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54 Cards in this Set
- Front
- Back
T4 and T3 can dissociate from binding proteins instantaneously.
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True
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T4 and T3 enter cells by specific processes that are
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Energy and Na+ dependent.
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What mediate most of the physiological actions of the thyroid hormones?
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Nuclear receptors(T3)
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Nuclear receptor binds to
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T3 more avidly.
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In vivo,virtually all of nuclear bound hormone is
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T3
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What are the 2 types of nuclear receptors?
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-Alpha:3 forms
-Beta:2 forms |
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B1 receptors found in
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Brain,kidney,liver and heart
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B2 receptors found in
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Pituitary and hypothalamus
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Thyroid hormone RECEPTOR(TR) binds to
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thryoid response element(TRE)
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What happens after TR binds to TRE?
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Stabilization by TRAP and transcription silencing by CoR.
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What happens then when T3 binds to TR?
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Formation of TR-T3 complex resulting in dissociation of CoR and recruitment of CoA.
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Regulation of thyroid hormone production is via
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Hypothalamic-pituitary-thyroid axis.
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Thyrotrophin is also known as
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TSH
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Sequence of actions leading to T3/T4 release:-
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TRH from hypothalamus--->TSH secretion from pituitary--->T4/T3 secretion(-ve feedback on hypothalamus and pituitary.
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What regulates extrathyroidal conversion of T4 to T3?
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Nutritional,hormonal and illness related factors.
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Is TRH measurement useful clinically?
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Not really measured except when given as injection to test ability of pituitary to produce TSH
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TRH synthetized as
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Large,pre-pro TRH protein in several tissues.
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TRH is released from
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Axon terminals close to hypo-pituitary portal plexus(which travel thru median eminence)
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TRH producing neurons are densely innervated by
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Catecholamines and Neuropeptide Y containing axons.(somatostain axon also involved)
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TRH receptor is
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G-protein coupled receptor with 7 transmembrane domains.
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TRH binds to
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Specific receptor in plasma mb of thyrotroph
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TSH?
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heterodimer with alpha and beta subunits
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Alpha subunit of TSH is
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Common to those of FSH,LH and hCG.
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What subunit of TSH confers specifity?
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Beta
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Normal bioactivity of TSH requires
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Glycosylation
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TSH regulated by
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-TRH
-T3 -T4 |
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TSH receptor is usually
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A target of immune system.
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What are the 2 forms of TSH receptor?
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-Low affinity
-High affinity |
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Mediation of TSH actions
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-activation of adenyl cyclase
-CAMP activates cAMP dependent kinases. |
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TSH actions?
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-stimulates every step in thyroid hormone synthesis and secretion in thyroid.
-Stimulates intermediary metabolites. -stimulates expression of many genes in thyroid tissues. -causes thyroid hyperplasia and hypertrophy. |
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Signs and Symptoms of Graves disease:
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-diarrhoea
-vomiting -shaking arms,legs,hands -weight loss -palpitations -sore eyes with blurry vision -marked hunger -extreme heat intolerance |
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On examination of pt with Graves?
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-Tachy
-Sweaty -Restless -Thyroid diffusely enlarged with bilateral bruits. -Obvious stare with lid retraction -Brisk ankle jerks |
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Presence of which antibodies in Graves?
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TSH receptor antibodies in higher levels.
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Thyroid hormones levels in Graves/
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High levels of T4 and T3 but low levels of TSH.
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Thyroid stimulating Ab stimulates
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Cells to produce T4/T3
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What is hot nodule?
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When Technecium injected in thyroid ,technecium accumulates where thyroid is working alot.-->black area.
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High levels of hCG having TSH like effects can cause
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hyperthyroidism.
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hCG and TSH beta subunits have
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considerable homology.
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Hyperemesis gravidarum ?
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Occurs during early pregnancy when hCG levels are high-->stimulate thyroid and increase T4 and T3.
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Presentation of H.gravidarum?
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-Severe vomiting
-thrytoxicosis. |
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High hCG levels can also cause
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Hydatidiform mole or choriocarcinoma.
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Treatments for hyperthyroidism:-
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-medical
-Ablative:either surgery or radioactive iodine. |
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Name 2 antithyroid drugs.
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-Propylthiouracil
-Carbimazole |
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Effects of antithyroid drugs?
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-Inhibit peroxidase system,preventing oxidation of trapped iodine & subsequent incorporation into iodotyrosines and ultimately iodothyronine.
-Inhibit coupling with iodotyrosines -Inhibit conversion of T4 to T3 in peripheral tissues -mild immunosuppressive effects |
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Antithyroid drugs bind to peroxidase at
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Same site as iodide or nearby preventing binding of iodine.
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What is resistance to thyroid hormones?
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Inherited syndrome with reduced responsiveness of target tissue to thyroid hormone.
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Where is the abnormality?
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mutations in the Thyroid hormone Receptor-beta in T3 binding domain(101 mutations recognised)
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What are the findings in this syndrome?
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high T4 and T3 with normal or increased TSH.
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Clinical features?
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-Clinically euthyroid
-Goitre(due to high TSH) -attention deficit disorder -emotional disturbances -recurrent ear and throat infections -delayed bone age -learning disability and retardation -short stature. |
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Clinical findings with hypothyroidism due to pituitary failure?
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-Low T4 and T3 + low TSH
-deteriorating eyesight -feeling cold more than usual weight gain,CONSTIPATION,dry skin. |
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What type of hypothyroidism is it ?
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Secondary due to pituitary failure.
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Somatostatin release from hypothalamus
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Inhibits pituitary
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Causes of hypothyroidism
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-Central hypothyroidism:TSH/TRH deficiency:
-->pituitary tumour --> pituitary surgery or radiation --> infiltration-hypophysitis,hemochromatosis -->infarction/apoplexy(sheehan syndrome) -->craniopharyngioma -->CNS radiation -->infiltrative lesions -->trauma |
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Lab dx. of hypothyroidism
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-total and free T4 decreased
-total and free T3 decreased -TSH up(EXCEPT in central hypo. where it is normal or decreased) |