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Endocrinology > Thyroid Physiology > Flashcards

Thyroid Physiology Flashcards

1
Q

What hormones does the thyroid secrete?

A

T3-triiodothyronine
T4-thyroxine
Calcitonin

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2
Q

What is the biologically active thyroid hormone? Why?

A

free T3

more potent

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3
Q

How do thyroid hormones travel in the blood?

A

Bound to plasma proteins
TBG ( thyroxine binding globulin)
TBPA (thyroxine binding prealbumin)
Albumin

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4
Q

What is the most abundant plasma protein transporter for thyroid hormones?

A

TBG (70 %)

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5
Q

What is the most abundant thyroid hormone?

A

T4

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6
Q

What thyroid hormone levels correlate with metabolic state?

A

Free thyroid hormone levels

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7
Q

What are the cellular components of the thyroid gland?

A

Follicle
Follicular cells (make ring like structure around colloid)
Colloid (tyrosine containing, thyroglobulin filled)
Parafollicular C cells (secrete calcitonin)

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8
Q

How are thyroid hormones released from the cell?

A 
Iodine uptake into follicular cells
Move into colloidal cells:
Iodine attached to tyrosine residue from thyroglobulin makes MIT and DIT 
(MIT+DIT=T3, DIT+DIT=T4)
Stored in colloid thyroglobulin
Pinocytosis of T3 and T4
T3 and T4 released into bloodstream
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9
Q

What is the thyroid hormone axis?

A

Hypothalamus: TRH secreted (thyrotrophin releasing hormne)
Pituitary: TSH production increased (thyroid secreting hormone)
Thyroid: TSH binds to follicular cells, T3/4 secreted from colloid
Negative feedback from hormones to hypothalamus and pituitary

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10
Q

What action do thyroid hormones have on lipid metabolism?

A

increase fatty acid oxidation

mobilises adipose tissue fat

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11
Q

What action do thyroid hormones have on growth?

A 
GHRH requires thyroid hormones for secretion (permissive action)
GH somatomedins (permissive action)
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12
Q

What action do thyroid hormones have on CHO metabolism?

A

increases blood glucose
increases insulin dependant glucose uptake in cells
increase gluconeogenesis and glyconeogenesis

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13
Q

What action do thyroid hormones have on thermogenesis?

A

hormones released in response to cold stress

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14
Q

What action do thyroid hormones have on basal metabolic rate?

A

increases number and size of mitochondria
increased respiratory chain enzymes
increased O2 and ATP hydrolysis

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15
Q

What action do thyroid hormones have on foetal/neonatal brain development?

A

myelinogenesis

axonal growth

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16
Q

What action do thyroid hormones have on protein metabolism?

A

increased protein synthesis

17
Q

What permissive sympathetic actions do thyroid hormones have?

A

increases response to adrenaline
increases response to sympathetic neurotransmitters
increases no. sympathetic receptors

18
Q

How will change in levels of TBG affect levels of thyroid hormones in blood?

A

Changes total hormone level

Not free hormone level (weren’t bound anyway)

19
Q

What conditions can cause an increase in TBG?

A 
Pregancy 
Hep A
Newborn
Chronic active hepititis
Genetics
Heroin usage
20
Q

What conditions can cause a decrease in TBG?

A 
Large dose glucocorticoids 
Cushing's
Androgens
Active acromegaly
Nephritic syndrome
genetics
21
Q

How does a change in temperature affect thyroid hormone levels?

A

Cold temperature
TRH release
TSH release
T3/T4

22
Q

How does stress affect thyroid hormone release?

A

Inhibits TRH and TSH

23
Q

How do changes in levels of thyroid hormone throughout the day demonstrate a circadian rhythm?

A

Highest levels at night

Lower levels in am

24
Q

How do hyper/hypo thyroidism affect ovulation?

A

anovulation

25
Q

What effect does pregnancy have on thyroid hormones?

A

thyroxine important for development
increased demand on thyroid
plasma protein binding increases

26
Q

How does pregnancy effect hypothyroid patients?

A

thyroid can’t meet extra demand

27
Q

How does hCG effect thyroid function?

A

Acts like TRH

28
Q

How is TBG effected in pregancy?

A

increased protein binding
normally fT4 normal
fT4 may be low if hypothyrioid

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