What effect do the thyroid hormones have on metabolism?
net catabolic effects
what would the blood cholesterol concentration look like in a patient with hypothyroidism?
it would be increased
deficiency of thyroid hormones during the perinatal period leads to what?
abnormal development of synapses and decreased myelination
what is primary hyperthyroidism?
grave’s disease
what is secondary hyperthyroidism?
a TSH secreting tumor
what occurs in grave’s disease?
thyroid-stimulating immunoglobulins stimulate the TSH receptor without TSH hormone
What would expect the levels of TSH to be in a patient diagnosed with Grave’s disease?
low
What are some signs/symptoms of graves disease?
exophthalmos, elevated serum free and total T4 or T3 levels, goiter, presence of circulating thyroid-stimulating immunoglobulin
What is the most common primary hypothyroidism?
hashimoto’s thyroiditis
what could be an adverse effect of over prescribing T4 to women beyond menopause?
development of osteoporosis
in hoshimoto’s, what is thyroid synthesis impaired by?
thyroglobulin or TPO antibodies
what would you expect the TSH levels to be of a patient with hoshimoto’s?
high
what are some signs of congenital hypothyroidism?
feeding problems, protruding tongue, growth retardation
what are some causes of congenital hypothyroidism?
iodide deficiency, maternal intake of anti-thyroid medication
what is sheehan syndrome?
postpartum hypopituitarism due to necrosis of the pituitary gland
