Lect. 4 - Thyroid

Question 1

cretinism

Answer 1

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.

Question 2

predominant structures in the thyroid glands

Answer 2

thyroid follicles filled with protein rich colloid

Question 3

thyroid medullary cells produce what

Answer 3

also known as C cells

produce calcitonin

Question 4

identify the molecule

Answer 4

thyroxine, T4

Theterm‘thyroidhormone’
encompassesbothT3and T4.

Question 5

identify the molecule

Answer 5

triiodothyronine or T3

Theterm‘thyroidhormone’encompassesbothT3and T4.

Question 6

themainprecursortothyroid
hormones

Answer 6

Thyroglobulin

is the main precursor to thyroid hormones.

Iodineistransferredacross
theepithelium
intothethyroid follicular lumen
whereitisconjugated
tothyroglobulin.

Question 6

percentage output of T4 and T3 from thyroid gland

Answer 6

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

Question 7

What secretes thyrotropin releasing hormone?

Answer 7

NeuronsofthehypothalamussecreteTRH whichiscarriedtothe anteriorpituitary.

Question 8

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

Answer 8

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

Question 9

Thehypothalamic‐pituitary‐thyroid(HPT)axis

Answer 9

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

Question 10

Function of TSH

Answer 10

TSHbindstoreceptorsonthebasolateral

membraneofthyroidfollicularcells,

stimulates the synthesis and

release of T4andT3.

Question 11

Cell receptors for T3 & T4 are located where?

Answer 11

areintracellular so T3 andT4 arehydrophobic/lipophilic(cancrosscellmembranes)

Question 12

thyroid hormone primary binding protein differs between species

Answer 12

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

Question 13

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

Answer 13

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)

Question 14

the most significant metabolic transformation of the thyroid
hormones is

Answer 14

deiodination

About80%ofthesecreted
T4isdeiodinated toformT3andreverse-T3,
predominantlyin
theliverandkidney.

Question 15

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

Answer 15

plasma reverseT3 concentration.

Question 16

name 2 FactorsthatimpairT3formation

Answer 16

fastingandnonthyroidaldisease,
almostinvariablyincrease
plasmarT3concentration.

Illnessleadstoincreased
activityoftype3
deiodinase(D3),
whichprimarily
deiodinates theinnerring.

Question 17

T4bindsmoretightlyto
bindingproteinsinplasmathandoes what?

Answer 17

T3,whichresultsinT4havinga
lowermetabolicclearance
rateandlongerhalf‐life.

Question 18

Overall,thekineticsofthyroid
hormonedistributionand
turnoveraremuchmorerapidin

Answer 18

dogsthaninhumans,
inpartbecauseofthe
lowerbindingofbothT4
andT3incanineplasma.

Theplasmahalf-lifeofT4isabout
0.6daysindogs,
comparedtoabout
sevendaysinhumans.

Thus, levothyroxine(T4)
therapeuticdosage
ishigherindogs
comparedtohumans.

Question 19

T3orT4activatesthyroidhormonereceptorsandregulatesthetranscriptionofthesegenes to produce proteins with what effects?

Answer 19

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

Promote nervoussystem and skeletaldevelopment.

Question 20

Clinically,thyroidhormones help maintainnormal:

Answer 20

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

Inmanyrespectsthyroidhormonescanbeviewedastissuegrowthfactors,thisbeingbestexemplifiedbytheconsequencesofthyroidhormonedeficiencyatayoungage.

Question 21

Hypothyroidisminyounganimals, name the potential diseases (4)

Answer 21

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

Question 22

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

Answer 22

hypothyroidism due to iodine deficiency.

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

Question 23

Earlyinlifethepresenceofthyroid
hormonesiscrucialfor

Answer 23

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.

Question 24

cause of Acquired juvenile hypothyroidism

Answer 24

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

Question 25

lack of iodine results in

Answer 25

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.

Question 26

Lymphocytic thyroiditis is

Answer 26

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

Question 27

Thyroid dysgenesis

Answer 27

the term used to describe abnormalities in thyroid gland development Ectopia of thyroid tissue is common in the dog and is also known to occur in cats. Ectopia meaning the thyroid tissue is outside of the gland. Gland function may be insufficient to prevent hypothyroidism. Complete athyreosis has also been found meaning no thyroid gland whatsoever present (but in some cases thyroid tissue can still be present somewhere).

Question 28

Abnormalities in the newborn that may suggest hypothyroidism include: (5)

Answer 28

A large fontanel (soft spot in the skull which should be closed at birth in dogs but not in cats) * Hypothermia * Hypoactivity * Suckling difficulties * Abdominal distension

Question 29

besides dysngenesis, Congenital hypothyroidism may occur because of

Answer 29

an enzyme deficiency that prevents synthesis of thyroid hormones. Such congenital defects are rare and although in principle any step in thyroid hormone synthesis may be affected. Only unresponsiveness to TSH and defective peroxidase activity (thyroid hormone is not synthesized) have been found thus far in the dog and the cat.

Question 30

Central (secondary) hypothyroidism is due to

Answer 30

thyroid-stimulting hormone deficiency

Question 31

central hypothyroidism can be classified as one of two

Answer 31

pituitary (secondary hypothyroidism) hypothalamic (tertiary hypothyroidism) (primary would indicate an issue with the gland itself)

Question 32

Hyposecretion of TSH is usually accompanied by what other hormonal issues?

Answer 32

Hyposecretion of TSH is usually accompanied by decreased secretion of other pituitary hormones. e.g. lack of GH usually go hand in hand with lack of T4 " combined deficiency of  adenohypophyseal hormones." (e.g. in the dwarf animals)

Question 33

hypothyroidism due to central causes is less severe than

Answer 33

 primary hypothyroidism.

Question 34

Hypothyroidism in adult animals is classed as either

Answer 34

primary or central

Question 35

In about 95% of cases of adult onset hypothyroidism, it is 

Answer 35

a primary thyroid disorder.

Question 36

In about 5% or less of cases of adult onset hypothyroidism, it is due to

Answer 36

a TSH deficiency (so pituitary or hypothalamic).

Question 37

Primary hypothyroidism pathogenesis is typically due to

Answer 37

progressive autoimmune process which leads to lymphocytic infiltration and disappearance of thyroid follicles. The immune‐mediated destruction is a slow process and clinical manifestations of thyroid hormone deficiency only become evident after destruction of about 75% of the thyroid follicles.

Question 38

In primary hypothyroidism, as the autoimmune destruction progresses, thyroid follicles are replaced by 

Answer 38

fibrous and adipose tissue and the inflammatory cells disappear, resulting in  the histological appearance of noninflammatory atrophy.

Question 39

 polyglandular failure syndrome

Answer 39

involve deficiencies in the function of several endocrine glands, which may occur simultaneously or sequentially. E.g. in primary hypothyroidism the immunologic damage may also involve one or more other endocrine glands and lead to multiple endocrine deficiencies, known as polyglandular failure syndrome. 

Question 40

 Schmidt’s syndrome

Answer 40

The combination of hypothyroidism and hypoadrenocorticism (Addison's) is known as Schmidt’s syndrome. a polyglandualr failure syndrome Sometiems includes type 1 diabetes mellitus too.

Question 41

Iatrogenic hypothyroidism

Answer 41

Iatrogenic hypothyroidism (IH) results when treatment for hyperthyroidism causes suppression of total thyroxine (T4) below normal levels. occurs specially in cats treated for hyperthyroidism The hypothyroidism may be an adverse effect of radioiodine therapy or bilateral surgical thyroidectomy.

Question 42

Acquired primary hypothyroidism typical to what type of animal

Answer 42

a condition of young‐adult and middle‐aged dogs. dogs of large breeds may be affected more frequently than those of small breeds. there is no pronounced breed predispositionand is equally distributed between males and females. either due to lymphocytic thyroiditis or thyroid gland atrophy.

Question 43

Clinical manifestations of primary hypothyroidism in adult dogs

Answer 43

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

Question 44

95% of hypothyroidism is

Answer 44

primary hypothyroidism meaning the issue is on the glandular level e.g. autoimmune thyroiditis, thyroid tumor etc.

Question 45

secondary hypothyroidism refers to

Answer 45

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

Question 46

tertiary hypothyroidism refers to ?

Answer 46

Deficiency in the secretion of TRH (extremely rare in dogs) (thyrotropin releasing hormone)

Question 47

Hyperthyroidism in cats is typically caused by

Answer 47

A thyroid hormone excess produced by thyroid adenomatous hyperplasia or adenoma, involving one or, more often, both thyroid lobes. 

Question 48

Thyroid carcinoma, in dogs vs cats

Answer 48

Thyroid carcinoma is the main cause of hypeRthyroidism in dogs, but accounts for only 3 % of cases in cats. Feline hyperthyroidism is typically caused by thyroid adenomatous hyperplasia or thyroid adenomas.

Question 49

The classic presentation of a hyperthyroid cat is 

Answer 49

a skinny, restless, elderly cat with an increased appetite and polyuria.

Question 50

Clinical manifestations of hyperthyroidism in cats (6)

Answer 50

weight loss despite polyphagia tachycardia with potential LV hypertrophy restlessness panting poor hair coat polyuria leukocytosis elevated ALT etc.

Question 51

In about 10% of feline hyperthyroid cases the clinical picture may be atypical in what way?

Answer 51

In these cats weight loss remains an important feature but there is lethargy and anorexia rather than hyperactivity and increased appetite. This form is called apathetic hyperthyroidism.

Question 52

 “apathetic hyperthyroidism”, and may represent what?

Answer 52

an end‐stage of the disease and may also be associated with cardiac disorders.

Question 53

“thyroid storm”

Answer 53

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. The severe form of feline hyperthyroidism has also been called “thyroid storm”.

Question 54

Radioactive iodine therapy, thyroid surgery, vigorous thyroid  palpation, and stress may cause

Answer 54

acute elevation of plasma thyroid  hormone concentration and have been implicated as possible  precipitating factors for “thyroid storm”.

Question 55

diagnosis of feline hyperthyroidism (3)

Answer 55

clinical picture palpation of thyroid for enlargement direct measurement of thyroid function, biochemistry

Question 56

Radioiodine uptake studies

Answer 56

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.

Question 57

Scintigraphy

Answer 57

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.

Question 58

There are three options for eliminating the excess production of T4:

Answer 58

(1) radioiodine ablation of the thyroid (2) surgical thyroidectomy (3) inhibition of secretion by antithyroid drugs (methimazole, carbimazole) the first option is preferable if available

Question 59

How does radioiodine therapy of the thyroid work?

Answer 59

Radioiodine (131I) by its β‐radiation selectively destroys hyperfunctioning thyroid cells while sparing the suppressed normal thyroid tissue and the parathyroid glands.  The normal follicles gradually resume function and there is usually no need for administration of thyroxine. prognosis is excellent if no concurrent disease (renal etc.). recurrence possible.

Question 60

Thyroid neoplasia accounts for what % of all canine tumors.

Answer 60

about 2 % of all canine tumors. Most of the benign tumors (adenomas) are small and commonly not detected during life. Benign can also cause hyperthyroidisim.

Question 61

Thyroid tumors arise not only from follicular epithelium but also from 

Answer 61

the parafollicular C cells. These so‐called medullary thyroid tumors are relatively rare in dogs.

Question 62

Clinical features of dogs presenting with thyroid tumors (4)

Answer 62

mean age is nine years (range 5–15 years) Boxers are overrepresented. There is no sex predilection. The signs and symptoms are due to: * (1) thyroid enlargement * 2) hypersecretion of thyroid hormones.

Question 63

What is a "cold" thyroid tumor?

Answer 63

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

Question 64

A large and invasive thyroid tumor may even damage the cervical sympathetic trunk, causing what

Answer 64

 Horner’s syndrome

Question 65

Manifestations of NONhypERfunctioning thyroid tumors in dogs (?)

Answer 65

a usually large unilateral tumor is common. can be accompanied by weight loss, resp. distress, dysphagia, anorexia, horner's syndrome

Question 66

Hypersecretion of thyroid hormone occurs in what percentage of canine thyroid tumors?

Answer 66

about 10% of cases of thyroid tumor in dogs. It may result in the syndrome of hyperthyroidism, very similar to that cats but often less severe.

Question 67

Manifestations of hyperfunctioning thyroid tumors in dogs (?)

Answer 67

Question 68

Occasionally therere symptoms of hyperthyroidism without palpable thyroid  enlargement, in which case what should be considered?

Answer 68

an intrathoracic hyperfunctioning  tumor in ectopic thyroid tissue  should be considered.

Question 69

A low plasma T4 and high plasma TSH, indicates?

Answer 69

hypothyroidism can be found in dogs in which the normal thyroid tissue is replaced by bilateral thyroid carcinoma or preexisting thyroiditis. 

Question 70

high plasma T4 and low plasma  TSH, indicates?

Answer 70

hyperthyroidism from e.g. hyperfunctioning thyroid tumor

Question 71

treatment of Hyperfunctioning thyroid tumors (in dogs)

Answer 71

undelayed surgical resection if possible. Administration of radioiodide is an alternative. Chemotherapy with either doxorubicin or cisplatin may be considered in dogs with a high risk of developing metastases, namely those with large and bilateral thyroid carcinoma.

Question 72

Thyroid‐cell proliferation is dependent on what

Answer 72

TSH dependent and carcinomatous thyrocytes do have TSH receptors too. prognosis of treatment of thyroid carcinoma in dogs can be influenced favorably by TSH‐suppressive treatment with l‐thyroxine.

Question 73

TSH‐suppressive treatment has two objectives:

Answer 73

(1) hormone replacement (correction of induced hypothyroidism) (2) hormone suppression (reduction of plasma TSH levels that might stimulate  growth of persistent or recurrent neoplastic tissue). 

Question 74

what mediators inhibit TSH release

Answer 74

Somatostatin and dopamine released by hypothalamic neurons inhibit TSH  release

Question 75

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

Answer 75

* Transthyretin * Albumin * Prealbumin (primary binder in cats).  * Lipoproteins Protein‐bound hormones are inactive, serving as a reservoir. Free and bound forms are in an equilibrium → levels of free hormone remain fairly constant.

Question 76

Of the total thyroid hormone bound to intracellular receptors, what percentage is T3 

Answer 76

~90%

Question 77

explain central hypothyroidism

Answer 77

the thyroid is not affected primarily but are deprived of stimulation by TSH. The condition is rare compared with primary thyroid failure. Spontaneous causes include tumor of the pituitary or adjacent regions and head trauma. Central hypothyroidism can also result from surgical removal of a pituitary tumor.