Thyroid Flashcards
which is greater, BMR or RMR?
RMR (includes a few more activities than BMR)
what are the biologically active forms of thyroid hormone?
thyroxine (T4) and triiodothyronine (T3)
where is TH stored? where is it synthesized?
as a precursor in the lumen of the thyroid follicles
synthesized in the follicular cells surrounding the follicle
what stimulates the release of TH?
TSH that is released from the anterior pituitary
what is TH stored with in the follicles?
it is associated with thyroglobulin in the colloid
what do the C cells do?
they secrete calcitonin
what are T3 and T4 synthesized from? which one is made more abundantly?
tyrosine and iodide
synthesized on thyroglobulin
T4 is made preferentially
how is iodide concentrated in the thyroid gland? how is it regulated?
by a 2Na/I symporter that uses the inward Na gradient
thyroid autoregulates according to its needs
describe the TH synthesis events that happen within the follicle.
thyroglobulin is produced and exocytosed into the follicle
iodide is pumped into the lumen and is oxidized to iodine
iodine replases H+ on benzine rings of tyrosine
what is organification?
binding of iodine moities sequentially onto thyroglobulin
what is the function of thyroid peroxidase?
substitutes I for H+ on tyrosine, catalyzes DIT (diiodotyrosine) dimerization forming T4
how is T3 formed?
a MIT (monoiodotyrosine) is coupled with a DIT (diiodotyrosine)
what happens to T3 and T4 after they are made in the lumen?
they remain bound to thyroglobulin in the follicle and then are endocytosed back into the follicle cell
what does TSH stimulate?
colloid proteolysis creating free T3 and T4 to enter the blood stream
what is reverse T3?
biologically inactive TH that is derived from T4 by peripheral diodinases
how does thyroid hormone travel in the blood?
mostly bound to thyroid binding globulin
some bound to transthyretin and albumin
0.03% is free
what can free TH do that bound TH cannot?
it can enter target tissues and bind to TH receptors in the nucleus
what is T4 referred to and why?
it is referred to as prohormone because it is converted to T3 to produce a more biologically active form
how is hyperthyroidism treated?
with pharmacological agents that prevent T4>T3 conversion
why is T4 used to treat hypothyroidism if it is less active?
because it has a longer half life and greater stability
what does TH regulate? where?
metabolism of carbohydrates, proteins and lipids (increase basal metabolic rate)
in all tissues of the body
describe the kinetics of THs
slow onsets and long duration of action
T3 acts 4X more rapidly than T4
what hormones act synergistically with TH?
growth hormone and somatomedin
in hypothyroidism, what is the impact on bone? what can excessive hormone therapy do?
bone age is less than chronological age
hormone tharapy can lead to bone loss
when is TH neccessary for the CNS?
during CNS development in perinatal period
what effect does TH have on bone?
promotes ossification and fusion of bone plates and bone maturation
what does TH deficiency in infants result in?
growth (can be fixed) and mental retardation (only fixed if caught quickly)
what does TH do to the cardiovascular system?
increases cardiac output
what does TH do metabolically?
increases O2 consumption, increases heat production, increases glucose absorption, glycogenolysis, gluconeogenesis, lypolysis and protein synthesis and degredation
(catabolic and anabolic effects)
what is the effect of complete lack of TH secretion and extreme excess secretion on BMR?
lack- BMR falls 40-50%
excess- BMR rises 60-100% above normal
protein synthesis and degredation occur with TH action. which predominates?
degredation
in hypothyroidism, what two factors increase atherosclerosis risk?
decreased LDL receptor expression and cholesterol excretion in bile
what is the effect on the SNS in hyperthyroidism?
increased expression of beta adrenergic receptors leads to enhanced SNS sensitivity
what are the hypothalamic and pituitary releasing factors of thyroid hormone?
thyrotropin releasing hormone (TRH)-hypothalamus
thyroid stimulating hormone (TSH)- pituitary
how does TRH stimulate TSH release?
activates g protein linked to PLC> IP3 generation> increase in intracellular calcium (vesicular fusion)
how does TSH stimulate TH release?
g protein linked to adenylate cyclase> cAMP
what is the control mechanism of TH release
T3 (especially) and T4 exert negative feedback to the anterior pituitary and hypothalamus
dopamine and somatostatin inhibit TSH release
how is thyroid disease diagnosed?
serum TSH levels
elevated levels- hypothyroidism
reduced levels- hyperthyroidism
