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ENRE > Thyroid Gland > Flashcards

Thyroid Gland Flashcards

1
Q

What is colloid made of?

A

newly synthesized thyroid hormones attached to thyroglobulin

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2
Q

What is the general reaction to make T4 or T3?

A

tyrosine + I2 –> MIT (one I) or DIT

DIT + DIT –> T4

DIT + MIT –> T3 (outer ring w/ 1 I)

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3
Q

What is the difference btw T3 and reverse T3?

A

reverse T3 = inactive, minor product of rxn

2 I atoms on outer ring of rT3

1 I atom on outer ring of T3

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4
Q

What is the major secretory product of the thyroid?

A

T4

converted to T3 in periphery

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5
Q

What occurs in TH synthesis when the availability of iodide is restricted?

A

formation of T3 is favored

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6
Q

What is the I trap?

A

I- is brought into follicle cells from blood w/ 2 Na

I- moves against its electrochemical gradient - powered by Na/K ATPase pump on basolateral membrane

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7
Q

What does pendrin do?

A

on lumen side of follicle cell

pumps out I- into lumen and Cl into follicle cell

peroxidase simultaneously turns I- to I2 –> Iodide pumped out to lumen to be added to TH

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8
Q

What does TSH stimulate specifically in a follicle cell?

A

Stimulates it to pinocytose TG bound to T4 and T3

(then is secreted to blood)

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9
Q

What occurs when there are high levels of I-?

A

inhibits organification and addition of I2 to MIT and DIT

*Wolff-Chaikoff effect

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10
Q

What do perchlorate and thiocynate do?

A

inhibit the NIS pump –> I- cannot be pumped into follicle cells to synthesize T3 and T4

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11
Q

What does PTU do?

A

inhibits the peroxidases that turn I- to I2 on the lumenal membrane of follicle cells –> can’t add I to thyroxine

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12
Q

Where is most of iodine in the thyroid gland located?

A

most stored iodinated as tyrosines of thyroglobulin

8000 ug total

600 ug is T4 and T3

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13
Q

How much new thyroid hormone is secreted a day?

A

60 ug

enough stored as iodinated TG in colloid to last the body 2-3 months

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14
Q

What does radioactive I123 allow you to see?

A

activity of thyroid gland

highest in hyperthyroidism

slightly lower in graves (idk why though)

normal = 25%

depressed in hypothyroidism

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15
Q

How is Thyroid hormone found in the blood?

A

equilibrium btw bound and free T3 and T4

TBG = main binding protein, higher affinity for T4

TTR and albumin minor binding protein

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16
Q

How does the affinity of TBG for T4 and T3 relate to their half-lives?

A

higher affinity for T4 –> contributes to its longer half life

T4 = 6 days

T3 = 1 day

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17
Q

What does the T3 resin uptake test do?

A

measures circulating levels of TBG

unbound and labeled T3 added –> binds to unbound TBG spots –> anti T3 Ab/resin absorbs leftover labeled T3

higher the T3 uptake –> lower the TBG

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18
Q

What occurs to TBG and and T3 uptake levels during pregnancy?

A

increased TBG = increased total T4, FT4 usually normal

decreased T3 uptake

19
Q

What occurs to T3 uptake levels in hypothyroidism?

A

decreased T4

decreased T3 uptake

20
Q

What occurs to TGB during hepatic failure?

A

decreased blood levels of TBG

transient increase in FT3 and FT4 –> followed by negative feedback and inhibition of sythesis –> levels normalize

21
Q

How does TSH regulate the thyroid gland?

A

TSH regulates growth and secretion of thyroid hormones

regulated by TRH form hypothalamus and - feedback of FT3

TSH secretion occurs at steady rate

22
Q

What type of receptor does TSH bind on follicle cells?

A

Gs GPCR –> increases adenylyl cyclase –> cAMP –> increases synthesis, secretion, and growth of cells

23
Q

What occurs to Na/K ATPase in response to T3?

A

synthesis of Na/K ATPase increased in most tissues

24
Q

How does T3 affect cells?

A

binds to thyroid hormone receptor-retinoid X receptor complex in the nucleus –> stimulates synthesis of new proteins

25
Q

What is the affinity for thyroid hormone receptor for T3 and T4?

A

higher affinity for T3

26
Q

How does T3 affect cardiac tissue?

A

increases synthesis of:

myosin

B1 adrenergic receptors

Ca+ ATPase

27
Q

How is T3 deiodinated in peripheral tissue?

A

by 5’ iodinase

28
Q

How does T3 affect metabolism?

A

increases glucose absorption

increases gluconeogenesis and glycogenolysis

increases lypolysis

increases protein synthesis and degradation (net catabolic)

29
Q

How does T3 affect basal metabolic rate?

A

increased synthesis of Na/K ATPase –> more O2 consumption and heat production –> increased BMR

30
Q

How does a dose of T4 affect BMR?

A

increases BMR after several hours

long lasting

31
Q

How does T3 affect lipid metabolism?

A

stimulates fat mobilization –> increased FAs in plasma

enhanced oxidation of FAs

the more T3 –> the less cholesterol and triglycerides in blood

converts carotene to vitamin A

32
Q

Why can hypothyroid pts have blidness and yellowing of the skin?

A

T3 required to convert carotene to vit A

33
Q

Why is blood volume increased in response to T3?

A

stimulates RAS system

34
Q

How does T3/T4 indirectly affect the cardiovascular system?

A

increased heat production and CO2 in tissues –> decreased peripheral vascular resistance –> decreased diastolic BP –>

reflexive indreased adrenergic stim

35
Q

How does T3 affect bones?

A

work w/ GH and somatomedins –> promote bone formation

36
Q

How does T3 affect the CNS?

A

important for maturation

deficiency during perinatal development –> abnormal synapses and decreased dendritic branching and myelination

37
Q

What occurs in Graves’ disease?

A

TSI –> binds TSH receptor –> unregulated overproduction of thyroid hormones

38
Q

What is exophthalmos?

A

abdormal protrusion of the eyeball

seen w/ periorbital edema in graves disease

due to recognition by anti-TSH receptor Abs of similar epitope w/in orbital cells

39
Q

What are the primary causes of hypothyroidism?

A

agenesis

gland destruction

inhibition of hormone synthesis and releae (iodine deficiency, inherited enzyme defects, drugs)

transient (afer surgery or therapeutic radioiodine, postpartum, thyroiditis)

40
Q

What causes hashimoto’s thyroiditis?

A

autoimmune

thyroglobulin or TPO Abs –> decreased T3 and T4 secretion

41
Q

What is Cretinism?

A

untreated postnatal hypothyroidism; caused by:

iodide def, maternal intake of anti-thyroid meds, impaired dev of thyroid, inherent def in synthesis of thyroid hormones

42
Q

What can cause a goiter?

A

hyperthyroidism

primary hypo - lack of iodine, idiopathic, chronic thyroiditis (hashimotos)

43
Q

What do you see in pituitary abnormality causing hypothyroidism?

A

low TSH

Low T3/T4

negative antithyroid antibody

ENRE (21 decks)

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