Thyroid

Question 1

which is greater, BMR or RMR?

Answer 1

RMR (includes a few more activities than BMR)

Question 2

what are the biologically active forms of thyroid hormone?

Answer 2

thyroxine (T4) and triiodothyronine (T3)

Question 3

where is TH stored? where is it synthesized?

Answer 3

as a precursor in the lumen of the thyroid follicles

synthesized in the follicular cells surrounding the follicle

Question 4

what stimulates the release of TH?

Answer 4

TSH that is released from the anterior pituitary

Question 5

what is TH stored with in the follicles?

Answer 5

it is associated with thyroglobulin in the colloid

Question 6

what do the C cells do?

Answer 6

they secrete calcitonin

Question 7

what are T3 and T4 synthesized from? which one is made more abundantly?

Answer 7

tyrosine and iodide
synthesized on thyroglobulin
T4 is made preferentially

Question 8

how is iodide concentrated in the thyroid gland? how is it regulated?

Answer 8

by a 2Na/I symporter that uses the inward Na gradient

thyroid autoregulates according to its needs

Question 9

describe the TH synthesis events that happen within the follicle.

Answer 9

thyroglobulin is produced and exocytosed into the follicle
iodide is pumped into the lumen and is oxidized to iodine
iodine replases H+ on benzine rings of tyrosine

Question 10

what is organification?

Answer 10

binding of iodine moities sequentially onto thyroglobulin

Question 11

what is the function of thyroid peroxidase?

Answer 11

substitutes I for H+ on tyrosine, catalyzes DIT (diiodotyrosine) dimerization forming T4

Question 12

how is T3 formed?

Answer 12

a MIT (monoiodotyrosine) is coupled with a DIT (diiodotyrosine)

Question 13

what happens to T3 and T4 after they are made in the lumen?

Answer 13

they remain bound to thyroglobulin in the follicle and then are endocytosed back into the follicle cell

Question 14

what does TSH stimulate?

Answer 14

colloid proteolysis creating free T3 and T4 to enter the blood stream

Question 15

what is reverse T3?

Answer 15

biologically inactive TH that is derived from T4 by peripheral diodinases

Question 16

how does thyroid hormone travel in the blood?

Answer 16

mostly bound to thyroid binding globulin
some bound to transthyretin and albumin
0.03% is free

Question 17

what can free TH do that bound TH cannot?

Answer 17

it can enter target tissues and bind to TH receptors in the nucleus

Question 18

what is T4 referred to and why?

Answer 18

it is referred to as prohormone because it is converted to T3 to produce a more biologically active form

Question 19

how is hyperthyroidism treated?

Answer 19

with pharmacological agents that prevent T4>T3 conversion

Question 20

why is T4 used to treat hypothyroidism if it is less active?

Answer 20

because it has a longer half life and greater stability

Question 21

what does TH regulate? where?

Answer 21

metabolism of carbohydrates, proteins and lipids (increase basal metabolic rate)
in all tissues of the body

Question 22

describe the kinetics of THs

Answer 22

slow onsets and long duration of action

T3 acts 4X more rapidly than T4

Question 23

what hormones act synergistically with TH?

Answer 23

growth hormone and somatomedin

Question 24

in hypothyroidism, what is the impact on bone? what can excessive hormone therapy do?

Answer 24

bone age is less than chronological age

hormone tharapy can lead to bone loss

Question 25

when is TH neccessary for the CNS?

Answer 25

during CNS development in perinatal period

Question 26

what effect does TH have on bone?

Answer 26

promotes ossification and fusion of bone plates and bone maturation

Question 27

what does TH deficiency in infants result in?

Answer 27

growth (can be fixed) and mental retardation (only fixed if caught quickly)

Question 28

what does TH do to the cardiovascular system?

Answer 28

increases cardiac output

Question 29

what does TH do metabolically?

Answer 29

increases O2 consumption, increases heat production, increases glucose absorption, glycogenolysis, gluconeogenesis, lypolysis and protein synthesis and degredation
(catabolic and anabolic effects)

Question 30

what is the effect of complete lack of TH secretion and extreme excess secretion on BMR?

Answer 30

lack- BMR falls 40-50%

excess- BMR rises 60-100% above normal

Question 31

protein synthesis and degredation occur with TH action. which predominates?

Answer 31

degredation

Question 32

in hypothyroidism, what two factors increase atherosclerosis risk?

Answer 32

decreased LDL receptor expression and cholesterol excretion in bile

Question 33

what is the effect on the SNS in hyperthyroidism?

Answer 33

increased expression of beta adrenergic receptors leads to enhanced SNS sensitivity

Question 34

what are the hypothalamic and pituitary releasing factors of thyroid hormone?

Answer 34

thyrotropin releasing hormone (TRH)-hypothalamus

thyroid stimulating hormone (TSH)- pituitary

Question 35

how does TRH stimulate TSH release?

Answer 35

activates g protein linked to PLC> IP3 generation> increase in intracellular calcium (vesicular fusion)

Question 36

how does TSH stimulate TH release?

Answer 36

g protein linked to adenylate cyclase> cAMP

Question 37

what is the control mechanism of TH release

Answer 37

T3 (especially) and T4 exert negative feedback to the anterior pituitary and hypothalamus
dopamine and somatostatin inhibit TSH release

Question 38

how is thyroid disease diagnosed?

Answer 38

serum TSH levels
elevated levels- hypothyroidism
reduced levels- hyperthyroidism