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endocrine term 2 quiz 2 - julia's notes > chapter 9 - thyroid > Flashcards

chapter 9 - thyroid Flashcards

1
Q

active thyroid hormones

A

T3 more active than T4 but most output is T4, but target tissues convert T4 to T3
only difference between two structures is iodine atom
also a reverse T3 that has no biological activity

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2
Q

follicular epithelial cells

A

synthesize thyroid hormones
arranged in circular folicles
have basal membrane facing blood and apical membrane facing follicular lumen
make colloid

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3
Q

colloid

A

made by thyroid follicular epithelial cells
secreted into lumen
newly synthesized thyroid hormones attached to thyroglobulin

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4
Q

secretion of colloid

A

when gland stimulated, colloidal thyroid hormone absorbed into follicular cells by endocytosis

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5
Q

features of thyroid hormone synthesis (3)

A

1: need large amounts of iodine from diet
2: partially intercellular and partially extracellular - completed hormones stored extracellularly until secretion stimulated
3: major secretory product (T4) not most active form of hormone

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6
Q

steps in thyroid hormone biosynthesis

A

1: thyroglobulin made in rough ER and golgi of thyroid follicular cells
2: thyroglobulin incorporated into secretory vesicles and extruded across follicular lumen
3: I- is pumped into cell by I-/Na+ cotransporters
4: at apical membrane, I- is oxidized to I2 by thyroid peroxidase
5: thyroid peroxidase now catalyzes the combination of I2 with thyroglobulin - makes monoiodotyrosine (MIT) or diiodotyrosine (DIT)
note: MIT and DIT still attached to thyroglobulin
6: thyroid peroxidase now catalyzes either the combination of two DIT molecules to make D4 or one DIT and MIT to make T3 (D4 reaction faster) - stored as colloid
note: some MIT and DIT does not get converted and so remains attached to thyroglobulin as MIT and DIT
7: gland stimulated and iodinated thyroglobulin endocytosed into follicular endothelial cells by pseudopods
8: microtubules transport thyroglobulin to basal membrane
9: thyroglobulin droplets fuse with lysosomal membranes
10: lysosomal proteases hydrolyze peptide bonds to release T3, T4, MIT and DIT
11: T3 and T4 transported across basal membrane into nearby capillaries
12: MIT and DIT remain in cell and are recycled - deiodinationed by thyroid deiodinase - I- added to intercellular pool

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7
Q

thyroid peroxidase

A

enzyme that catalyzes the oxidation of I- to I2, the combination of I2 with thyroglobulin (formation of MIT and DIT), and the coupling of MIT and DIT to make T3 and T4
inhibited by propylthiouracil (PTU)

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8
Q

propylthiouracil (PTU)

A

blocks thyroid peroxidase

effective treatment for hyperthyroidism

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9
Q

Wolff-Chaikoff effect

A

when high levels of I- inhibit organification and synthesis of thyroid hormones

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10
Q

I-trap

A

the Na/I cotransporter
in blood side membrane of follicular epithelial cells
transports I- and Na+ into cell against both chemical and electrical gradients
regulated by I- levels in the body - low levels stimulate - so if dietary deficiency will work to compensate, but if there’s a severe deficiency won’t be able to keep up and there will be a reduction in thyroid hormone production
competitively inhibited by anions thiocyanate and perchlorate

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11
Q

thyroid deiodinase

A

removes iodine from MIT and DIT in follicular epithelial cells
allows iodine to be salvaged
deficiency in this enzyme will mimic dietary I- deficiency

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12
Q

perichlorate

A

inhibits Na/I cotransporter

will reduce iodine uptake and so thyroid hormone production

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13
Q

thiocyanate

A

inhibits Na/I cotransporter

will reduce iodine uptake and so thyroid hormone production

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14
Q

thyroxine-binding globulin (TBG)

A

protein that most T3 and T4 binds to in circulation
come circulates unbound and some binds to albumin and prealbumin
changes in blood levels can alter the fraction of free (and so physiologically active) thyroid hormones

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15
Q

hepatic failure and thyroid hormone levels

A

blood levels of TBG decrease because there’s decreased hepatic protein synthesis
results in a transient increase in level of free thyroid hormones
results in inhibition of synthesis of thyroid hormones

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16
Q

pregnancy and thyroid hormone levels

A

high level of estrogen inhibits hepatic breakdown of TBG, increases TBG levels
high TBG means that more thyroid hormone is bound to TBG and less is free and unbound
increased level of synthesis and secretion but low levels of active hormone = clinically euthyroid

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17
Q

T3 resin test

A

way to indirectly assess levels of TBG
measures binding of radioactive T3 to a synthetic resin
add standard amount of radioactive T3 to an assay system that has a sample of patient’s serum and the T3-binding resin
radioactive T3 will bind to unoccupied sites on patient’s TBG and any leftover radioactive T3 will bind to the resin
so T3 resin uptake associated with amount of TBG present and endogenous T3 levels

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18
Q

5’-iodinase

A

enzyme that converts T4 to T3 by removing one atom of I2
also convert portion of it to reverse T3 (rT3) - inactive
inhibited in starvation in skeletal muscle and other tissue but not in brain

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19
Q

5’-iodinase during starvation

A

inhibited in most tissues - lowers O2 consumption and basal metabolic rate
but brain 5’ iodinase not inhibited so brain levels of T3 are protected

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20
Q

regulation of thyroid hormone production

A

hypothalamus releases TRH
activates anterior pituitary release of TSH
activates thyroid gland release of T4 and T3
T4 and T3 have negative feedback effect on anterior pituitary

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21
Q

stimulatory factors for thyroid hormone

A

TSH
Thyroid stimulating inmmunoglobulins
increases TBG levels (like during pregnancy)

22
Q

inhibitory factors for thyroid hormone secretion

A

I- deficiency
deiodinase deficiency
excessive I- intake (Wolff-Chaikoff effect)
Perchlorate, thiocyanate (inhibit Na/I cotransport)
propylthiouracil (inhibits peroxidase enzyme)
decreased TBG levels (liver disease)

23
Q

TRH

A

secreted by parventricular nuclei of hypothalamus
acts on thyrotrophs of anterior pituitary to stimulate transcription of TSH gene and secretion of TSH
also stimulates secretion of prolactin by the anterior pituitary

24
Q

TSH

A

glycoprotein
secreted by anterior lobe of pituitary in response to stimulation by TRH
regulates growth of thyroid gland and secretion of thyroid hormones
begins being secreted around gestational week 13 - same time that fetal thyroid gland begins secreting thyroid hormones

25
Q

regulation of TSH secretion

A

regulated by:

1: TRH from the hypothalamus (stimulates)
2: thyroid hormone, mediated by T3 - anterior lobe contains thyroid deiodinase (inhibit)

26
Q

actions of TSH on the thyroid gland

A

binds receptor coupled to adenylyl cyclase via Gs protein
generates cAMP

results in:

1: increased secretion and synthesis of thyroid hormones by stimulating each step of the biosynthetic pathway
2: trophic effect on gland - raised for extended periods of time results in hypertrophy and hyperplasia of thyroid follicular cells and increased thyroidal blood flow

27
Q

thyroid stimulating immunoglobulins

A

activate TSH receptor on thyroid cells - antibodies to the receptor
IgG
one example = graves’ disease

28
Q

Graves’ disease

A

form of hyperthyroidism
due to increased circulating levels of thyroid-stimulating immunoglobulins
get high circulating levels of thyroid hormones
lower TSH levels than usual because TSH has negative feedback effect on its secretion
get hypertrophy of the thyroid gland

29
Q

actions of thyroid hormones

A 
growth: 
	growth formation
	bone maturation
CNS:
	maturation of the CNS
BMR:
	increases Na/K ATPase activity
	increases O2 consumption
	increases heat production
	increases BMR
metabolism
	increases glucose absorption
	increases glycogenolysis
	increases gluconeogenesis
	increases lipolysis
	increases protein synthesis and degradation (net catabolic)
cardiovascular
	increases cardiac output
30
Q

rT3 levels

A

normally tissues make about equal amounts of T3 and rT3
but pregnancy, fasting, stress, hepatic and renal failure, and beta-adrenergic blocking agents all decrease conversion to T3 and increase conversion to rT3 => decrease in amount of active hormone

31
Q

T3 actions in cell

A

binds to nuclear receptor in nucleus
T3-receptor complex binds to regulatory element on DNA
stimulates DNA transcription

32
Q

effects of thyroid hormone on BMR

A

increases O2 consumption => increase in BMR and body temperature
in all tissues except brain, gonads and spleen
does so by inducing the synthesis of Na/K ATPase

33
Q

effects of thyroid hormone on metabolism

A

increase glucose absorption from GI tract and potentiate effects of other hormones on glyconeogenesis, lipolysis, and proteolysis
increase both protein synthesis and degradation but the overall effect is catabolic (degradation) => loss of muscle mass
done via increased synthesis of metabolic enzymes including cytochrome oxidase, NADPH cytochrome C reductase, alpha-glycerophosphate dehydrogenase, malic enzyme, some proteolytic enzymes

34
Q

thyroid hormone effects on cardiovascular and respiratory systems

A

increase O2 consumption so create higher demand for O2
to allow for this, also create increase in cardiac output and ventiallation
increase HR and increase contractility (so increase stroke volume) by inducing synthesis of cardiac beta1-adrenergic receptors - mediate effects of sympathetic nervous system
also induce synthesis of cardiac myosin and sarcoplasmic reticulum Ca2+ ATPase

35
Q

thyroid hormone effect on growth

A

required for growth to adult stature
act synergistically with growth hormone and somatomedins to promote bone formation
promote ossification and fusion of bone plates and bone maturation

36
Q

thyroid hormone effects on CNS

A

age dependent:
- in perinatal period: essential for normal maturation of CNS
hypothyroidism results in irreversible mental retardation, but if detected in newborn can be treated
- in adults:
hypothyroidism causes listlessness, slowed movement, somnolence, impaired memory, decreased mental capacity
hyperthyroidism causes hyperexcitability, hyperreflexia and irritability

37
Q

thyroid hormone effects on autonomic nervous system

A

synergistic effects with catecholamines on heat production, cardiac output, lipolysis and gluconeogenesis - beta-adrenergic blocking agents can treat symptoms of hyperthyroidism
effects on BMR, heat production, HR, and SV similar to those produced by catecholamines via beta-adrenergic receptors

38
Q

symptoms of hyperthyroidism

A 
increased basal metabolic rate
weight loss
negative nitrogen balance
increased heat production
sweating
increased cardiac output
dyspnea (shortness of breath)
tremor, muscle weakness
exophthalmos
goiter
39
Q

symptoms of hypothyroidism

A 
decreased basal metabolic rate
weight gain
positive nitrogen balance
decreased heat production
cold sensitivity
decreased cardiac output
hypoventilation
lethargy, mental slowness
drooping eyelids
myxedema
growth retardation
mental retardation (if perinatal)
goiter
40
Q

causes of hyperthyroidism

A

Graves’ disease
thyroid neoplasm
excess TSH secretion
exogenous T3 or T4

41
Q

causes of hypothyroidism

A 
thyroiditis (autoimmune or Hashimoto's thyroiditis)
surgery for hyperthyroidism
I- deficiency
congenital (cretinism)
decreased TRH or TSH
42
Q

TSH levels in hyperthyroidism

A

decreased - due to feedback inhibition of T3 on the anterior lobe
but if defect is in the anterior pituitary, levels will be increased

43
Q

TSH levels in hyperthyroidism

A

increased by negative feedback if primary defect is in the thyroid gland
decreased if defect is in hypothalamus or anterior pituitary

44
Q

treatment for hyperthyroidism

A

propylthiouracil - inhibits peroxidase enzyme and thyroid hormone synthesis
thyroidectomy
131I- destroys thyroid
beta-adrenergic blocking agents (adjunct therapy)

45
Q

treatment for hypothyroidism

A

thyroid replacement therapy

46
Q

diagnosis of hyperthyroidism

A

if cause is Graves’ disease, thyroid neoplasm, or exogenous administration of thyroid hormones then TSH levels decreased by negative feedback of T3 on anterior pituitary
if cause is increased secretion of TRH or TSH then TSH levels will be increased

47
Q

goiter

A

enlargement of thyroid gland
happens in both hyperthyroidism and hypothyroidism
in hypothyroidism when cause is in the thyroid - due to unrelenting stimulation of the thyroid gland by high levels of TSH

48
Q

thyroiditis

A

autoimmune destruction of the thyroid gland

antibodies either destroy gland or block synthesis of thyroid hormone

48
Q

diagnosis of hyperthyroidism

A

decreased levels of T3 and T4
if defect is in thyroid gland, will have increased TSH
if defect in hypothalamus or pituitary, TSH levels decreased

49
Q

myxedema

A

increased filtration of fluid out of the capillaries and edema due to accumulation of osmotically active mucopolysaccharides in interstitial fluid

50
Q

cretinism

A

form of growth and mental retardation caused by hypothyroidism during the perinatal period that is untreated
irreversible but can be treated at birth if detected

endocrine term 2 quiz 2 - julia's notes (16 decks)

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