Thyroid Hormone Physiology

Question 1

describe how thyroid hormones are transported in the body

Answer 1

vast majority bound to serum proteins, very small amount is free

Question 2

what serum proteins do thyroid hormones bind to

Answer 2

thyroxine binding globulin(TBG) 70%
thyroxine binding pre-albumin(TBPA) 20%
albumin 5%

Question 3

what form of T3 and T4 is available to the tissues

Answer 3

only the free T3 and T4, not the vast majority of hormones bound to serum proteins

Question 4

describe the readiness of binding by T3 to serum proteins, and what this results in

Answer 4

bound 10-20 times less avidly to TBG, and not significantly by TTR, means T3 has more rapid onset and offset of actions

Question 5

how does the metabolic state correlate to thyroid hormone levels

Answer 5

correlates more closely with free than the total conc. in plasma

Question 6

what is the effect on T4 when TBG is increased or decreased in certain states

Answer 6

when TBG increased = total T4 increased but free T4 unchanged
when TBG decreased = total T4 decreased but free T4 unchanged

Question 7

what are some states that result in decreased and increased TBG

Answer 7

increased = pregnancy, Hep A, newborn
decreased = Cushings, chronic liver disease

Question 8

what affects do thyroid hormones have on all cells, especially neurons

Answer 8

increase metabolic rate, and increase glucose uptake

Question 9

what effect do thyroid hormones have in the liver

Answer 9

increased glycogenolysis and gluconeogenesis, decreased glycogenesis

Question 10

what effect do thyroid hormones have in adipose tissue

Answer 10

increased lipolysis and decreased lipogenesis

Question 11

what effect do thyroid hormones have on the heart and lungs

Answer 11

heart = increase HR and increase contractile strength
lungs = increased breathing rate

Question 12

in what ways do thyroid hormones increase the basal metabolic rate

Answer 12

increase number + size of mitochondria, increase O2 use and rates of ATP hydrolysis, increase synthesis of respiratory chain enzymes

Question 13

what effect to thyroid hormones have on temp. regulation

Answer 13

~30% temp. regulation is due to thyroid hormone thermogenesis

Question 14

what effects do thyroid hormones have on carbohydrate metabolism

Answer 14

increase blood glucose and increase insulin-dependant uptake of glucose into cells

Question 15

what effects do thyroid hormones have on lipid metabolism and protein metabolism

Answer 15

lipid = mobilise fats from adipose tissues and increase fatty acid oxidation in tissues
protein = increase protein synthesis

Question 16

what role do thyroid hormones have in growth

Answer 16

growth hormone releasing hormone(GHRH) production + secretion, glucocorticoid-induced GHRH release and GH activity all requires thyroid hormones
(permissive actions)

Question 17

what role do thyroid hormones have in development of foetal and neonatal brains

Answer 17

myelinogenesis and axonal growth requires thyroid hormones

Question 18

what permissive sympathomimetic action do thyroid hormones have

Answer 18

increase response to adrenaline + sympathetic NS neurotransmitter, noradrenaline, by increasing number of receptors

Question 19

what is responsible for the degradation of thyroid hormones

Answer 19

the de-iodinases, 3 types D1, D2 and D3

D3 main one

Question 20

where are D1, D2 and D3 found

Answer 20

D1 = mostly liver and kidney
D2 = thyroid, pituitary, heart, some peripheral tissues
D3 = foetal tissue, placenta, brain(not pituitary)