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Flashcards in thyroid hormones Deck (40):

where is T3 and T4 synthesized from

iodinated derivatives of tyrosine


what enzyme is responsible ofr "coupling", and what is the coupling process?

thyroid peroxididase: couples DIT with MIT or DIT with DIT to form T3 and T4


where is iodinated tyrosine residue contained?



where was thyroglobulin made and secreted to?

made in follicular cells and secreted through apical membrane into colloid. it's a major component of colloid


what is "organification" of iodine

incorporation of iodine into thyroglobulin


Low dietary iodine intake can cause what? what is the characteristic of this disease?

Goiter: enlarged thyroid gland

elevated TSH drives the growth


Where is iodide derived from

iodine from diet


what is iodide used for during oxidation

to reduce H2O2 into water

and I- gets oxidized into I or I+ to react with tyrosine in thyroglobulin to form MIT or DIT


what enzyme is responsible for organification

thyroid peroxidase


what does very high thyroglobulin content mean?

thyroid gland has significant storage capacity: 2-3 months supply


what is required for T3 and T4 release from thyroglobulin

proteolysis of thyroglobulin where it is taken back up into follicular cells via endocytosis and broken down by lysosome

Proteolysis releases T3, T4, MIT, DIT and peptides


How does thyroglobulin get into bloodstream? and what can this be used as?

Thyroglobulin can be TRANSCYTosED into blood stream,

this could be used as tumor marker for increased level indicate graves disease or thyroiditis


what releases thyrotropin releasing hormone (TRH)?

parvicellular neurons


what is the process of TRH for thyroid hormone release

TRH released from hypothalamus goes to anterior pituitary by portal hypophysial system. TRH binds to G protein coupled receptor-->activates phospholipase C-->Ca release triggers exocytosis of vesicles containing TSH


HOw is T4 converted to T3 and where does thi conversion occur

deodination type II, and occurs in liver and kidneys


what causes increases levels of free thryoid hormones? what reduces free thyroid hormone in circulation?

disease states decrease protein synthesis (thyroid hormone binds to serum protein--albumin/transtheyretin--in circulation) and will promote increased levels of free thyroid hormone

Preganancy reduce free thryoid hormone bc it increases plasma protein synthesis.


what hormones inhibit TSH secretion

somatostatin and dopamine


what is T3 and T4 usually bound to? why?

they are hydrophobic so needs to bind to carrier proteins in order to INCREASE half life, bc free T3 and T4 cleared in kidneys
-thyroid binding globin


does T3 or T4 have longer 1/2 life

T4 because it it binds to protein more tightly


what are the differences between T3 and T4

T3 is more biologically active
-it binds to serum portein less tightly
-binds to thyroid hormone receptors MORE tightly
-deiodination converts T4 to T3-->cause activation


what does deiodination mean? what do deiodinases do?

modifying thyroid hormone activity. it helps control thyroid hormone levels


major sites of deoidination?

liver and kidney


what is the role of TSH

Increase Na/I symporter--> increase iodide transport into follicular cells to produce more thyroid hormones .
-increase thyroglobulin synthesis
-thyroid peroxidase action promoting oxidation of iodide and transport onto thyroglobulin


what are the various levels of feedback control

TRH release is inhibited by T2 in circulation and local deiodinase activity in hypothalamus and anterior pituitary gand.

T3 regulates TRH receptor expression on ant pituitary gland thyrotrophs reducing the ability to produce/secrrete TSH.

T3 also regulates amt of mRNA for TSH


which class is the major source of circulating T3; production of T4-->T3 in peripheral tissues

Type I


which class is MOST important in control of thyroid hormone release.

Type II


which class is considered to be MAJOR physiological terminator of thyroid action in peripheral tissue

type III


which class Converts T4-->T3

Type II


which class does NOT convert T4 to T3

Type III


which class inactivates both T4 and T3

type III


if theres high T3, what does it induce?

expression of Type III deodinase to promote destruction of T3


what role does glucuronidation play in thyroid hormone metabolism? where does it occur?

Occurs in liver;
Glucuronic acid is added to phenolic hydroxyl group of thyroid hormone promoting water solubility of thyroid hormones.

If also facilitates in elimination in bile


What are some processes regulated by Thyroid hormone

its virtually in all tisues

-increase basal metabolic rate
-increase C.O.(increase HR, decrease peripheral vascular resistance)
-regulate carb and lipid metabolism
-bone turnover (increased bone resorption and bone formation)

-hematopoietic effects: increase O2 demand and erythropoietin production -->increase erythrocytes

-increase gut motility

-increase endocrine metabolic synthesis clearance, responsiveness


hyperthyroid symptoms

excessive head production, nervousness, insonmia, increased HR, weight loss, increased appetite


Graves disease

autoimmune disease
-antibodies bind to TSH receptor: release Thyroid hormone irrespective of TSH levels

(TSH hypersecretion/hyperthyroidism)


Hypothyroid symptoms

-increased sensitivity to cold
-decreased basal metabolic rate
-weight gain w/o increase caloric intake


Hashimoto thyroiditis

-autoimmune disease
-antibodies to thyroid peroxidase, thyroglobulin, TSSSH receptor blocking antibodies-->decrease T3/T4 hormone production

decreased hormone production/secretion


how do thyroid hormones enter target cells

cross plasma membrane via entering cell and binding to soluble receptor molecules due to high affinity/low capcity transport protein.

uptake is an active process and require energy of sodium electrochemical gradient


Thryoid hormone binding process

binds to DNA as heterodimer. other components of dimer is often retinoid X receptor (RXR)--> thryoid hormone triggers confirmational change of receptor-->HDAC is release and histone acetylase (HAT) replaces it-->relaxation of chromatin and enhance transcription


what happens to the thyroid hormone receptor complex with the absence of thyroid hormone bounded?

Thyroid hormone receptor/RXR complex recrruits a histone deacetylase (HDAC)-->condensation of chromatin and inhibit transcription