Thyroid Physiology

Question 1

what does the thryoid gland secrete

Answer 1

thyroxine -T4
tri-iodothyroxine -T3
calcitonin

Question 2

what do the parathryoid glands secrete

Answer 2

parathyroid hormone

Question 3

what is the isthmus of the thyroid

Answer 3

connects the two lobes across the trachea

Question 4

where is the thyroid gland

Answer 4

between the 5th cervical and 1st thoracic vertebrae / 2-4th tracheal rings

Question 5

how big and heavy is the normal thyroid gland

Answer 5

12-15mm

25-30 grams

Question 6

is there always an isthmus

Answer 6

no- can just be 2 separate lobes

Question 7

what can cause the thyroid to change in size without being pathological

Answer 7

increases in size during pregnancy/ menstruation

Question 8

what innervates the thyroid gland

Answer 8

autonomic nerve supply- parasympathetic from vagus nerves

sympathetic fibres of superior, middle and inferior ganglia of the sympathic trunk

Question 9

what is the blood supply to the thyroid (+ veins)

Answer 9

superior and inferior thyroid arteries (branch of external carotid) +/- thyroidea ima

3 pairs of veins- superior/ middle thyroid vein (goes into internal jugular)
inferior thyroid vein (goes into brachiocephalic veins)

Question 10

what supports the thryoid gland

Answer 10

ligaments and strap muscles

posteromedial aspect of the gland is attached by the posterior suspensory ligament (berry ligament)

Question 11

what is the relevance of the anatomy of the thyroid gland for any surgical proceudres

Answer 11

Isthmus encountered during routine tracheostomy (needs retracted)
Recurrent Laryngeal Nerve/ vascularity - surgery

Question 12

what happens if the recurrent laryngeal nerve is bruised

Answer 12

causes temporary hoareness of voice

Question 13

what are the componnents of the thryoid gland

Answer 13

follicle, follicular cells, colloid, parafollicular cells

Question 14

what is a colloid thryoglobulin

Answer 14

tyrosine containing thyroglobulin filled spheres enclosed by follicular cells

Question 15

what do parafollicular cells do

Answer 15

secrete calcitonin

Question 16

what is the function of thyroid follicles

Answer 16

store the thyroid hormones produced by the follicular cells

Question 17

why does it take 2-3 weeks to notice that a hormone has stopped being produced

Answer 17

as hormones stored in follicles and slowly released into blood stream

Question 18

what mineral is need to make thyroid hormones

Answer 18

iodine

Question 19

how is iodine taken into follicular cells

Answer 19

from blood stream via iodine transporters

Question 20

what does iodine get trapped in to make thryoid hormones

Answer 20

pinocytes

Question 21

what does iodine form when in follicular cells

Answer 21

attaches to tyrosine residues on thyroglobulin to form MIT (monoiodotyrosine unit) and DIT (di-iodotyrosine unit)

Question 22

how do MIT and DIT form thyroid hormones

Answer 22

MIT and DIT couple to make triiodothyronine= T£

DIT and DIT couple to make thryoxine= T4

Question 23

what drugs can stop the formation of thyroid hormones from iodine and how

what are they used to treat

Answer 23

carbimazole and propylthioracil

stops iodine binding to tyrosine residues on thryoglobulin

used to treat hyperthyroidism

Question 24

what makes up the majority of thyroid hormones secreted

Answer 24

T4 thyroxine (90%)
(T3 10%)

Question 25

which thyroid hormone is more potent

Answer 25

T3 is 4x more potent than T4

Question 26

what makes T4 more potent

Answer 26

is converted to T3 by any cell (take off the iodine)

Question 27

what is the major biologically active thyroid hormone

Answer 27

T3- triiodothyronine

Question 28

what causes the release of thyroid hormones

Answer 28

thyroid stimulating hormone (from pituitary gland)

Question 29

how do T3 and T4 travel in the blood

Answer 29

are hyrdophobic/ lipophobic so bind to plasma proteins

Question 30

what plasma proteins do T3 and T4 bind to

Answer 30

70% thyroxine binding globulin (TBG) 20% thryoxine binding prealbumin (TBPA) 5% albumin

Question 31

what receptors do T3 and T4 work on

Answer 31

nuclear receptors

Question 32

wha is the biologically active form of thyroid of thryoid hormone- bound or unbound to plasma proteins why

Answer 32

unbound as only the free hormone is available to the tissues

Question 33

why is T3 the more active thyroid hormone

Answer 33

as is bound 10-20 times less by TBG and not significantly by the others so has a more rapid onset and offset of actions metabolic state correlates more closely with the free than the total concentration in the plasma

Question 34

what is the role of the bound plasma

Answer 34

acts a buffer to maintain free hormone concentration

Question 35

what can increase total T4 but not free T4 by increasing TBG

Answer 35

pregnancy, newborn state, contraceptive pill/ other sources of oestrogen hepatitis A and chronic active hepatitis, billiary cirrhosis

Question 36

what can decrease total T4 but not free T4 by decreasing TBG

Answer 36

androgens larges doses of glucocortcoids, cushings syndrome, active acromegaly severe systemic illness, chronic liver disease, nephrotic syndrome

Question 37

what are the effects of thyroxine

Answer 37

``` affects every tissue moist, sweaty skin increased HR, palpitations anxiety loose stools lessens/ loss of periods loss of muscle mass as you burn up stores ```

Question 38

what do thryoid hormones do to the basal metabolic rate? | how?

Answer 38

increase it increase: - number and size of mitochondria - oxygen use an rate of ATP hydrolysis - synthesis of respiratory chain enzymes

Question 39

how do thyroid hormones affect thermogenesis

Answer 39

increase it

Question 40

how do thyroid hormones affect carbohydrate metabolism

Answer 40

increase blood glucose- stimulate glyconeogenesis and gluconeogenesis increase insulin dependent glucose uptake to cells

Question 41

how do thyroid hormones affect lipid metabolism

Answer 41

mobilise fats from adipose tissue increase fatty acid oxidation in tissues

Question 42

how do thyroid hormones affect protein metabolism

Answer 42

protein synthesis

Question 43

how do thyroid hormones affect growth

Answer 43

growth hormone releasing hormone (GHRH) production and secretion requires thyroid hormones glucocorticoid-insulin GHRH release also dependent on thyroid hormones GH/ somatomedins require presence of thyroid hormone for activity

Question 44

how do thyroid hormones influence development of foetal and neonatal brain

Answer 44

myelinogenesis and axonal growth require thyroid hormones

Question 45

what are the effects of hypothyroidism on the CNS activity

Answer 45

slow intellectual functions

Question 46

what are the effects of hyperthryoidism on the CNS activity

Answer 46

nervousness, hyperkinesis and emotional lability (quick changes in mood)

Question 47

what is the permissive sympathetic action of thyroid hormones

Answer 47

thyroid hormones increase responsiveness to adrenaline and sympathetic NS neurotransmitter, noradrenaline- increases number of receptors cardiovascular responsiveness also increased due to this effect - increased force of rate of contraction of the heart

Question 48

what drug is used to treat symptoms of the permissive sympathomimetic action of thyroid hormones in hyperthyriodism

Answer 48

beta blockers (propanalol)

Question 49

what stimulates the release of TSH

Answer 49

thyrotrophin releasing hormone from the hypothalamus

Question 50

what releases TSH

Answer 50

anterior pituitary

Question 51

what is the negative feedback loop of the thyroid hormone regulation

Answer 51

T3 and T4 exert negative feedback control of release of TRH and TSH

Question 52

what effect does temperature have on thyroid hormone regulation

Answer 52

in babies and young children cold temperatures stimulate TRH release (stimulates TSH release- T3 and T4 release from thyroid)

Question 53

what effect does stress have on thyroid hormone regulation

Answer 53

inhibits TRH and TSH release

Question 54

what is the 'rhythm' of thyroid hormone release

Answer 54

circadian rhythm- highest late at night, lowest in the am

Question 55

what are de-iodinase enzymes

Answer 55

3 enzymes (type 1,2,3), important in the activation (t4 - t3) and deactivation (t3- t4, t3- t2, t4-rt3) of thyroid hormone add or remove iodine atom in the outer ring

Question 56

where is de-iodinase type 1 found

Answer 56

in the liver and kidney (acts a reserve enzyme)

Question 57

where is de-iodinase type2 found

Answer 57

heart, skeletal muscle, CNS, fat, thyroid and pituitary

Question 58

where is de-iodinase type 3 found

Answer 58

fetal tissue and placenta and brain (except pituitary)

Question 59

what is the most active de-iodinase ezyme

Answer 59

type 2

Question 60

what can hypothyroidism be associated with

Answer 60

primary gland failure can be associated with enlarged thyroid gland (goitre) secondary to TRH or TSH (no goitre) lack of iodine in diet (goitre)

Question 61

what are the symptoms of hypothyroidism

Answer 61

``` reduced BMR slow pulse rate fatigue lethargy slow response times mental sluggishness cold intolerance put on weight easily ```

Question 62

what is myxoedema

Answer 62

sign of hypothyroidism in adults - puffy face, hands and feet

Question 63

what is cretinism

Answer 63

sign of hypothyroidism in babies- dwarfism and limited mental function due to deficiency of thyroid hormones present at birth

Question 64

what is the most common cause of hypothyroidism

Answer 64

autoimmune disease- destroys cells of the thyroid

Question 65

what is graves disease

Answer 65

hyperthyroidism

Question 66

what is hyperthyroidism associated with

Answer 66

autoimmune disease- thyroid stimulating immunoglobulin (acts like TSH but unchecked T3 and T4) exophthalmos- bulging eyes due to water retaining carbohydrate build up behind eyes goitres

Question 67

what are the symptoms of hyperthyroidism

Answer 67

``` increases BMR very fast pulse rate increased nervousness and excessively emotional insomnia sweating and heat intolerance lose weight easily ```

Question 68

does thyroxine affect affect cardiac muscle

Answer 68

yes- can affect every tissue in the body

Question 69

can thyroxine cause infertility

Answer 69

yes can cause loss of periods

Question 70

heat intolerance is in hyper/hypo thyroidism

Answer 70

hyperthyroidism

Question 71

cold intolerance is in hyper/hypo thyroidism

Answer 71

hypothyroidism