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Endocrinology > Lect. 4 - Thyroid > Flashcards

Lect. 4 - Thyroid Flashcards

1
Q

cretinism

A

a condition of abnormal mental and physical development resulting from a deficiency of thyroid hormone in fetal or early life.

typically characterized by intellectual disability, small stature, and thickening of the facial features.

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2
Q

predominant structures in the thyroid glands

A

thyroid follicles filled with protein rich colloid

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3
Q

thyroid medullary cells produce what

A

also known as C cells

produce calcitonin

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4
Q

identify the molecule

A

thyroxine, T4

Theterm‘thyroidhormone’
encompassesbothT3and T4.

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5
Q

identify the molecule

A

triiodothyronine or T3

Theterm‘thyroidhormone’encompassesbothT3and T4.

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6
Q

themainprecursortothyroid
hormones

A

Thyroglobulin

is the main precursor to thyroid hormones.

Iodineistransferredacross
theepithelium
intothethyroid follicular lumen
whereitisconjugated
tothyroglobulin.

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6
Q

percentage output of T4 and T3 from thyroid gland

A

T4(90%ofoutput)
T3(10%ofoutput)
arebiologicallyactive,
andT3isfarmoreactivethan
T4becauseofahigheraffinityfor
thyroidhormonereceptors

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7
Q

What secretes thyrotropin releasing hormone?

A

NeuronsofthehypothalamussecreteTRH whichiscarriedtothe anteriorpituitary.

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8
Q

After thyrotropin releasing hormone is secreted from thehypothalamusandcarriedtothe adenohypophysis, what occurs?

A

Thyrotrophs are stimulated by TRH to synthesize and release Thyroid-stimulating hormone (TSH).

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9
Q

Thehypothalamic‐pituitary‐thyroid(HPT)axis

A

thyrotropin releasing hormone TRH, hypothalamus
thyroid-stimulating hormone TSH, adenohypophysis
thyroxine & triiodothyronine T4 & T3, thyroid gland

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10
Q

Function of TSH

A

TSHbindstoreceptorsonthebasolateral

membraneofthyroidfollicularcells,

stimulates the synthesis and

release of T4andT3.

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11
Q

Cell receptors for T3 & T4 are located where?

A

areintracellular so T3 andT4 arehydrophobic/lipophilic(cancrosscellmembranes)

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12
Q

thyroid hormone primary binding protein differs between species

A

More than 99% of T3 and T4 are protein‐bound for transport within the blood by:

Thyroid‐binding globulin (TBG) (primary binder in dogs).

Cats do not appear to have a high‐affinity TBG.

Prealbumin is the primary binder in cats.

Other plasma proteins such as transthyretin and albumin bind thyroid hor

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13
Q

Factors affecting thyroid hormone concentration and/or capacity of circulating binding compounds (3)

A

diseases

Pharmacologicagents (e.g. glucocorticoids, acetylsalicyclic acid both lower plasma TT4 without affecting free T4)

breed differences (e.g. TT4 low in some whippets sled dogs & greyhounds; small breeds tend to have higher TT4)

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14
Q

the most significant metabolic transformation of the thyroid
hormones is

A

deiodination

About80%ofthesecreted
T4isdeiodinated toformT3andreverse-T3,
predominantlyin
theliverandkidney.

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15
Q

Factors that impair T3 formation,
such as fasting and nonthyroidal disease, almost invariably increase what

A

plasma reverseT3 concentration.

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16
Q

name 2 FactorsthatimpairT3formation

A

fastingandnonthyroidaldisease,
almostinvariablyincrease
plasmarT3concentration.

Illnessleadstoincreased
activityoftype3
deiodinase(D3),
whichprimarily
deiodinates theinnerring.

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17
Q

T4bindsmoretightlyto
bindingproteinsinplasmathandoes what?

A

T3,whichresultsinT4havinga
lowermetabolicclearance
rateandlongerhalf‐life.

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18
Q

Overall,thekineticsofthyroid
hormonedistributionand
turnoveraremuchmorerapidin

A

dogsthaninhumans,
inpartbecauseofthe
lowerbindingofbothT4
andT3incanineplasma.

Theplasmahalf-lifeofT4isabout
0.6daysindogs,
comparedtoabout
sevendaysinhumans.

Thus, levothyroxine(T4)
therapeuticdosage
ishigherindogs
comparedtohumans.

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19
Q

T3orT4activatesthyroidhormonereceptorsandregulatesthetranscriptionofthesegenes to produce proteins with what effects?

A

These proteins generally:
↑Cellularmetabolism
↑CellularoxygenconsumpƟon
↑Cellular glucose
↑ Circulation and respiration

Promote nervoussystem and skeletaldevelopment.

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20
Q

Clinically,thyroidhormones help maintainnormal:

A
  • Energylevels
  • Weight
  • Thermoregulation
  • Heartrates
  • Bowelmovements
  • Mood

Inmanyrespectsthyroidhormonescanbeviewedastissuegrowthfactors,thisbeingbestexemplifiedbytheconsequencesofthyroidhormonedeficiencyatayoungage.

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21
Q

Hypothyroidisminyounganimals, name the potential diseases (4)

A
  • Acquired juvenile hypothyroidism
  • Thyroid dysgenesis
  • Defective thyroid hormone synthesis
  • Central hypothyroidism (problem not in gland itself)
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22
Q

In all forms of hypothyroidism, treatment consists of oral administration of l‐thyroxine except in the case of

A

hypothyroidism due to iodine deficiency.

L-thyroxine administration will lower thyroid-stimulating hormone secretion and as a result the goiter will shrink.

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23
Q

Earlyinlifethepresenceofthyroid
hormonesiscrucialfor

A

growth and development of all body tissues and particularly the skeleton.

Disproportionate dwarfism may be a prominent sign of congenital or
juvenile‐onset hypothyroidism, in addition to the signs also seen in
adult‐onset hypothyroidism.

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24
Q

cause of Acquiredjuvenilehypothyroidism

A

Iodine deficiency is the classic cause of acquired juvenile hypothyroidism e.g. a diet consisting of meat alone is deficient in iodine.

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25
Q

lackofiodineresultsin

A

TSH‐induced thyroid hyperplasia.

In mild deficiencies the increased capacity for hormone production compensates sufficiently and euthyroidism is maintained.

In severe iodine deficiency there is insufficient production of thyroid hormone despite the compensatory thyroid hyperplasia.

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26
Q

Lymphocyticthyroiditis is

A

inflammation of the thyroid gland due to lymphocytic infiltration, an autoimmune disease. Thyroid tissue is destroyed and hypothyroidism is the result.

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27
Q

Thyroiddysgenesis

A

the term used to describe abnormalities in thyroid gland development

Ectopiaofthyroidtissueiscommoninthedogandisalsoknowntooccurincats. Ectopia meaning the thyroid tissue is outside of the gland.

Glandfunctionmaybeinsufficienttopreventhypothyroidism.

Completeathyreosishasalsobeenfound meaning no thyroid gland whatsoever present (but in some cases thyroid tissue can still be present somewhere).

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28
Q

Abnormalitiesinthenewbornthatmaysuggesthypothyroidisminclude: (5)

A

Alargefontanel(soft spot in the skull whichshouldbeclosedatbirthindogsbutnotincats)

  • Hypothermia
  • Hypoactivity
  • Sucklingdifficulties
  • Abdominaldistension
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29
Q

besides dysngenesis, Congenitalhypothyroidismmayoccurbecauseof

A

anenzymedeficiencythatpreventssynthesisofthyroidhormones.

Suchcongenitaldefectsarerareandalthoughinprincipleanystepinthyroidhormonesynthesismaybeaffected.
OnlyunresponsivenesstoTSHanddefectiveperoxidaseactivity(thyroidhormoneisnotsynthesized)havebeenfoundthusfarinthedogandthecat.

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30
Q

Central(secondary)hypothyroidism is due to

A

thyroid-stimulting hormone deficiency

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31
Q

central hypothyroidism can be classified as one of two

A

pituitary (secondaryhypothyroidism)
hypothalamic (tertiaryhypothyroidism)

(primary would indicate an issue with the gland itself)

32
Q

HyposecretionofTSHisusuallyaccompaniedby what other hormonal issues?

A

HyposecretionofTSHisusuallyaccompaniedbydecreasedsecretionofotherpituitaryhormones.

e.g. lack of GH usually go hand in hand with lack of T4 “combineddeficiencyof
adenohypophyseal hormones.” (e.g. in the dwarf animals)

33
Q

hypothyroidismduetocentralcausesislessseverethan

A

primaryhypothyroidism.

34
Q

Hypothyroidisminadultanimals is classed as either

A

primary or central

35
Q

Inabout95%ofcasesofadultonset hypothyroidism,itis

A

aprimarythyroiddisorder.

36
Q

Inabout5%or less of casesofadultonset hypothyroidism,itisdue to

A

a TSH deficiency (so pituitary or hypothalamic).

37
Q

Primaryhypothyroidism pathogenesis is typically due to

A

progressiveautoimmuneprocesswhichleadstolymphocyticinfiltrationanddisappearanceofthyroidfollicles.

Theimmune‐mediateddestructionisaslowprocessandclinicalmanifestationsofthyroidhormonedeficiencyonlybecomeevidentafterdestructionofabout75%ofthethyroidfollicles.

38
Q

In primary hypothyroidism, as theautoimmunedestructionprogresses,thyroidfolliclesarereplacedby

A

fibrousandadiposetissueandtheinflammatorycellsdisappear,resultingin
thehistologicalappearanceofnoninflammatoryatrophy.

39
Q

polyglandularfailuresyndrome

A

involve deficiencies in the function of several endocrine glands, which may occur simultaneously or sequentially.

E.g. in primary hypothyroidism theimmunologicdamagemayalsoinvolveoneormoreotherendocrineglandsandleadtomultipleendocrinedeficiencies,knownaspolyglandularfailuresyndrome.

40
Q

Schmidt’ssyndrome

A

Thecombinationofhypothyroidismandhypoadrenocorticism (Addison’s)isknownasSchmidt’ssyndrome.

a polyglandualr failure syndrome

Sometiems includes type 1 diabetes mellitus too.

41
Q

Iatrogenichypothyroidism

A

Iatrogenic hypothyroidism (IH) results when treatment for hyperthyroidism causes suppression of total thyroxine (T4) below normal levels.

occurs speciallyincatstreatedforhyperthyroidism

Thehypothyroidismmaybeanadverseeffectofradioiodinetherapyorbilateralsurgicalthyroidectomy.

42
Q

Acquiredprimaryhypothyroidism typical to what type of animal

A

aconditionofyoung‐adultandmiddle‐ageddogs.

dogsoflargebreedsmaybeaffectedmorefrequentlythanthoseofsmallbreeds.

thereisnopronouncedbreedpredispositionandisequallydistributedbetweenmalesandfemales.

either due to lymphocytic thyroiditis or thyroid gland atrophy.

43
Q

Clinicalmanifestationsofprimaryhypothyroidisminadult dogs

A

weight gain
course coat, nonpruritic symmetrical alopecia
myxedema (skin thickening)
bradycardia
lethargy
persistent anestrus

44
Q

95% of hypothyroidism is

A

primary hypothyroidism meaning the issue is on the glandular level

e.g. autoimmune thyroiditis, thyroid tumor etc.

45
Q

secondary hypothyroidism refers to

A

inadequate TSH secretion or thyroid trauma, cysts/nodules/tumors etc.

46
Q

tertiary hypothyroidism refers to ?

A

DeficiencyinthesecretionofTRH(extremelyrareindogs)

(thyrotropin releasing hormone)

47
Q

Hyperthyroidism in cats is typically caused by

A

Athyroidhormoneexcessproducedbythyroidadenomatoushyperplasiaoradenoma,involvingoneor,moreoften,boththyroidlobes.

48
Q

Thyroidcarcinoma, in dogs vs cats

A

ThyroidcarcinomaisthemaincauseofhypeRthyroidismindogs,but accountsforonly3%ofcasesincats.

Feline hyperthyroidism is typically caused by thyroid adenomatous hyperplasia or thyroid adenomas.

49
Q

Theclassicpresentationofahyperthyroidcatis

A

a skinny,restless,elderlycatwithanincreasedappetiteandpolyuria.

50
Q

Clinicalmanifestationsofhyperthyroidismincats (6)

A

weight loss despite polyphagia
tachycardia with potential LV hypertrophy
restlessness
panting
poor hair coat
polyuria

leukocytosis
elevated ALT etc.

51
Q

Inabout10%offeline hyperthyroid casestheclinicalpicturemaybe atypical in what way?

A

Inthesecatsweightlossremainsanimportantfeature butthereislethargyandanorexiaratherthanhyperactivityandincreasedappetite.

This form is called apathetic hyperthyroidism.

52
Q

“apathetichyperthyroidism”,andmayrepresent what?

A

anend‐stageofthediseaseandmayalsobeassociatedwithcardiacdisorders.

53
Q

“thyroidstorm”

A

Thyroid storm (also called thyroid crisis and thyrotoxic crisis) happens when your thyroid gland releases a large amount of thyroid hormone in a short amount of time. It’s a rare complication of hyperthyroidism. Thyroid storm is a medical emergency and is life-threatening.

Thesevereformoffelinehyperthyroidismhasalsobeencalled
“thyroidstorm”.

54
Q

Radioactiveiodinetherapy,thyroidsurgery,vigorousthyroid
palpation,andstressmaycause

A

acuteelevationofplasmathyroid
hormoneconcentrationandhavebeenimplicatedaspossible
precipitatingfactorsfor“thyroidstorm”.

55
Q

diagnosis of feline hyperthyroidism (3)

A

clinical picture
palpation of thyroid for enlargement
directmeasurementofthyroidfunction, biochemistry

56
Q

Radioiodineuptakestudies

A

Radioactive Iodine Uptake, or RAIU, is a test of thyroid function. The test measures the amount of radioactive iodine (taken by mouth) that accumulates in the thyroid gland.

57
Q

Scintigraphy

A

Scintigraphy, also known as a gamma scan, is a diagnostic test in nuclear medicine, where radioisotopes attached to drugs that travel to a specific organ or tissue (radiopharmaceuticals) are taken internally and the emitted gamma radiation is captured by external detectors (gamma cameras) to form two-dimensional images in a similar process to the capture of x-ray images.

58
Q

TherearethreeoptionsforeliminatingtheexcessproductionofT4:

A

(1)radioiodineablationofthethyroid

(2)surgicalthyroidectomy

(3)inhibitionofsecretionbyantithyroiddrugs(methimazole,carbimazole)

the first option is preferable if available

59
Q

How does radioiodine therapy of the thyroid work?

A

Radioiodine(131I)byitsβ‐radiationselectivelydestroyshyperfunctioningthyroidcellswhilesparingthesuppressednormalthyroidtissueandtheparathyroidglands.

Thenormalfolliclesgraduallyresumefunctionandthereisusuallynoneedforadministrationofthyroxine.

prognosis is excellent if no concurrent disease (renal etc.). recurrence possible.

60
Q

Thyroidneoplasiaaccountsforwhat%ofallcaninetumors.

A

about2%ofallcaninetumors.

Mostofthebenigntumors(adenomas)aresmallandcommonlynot detectedduringlife. Benign can also cause hyperthyroidisim.

61
Q

Thyroidtumorsarisenotonlyfromfollicularepitheliumbutalsofrom

A

theparafollicularCcells.

Theseso‐calledmedullarythyroidtumorsarerelativelyrareindogs.

62
Q

Clinicalfeatures of dogs presenting with thyroid tumors (4)

A

meanageisnineyears(range5–15years)
Boxersareoverrepresented.
Thereisnosexpredilection.

Thesignsandsymptomsaredueto:
* (1)thyroidenlargement
* 2)hypersecretionofthyroidhormones.

63
Q

What is a “cold” thyroid tumor?

A

Thyroid nodules that do not absorb the radioiodine are called cold.

typically not hormonally active

64
Q

Alargeandinvasivethyroid tumormayevendamagethecervicalsympathetictrunk,causing what

A

Horner’ssyndrome

65
Q

ManifestationsofNONhypERfunctioning thyroidtumorsindogs (?)

A

a usually large unilateral tumor is common.

can be accompanied by weight loss, resp. distress, dysphagia, anorexia, horner’s syndrome

66
Q

Hypersecretionofthyroidhormoneoccursinwhat percentage of canine thyroid tumors?

A

about10%ofcasesofthyroidtumorindogs.

Itmayresultinthesyndromeofhyperthyroidism,verysimilartothatcatsbutoftenlesssevere.

67
Q

Manifestationsofhyperfunctioningthyroidtumorsindogs (?)

A
68
Q

Occasionallythereresymptomsofhyperthyroidismwithoutpalpable thyroid
enlargement,inwhichcasewhat should be considered?

A

anintrathoracichyperfunctioning
tumorinectopicthyroidtissue
shouldbeconsidered.

69
Q

AlowplasmaT4andhighplasmaTSH,indicates?

A

hypothyroidism

canbefoundindogsinwhichthenormalthyroidtissueisreplacedbybilateralthyroidcarcinomaorpreexistingthyroiditis.

70
Q

highplasmaT4andlowplasma
TSH, indicates?

A

hyperthyroidism from e.g. hyperfunctioning thyroid tumor

71
Q

treatment of Hyperfunctioningthyroidtumors (in dogs)

A

undelayed surgical resection if possible.

Administrationofradioiodide isanalternative.

Chemotherapy witheitherdoxorubicinorcisplatinmaybeconsideredindogswithahighriskofdevelopingmetastases,namelythosewithlargeandbilateralthyroidcarcinoma.

72
Q

Thyroid‐cellproliferationisdependent on what

A

TSHdependentandcarcinomatousthyrocytesdohaveTSHreceptors too.

prognosisof treatment of thyroid carcinoma in dogs canbeinfluencedfavorablybyTSH‐suppressivetreatmentwithl‐thyroxine.

73
Q

TSH‐suppressivetreatment hastwoobjectives:

A

(1)hormonereplacement(correctionofinducedhypothyroidism)
(2)hormonesuppression(reductionofplasmaTSHlevelsthatmightstimulate
growthofpersistentorrecurrentneoplastictissue).

74
Q

what mediators inhibitTSHrelease

A

Somatostatinanddopamine releasedbyhypothalamicneurons inhibitTSH
release

75
Q

What plasma proteins in addition to thyroid-binding globulin (TBG) bind thyroid hormones?

A
  • Transthyretin
  • Albumin
  • Prealbumin(primarybinderincats).
  • Lipoproteins

Protein‐bound hormones areinactive,servingasa reservoir.

Freeandboundformsareinan equilibrium →levelsoffreehormoneremainfairlyconstant.

76
Q

Ofthetotalthyroidhormoneboundtointracellular receptors, what percentage is T3

A

~90%

77
Q

explaincentralhypothyroidism

A

thethyroid isnotaffectedprimarilybutaredeprivedofstimulationbyTSH.
Theconditionisrarecomparedwithprimarythyroidfailure.
Spontaneouscausesincludetumorofthepituitaryoradjacentregionsandheadtrauma.

Centralhypothyroidismcanalsoresultfromsurgicalremovalofapituitarytumor.

Endocrinology (16 decks)

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