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thyroid physiology Flashcards

1
Q

thyroid is located

A 
wraps around the trachea 
larger in females 
increases in size with pregnancy 
two large lateral lobes connected by a thin isthmus 
contains 4x parathyroid glands 
receives very high blood flow 
abundant nerve supply
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2
Q

thyroid gland structure

A

follicles - hollow vissicles

THs are synthesised in epithelial cells lining the follicle

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3
Q

follicle interior is filled with

A

thyroglobulin (glycoprotein)

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4
Q

each follicle is surrounded by

A

dense capillary network

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5
Q

thyroid hormones are derived from

A

tyrosine amino acid

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6
Q

2 thyroid hormones

A

thyroxine - T4

triiodothyronine - T3

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7
Q

thyroxine

A

major secretory product
low biological activity
usually transformed to T3 within target cells

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8
Q

need for iodine to produce thyroid hormone

A

thyroid hormones need large amounts of iodoine

- low levels are absorbed int the body and absorbed as iodide and need to be oxidised to iodine (thyroid peroxidase)

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9
Q

iodide oxidation to iodine

A

by thyroid peroxidase

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10
Q

how does the thyroid gland concentrate iodine

A

powerful iodide pumps

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11
Q

thyroglobulin precursor TGB

A

large protein
has lots of tyrosine residues
stores thyroid hormones as a colloid (holds 4 or 5 hormone molecules)

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12
Q

thyroglobulin precursor molecule is made in

A

follicular cells and exocytosed into the follicle lumen

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13
Q

steps in thyroid hormone production

A
  1. iodide trapping
  2. synthesis of thyroglobulin
  3. oxidation of iodide and iodination of tyrosine residues
  4. coupling of tyrosine residues
  5. endocytosis and digestion of colloid
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14
Q

iodide trapping

A

sodium iodide symporter
pumps 2x Na and 1x I- from plasma into the follicle cells
concentration of iodide in the follicle is 30x plasma concentration

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15
Q

synthesis of TGB

A

made on rough endoplasmic reticulum

exocytosed into the colloid where it waits for iodine

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16
Q

pendrin

A

passive transporter

iodide transported into the follicular lumen where thyroid peroxidase can oxidise the iodide

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17
Q

once the iodide is oxidised

A

once it is oxidise it can be added to the tyrosine residues present on the thyroglobulin

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18
Q

iodination of tyrosine molecules forms

A

DIT - diiodotyrosine

MIT - monoiodotyrosine

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19
Q

coupling of tyrosine

A

forms either T4 or T3
mature hormone is still attached to TGB storage molecules in the follicle lumen
DIT + DIT = T4
MIT + DIT = T3

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20
Q

iodine availability

A

TGB allows storage of large amounts of Th precursor so the body becomes independent on day to day iodine availability

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21
Q

secretion of thyroid hormone

A

when stimulated thyroglobulin is exocytossed
endocytotic vesicles fuse with lysosomes
proteases release T3 and T4 released from TGB storage molecule
thyroid hormones migrate to the basal membrane
actively transported into the circulation via monocarboxylate transporter8

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22
Q

thyroid hormones are transported into the circulation by

A

monocarboxylate transporter 8

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23
Q

thyroid hormone is mosty released as

24
Q

dominant plasma proteins binding thyroid hormones

A

TBG = thyroid binding globulin (main one)
transthyretin
albumin

needs to be unbound to work

25
to work, thyroid hormone must be
unbound
26
T3 half life
much shorter half life than T4 | 3.5 days
27
T4 half life
6.5 days - longer
28
thyroid hormones enter cells via
iodothyronine transporters organic anion transporters (passive, but probably involved ion exchange) OR monocarboxylate transporter (active transport - powerful scavenging)
29
TH receptors
10x more affinity for T3 | are ubiquitous
30
T4 conversion to T3
deiodinase enzymes convert T4 to T3
31
thyroid hormone functon
thyroid hormones bind intracellular nuclear receptors, and act to regulate the activity of specific genes
32
control of thyroid function
hypothalamus releases thyrotropin releasing hormone TRH acts on anterior pituitary to release thyroid stimulating hormone TSH acts on the thyroid gland to release thyroid hormone T3 and T4
33
hypothalamus stops anterior pituitary production of TRH with
somatostatin
34
stimuli to increase thyroid function axis
cold
35
thyrotropin releasing hormone stimulates production of
thyroid stimulating hormone / thyrotropin / TSH
36
TSH binds
TSHR to increase cAMP
37
TSHR
TSH receptor increases cAMP increases thyroid activity in 3 ways - increases hormone synthesis (increases activity of sodium iodide symporter pump and thyroglobulin production) - increases thyroid hormone secretion (vesicular reuptake and active transport) - increases blood flow to the thyroid
38
TH is main determinant of
basal metabolic rate - influences synthesis and degradation of CHOs, fats, and proteins - increases the sensitivity to sympathetic NS signal (increases receptors and receptor signal)
39
why doesn't ATP production cause negative feedback
1. (burn it off) ion gradients are expensive TH increases the activity of pumps involved in Na/K across the plasma membrane and Ca2+ gradients between cytoplasm 2. uncouple ADP phosphorylation in mitochondria - TH induces uncoupling protein expression which means protons are pumped but leak back out, burning fuel but not creating ATP both these processes produce lots of heat
40
CHO metabolism
T3 accelerates CHO utilisation by - increasing glucose absorption by the GIT - increasing glucose oxidation in liver, fat and muscle cells - amplifies signals from other signalling hormones (e.g. insulin) T3 acts by increasing the synthesis of specific metabolic enzymes
41
TH effects on lipid metabolism
optimal TH levels determine rates of lipolysis and lipogenesis in the liver stimulates mobilisation from fat cells (increase n free fatty acids and decrease in plasma TG and cholesterol)
42
TH effects on nitrogen metabolism
rate of protein synthesis and degradation is Th-dependant
43
Th effect on temperature
- energy utilisation - TH increases BAT theromegenesis - THs also act indirectly by stimulating the sympathetic nervous system to mobilise Cho and fat needed to fuel the shivering response and increase circulation to skeletal muscle and adjust blood flow to skin
44
Th effect on cardiovascularr regulation
increase tissue blood flow | increase cardiac output (increase heart rate and contractility
45
Th effects on skeletal system
T3 acts synergistically with growth hormone, IGF and other growth factors to signal increased bone formation bone maturation ie. closing of epiphysis (cartilaginous growth plates of bones)
46
TH effects of CNS
Th is essential for normal brain development
47
primary hypothyroidism
hashimotos disease - lack of energy, problem with fatty acid mobilisation causing increase in body weight - decreased sensory capacity, impaired memory and psychosis may occur - poor tolerance of cold - oedema due to accumulation of water-retaining hyaluronic acid and condition sulphate
48
TH deficiency in children
- short stature, bone retardation, malformed facial bone structure - severe, irreversible mental and physical retardation (cretinism)
49
treatment for goitre
thyroxine tablets
50
most common cause of hypothyroidism
decreased dietary iodine
51
deiodinase deficiency
cells can't convert T4 to T3 hypothyroidism goitre because negative feedback loop isn't triggered
52
severe selenium deificiency
deiodinases are selenoproteins - same as deiodinase deficiency
53
allan-herndon-dudley syndrome
MCT8 mutations - cells can't actively take up TH - particularly effects CNS - moderate to severe intellectual impairment, plus aphasia and ataxia - highly limited IQ, patients may never talk or walk, may need feeding tube
54
hyperthyroidism
Graves' disease elevated metabolic rate and heat production - poor heat tolerance increase protein degradation, severe catabolism of muscle, increase fatty acid mobilisation exaggerated autonomic responses goitre, high bone age, exophthalmos - oedematous fluid retention behind the eyes
55
autoimmune hyperthyroidism
production of thyroid stimulating immunoglobulins that bind to the TSH receptor and activate increase in TH
56
thyroid tumours
TH-secreting tumours are most common treated with antithyroid drugs to decrease TH production by blocking coupling of iodine or beta blockers or radioiodine which concentrated in the thyroid and kill of thyroid cells

physiology 2 (21 decks)

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