Thyroid Gland Flashcards
where are the thyroid hormones stored
in follicular lumen (colloid) in thyroid gland
which is synthesized faster T4 or T3
T4 so more is produced and secreted (20:1)
what enzyme converts T4 to T3 in the periphery
deiodinase, most T3 comes from peripheral conversion
what decreases the conversion of T4 to T3
Fasting, Aging, Pregnancy, Hepatic and renal failure, Beta-blockers, hypothyroid
what increases conversion of T4 to T3
hyperthyroid, obesity, cold exposure
how is thyroid hormones transported in the blood
99% bound to proteins
1% unbound, free
main binding proteins of T3 and T4
TBG, 70%, made in liver, higher affinity for T4 than T3 binds 1 each
- transthyretin
- albumin
T3 resin uptake test. what decreases T3 resin uptake
increase in TBG or a decrease in endogenous T3
T3 resin uptake test, what increases T3 resin uptake
When there is a decrease in TBG or an increase in endogenous T3
what does pregnancy do to TBG levels
- effect on T3 and T4 syn
- is this normal?
increases it, which decreases free T3 and T4 and stimulates synthesis of more T3 and T4
-increases levels of T3 and T4 but levels of free physiologyically active Thyroid hormones are normal
(person is clinically “euthyroid”)
hepatic failure and thyroid hormone syn
hepatic failure decreases TBG, which increases free T3 and T4
-negative feedback inhibits syn of more T3 and T4
which hormone has a negative feedback on the anterior pituitary
too much T3 inhibits the secretion of TSH from the AP
what is the second messenger for TSH
cAMP
what effects does TSH have on thyroid gland?
increases synthesis and secretion of thyroid hormones
trophic effect on thyroid gland
Graves disease and TSH levels
graves disease occurs when thyroid-stimulating immunoglobulins bind TSH receptor on thyroid cell and cause unregulated overproduction of thyroid hormones
-TSH levels are low bc T3 neg inhibits AP from releasing TSH
factors that stimulate thyroid hormone secretion
pregnancy (increased TBG decreases free T3 and T4 so make more)
TSH
thyroid stimulating Igs
how is heat production increased with thyroid hormones
thyroid hormones stimulate synthesis of Na+/K+ ATPase which leads to increased O2 consumption so BMR and heat production increase
what does TH stimulate in cardiac muscle cells
myosin, B1-adrenergic receptors and Ca+ ATPase
-thyroid increases HR and contractility
effect of TH on liver and adipose tissue
induce synthesis of metabolic enzymes, so alters carb, fat and protein metabolism
T3 effect on growth
growth formation and bone maturation
T3 effect on metabolsim
increase:
glucose absorption
glycogenolysis
gluconeogenosis
lipolysis
protein synthesis and degradation (net catabolic)
-decreased muscle massT3 effect on cardiovascular system
increase cardiac output
hypothyroidism ___ blood cholesterol
increases
hypothyroid patients can suffer from blindness and yellowing of skin. why is this?
TH stimulate lipid metabolism and this is required for conversion of carotene to vitamin A
