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Michaelmas 5th year > Thyroid > Flashcards

Thyroid Flashcards

1
Q

Hormonal cascade to produce thyroid hormones

A

Hypothalamus secretes TRH which stimulates anterior pituitary to make TSH
TSH stimulates the thyroid glands to produce T4 and T3

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2
Q

What are T4 and T3 and which one is made more

A

T4 is made in preference
T3 = triiodothyronine
T4 = thyroxine

T3 is the metabolically active form; T4 is converted to T3 in tissues

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3
Q

Functions of thyroid hormones

A

Control metabolism; increase metabolic rate
Stimulates growth phase of hair follicles
Stimulates haematopoiesis
STimulates heart sympathetically

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4
Q

Types of primary hypothyroidism

A

Acquired causes 95% of cases
Congenital

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5
Q

Types of acquired primary hypothyroid disease

A

50% = lymphocytic autoimmune thyroisitis
Almost 50% = idiopathic necrosis and fibrosis

+ rarely neoplasia, medication e.g potentiated sulphonamides

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6
Q

What medication might cause primary acquired hypothyroidism

A

Potentiated sulphonamides

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7
Q

What would a congenital hypothyroid dog look like

A

Disproportionate dwarf
= stunted with large heads, protruding tongue, small limbs, dull, alopecia

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8
Q

What causes secondary/central hypothyroidism

A

Pituitary dysfunction

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9
Q

Main clinical signs in classical hypothyroidism + what types of dogs are predisposed (signalment)

A

Signalment - middle aged pedigree dogs

Signs = lethargy, weight gain, alopecia + some other skin signs

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10
Q

Why do we see seborrhoea and pyoderma in some hypothyroid cases

A

Lack of thyroid hormone causes decreased humoral immune response and impair T cell function

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11
Q

What is myxoedema

A

Not pitting puffiness of skin in hypothyroid cases; due to deposition of hyaluronic acid in dermis
Called ‘tragic facial expression’

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12
Q

Biochemistry changes in hypothyroidism

A

Increased serum cholesterol is main one, increased triglycerides, some hepatic lipidosis (so get mild increases in liver enzymes), mild hyperglycaemia

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13
Q

Haematological changes in hypothyroidism

A

Mild non-regenerative anaemia, may see target cells

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14
Q

What are the mainstay tests for hypothyroidism

A

Serum T4: good as a screening test; if normal T4 then unlikely to have hypothyroidism
BUT lots of other conditions cause decreased T4

TSH (NB: 20% of hypothyroid dogs have normal TSH)

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15
Q

What might cause a decrease in serum T4 other than hypothyroidism

A

Illness, medications e.g trimethoprim sulphonamides, prednisolone, furosemide, phenobarbitone, anaesthesia
+ specific age/breed

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16
Q

What might cause increase in TSH that isn’t hypothyroidism

A

Sulphonamides
Anti-T4 antibodies
Non-thyroidal illness recovery

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17
Q

Why might measuring free T4 be a good idea

A

= biologically active non-protein bound T4
Less affected by non-thyroidal illnesses

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18
Q

Is anti-thyroglobulin antibodies a good thing to measure to look for hypothyroidism

A

Not really
Some think it is an early marker of hypothyroidism

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19
Q

What order do we expect to see clinical signs change in response to thyroid supplementation

A

First get improvement in lethargy and mentation, body weight reduction within 2 weeks, some skin improvements in 3 months

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20
Q

What is goiter

A

Non-neoplastic and non-inflammatory clinical enlargement of the thyroid gland

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21
Q

How can dietary iodide excess cause goitre

A

High blood iodide interferes with thyroid hormone synthesis so get low blood T4 and T3; then end up with compensatory increase in TSH secretion –> gland hyperplasia

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22
Q

How does thyroid hormone production work within the gland

A

Follicles are lined by cuboidal epithelial cells and filled with colloid and contain thyroglobin

Cuboidal epithelial cells take up iodine; enzyme thyroid peroxidase is involved

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23
Q

What is the histopathological changes seen in lymphocytic thyroiditis

A

Mononuclear cell infiltrate in the thyroid (mostly lymphocytes); starts to expand area between follicles and get fibrosis
- Thyroid may be enlarged or atrophic

24
Q

Thyroid atrophy histopath

A

Follicular cells shrink and stop making colloid
Gland replaced by fatty connective tissue

25
Q

What signs might longstnading hypercholesterolaemia as part of hypothyroidism lead to

A

Atherosclerosis
Hepatomegaly
Renal glomerular lipidosis
Corneal lipidosis

26
Q

What is myxoedema is a specific sign of

A

Hypothyroidism; specific but rare

27
Q

What type of thyroid neoplasms are seen in dogs vs cats

A

In dogs mostly carcinoma
In cats mostly adenoma

28
Q

What are causes of hyperthyroidism

A

Main ones = bilateral thyroid hyperplasia or adenomas

Rarely <5%: thyroid carcinomas

29
Q

What are the risk factors of hyperthyroidism in cats

A

Older cats (12 years), indoor cats esp using litter tray

30
Q

Clinical signs of hyperthyroidism

A

Main one = weight loss despite ravenous appetite (polyphagia)

Tachycardia, cardiac abnormalities, hyperactivitiy

Can feel a palpable goitre in almost all affected cats

31
Q

Blood work changes in a hyperthyroid cat

A
  • Elevated liver enzymes (unlike dogs don’t get raised by other non-specific things as often as dogs)
  • Mild to moderate azotaemia
  • Increase in PCV, RBC, Hb
  • Hyperphosphataemia
32
Q

Why might we see azotaemia when we start to treat hyperthyroidism in cats

A

Because the hyperthyroidism was masking CKD by artificially increasing sympathetic tone, CO and blood pressure so GFR is artifically high

33
Q

What is the first and second line tests for hyperthyroidism in cats

A

First go for total T4: 90% of hyperthyroid cats have elevated total T4
BUT can’t rule out hyperthyroidism from this

Then test free T4 which is more sensitive; 98% of hyperthyroid cats have elevated levels

34
Q

Why don’t we use free T4 as an initial test for hyperthyroidism

A

It is less specific than total T4; elevated in >10% of sick BUT euthyroid cats

35
Q

Why do we see normal total T4 levels in ~10% of hyperthyroid cats

A

Due to non-thyroidal illness suppressing the T4 level

36
Q

How does a T3 suppression test work

A

Measure T4; then give oral T3
Measure T4 a few days later
–> If cat is hyperthyroid, T4 won’t be very suppressed but would be in euthyroid cats

37
Q

What can thyroid scintigraphy be useful for

A

Detecting ectopic thyroid tissue

38
Q

Why do we measure blood pressure before and during hyperthyroid disease treatment

A

Because some hyperthyroid cats start in a relatively vasodilated state and then as it is managed, vascular diameter is normalited and they then become hypertensive

39
Q

What cardiac effect might hyperthyroidism be associated with

A

Reversible hypertrophic cardiomyopathy

40
Q

How do we monitor medical management of hyperthyroidism

A

Measure serum total T4 about 2 weeks after starting therapy or after changing a dose
Then once stable check every 3-6 months

41
Q

What is the aim with controlling serum T4 to when treating hyperthyroidism

A

Want to keep within the lower half of the reference interval
- Don’t want to make them hypothyroid because it makes them more likely to develop azotaemia

42
Q

What are some adverse effects of drugs for hyperthyroidism (carbimazole, methimazole)

A

Self-limiting GI effects
Mild, transient haematological abnormalities
Self induced excoriations of head and neck (should stop drug)
+ some serious effects

43
Q

What drugs are used to medically manage hyperthyroidism

A

Carbimazole (gets metabolised to methimazole)
Methimazole

44
Q

What can we use to perform radioablation of thyroid tissue

A

Iodine 131`

45
Q

How do hyperthyroid diets help

A

Contain very low levels of iodine

46
Q

Where do we look for metastasis from thyroid carcinomas

A

Cranial cervical LNs then retropharyngeal LNs then lungs

BECAUSE the drainage is cranial

47
Q

What is a necessary symptom for surgery to be an option for hyperthyroidism

A

Palpable goitre
(otherwise might be ectopic tissue)

48
Q

If one thyroid gland has goitre, what do we expect the other one to look like

A

Atrophied as responding to negative feedback
- If like this then don’t remove; if large then only remove if able to preserve the parathyroid on the other side

49
Q

Where do we autotransplant the parathyroid to if accidentally removed

A

Sternohyoid muscle

50
Q

What ion is especially important to monitor after thyroid removal

A

Calcium; due to risk of removing parathyroid
Esp if bilateral thyroidectomy or parathyroid autotransplantation

51
Q

How does canine hyperthyroidism present

A

Firm, non-painful mobile mass in cervical region
Non-functional so often incidental finding

52
Q

Why wouldnt we do an incisional biopsy on a thyroid mass

A

RIsk of uncontrollable bleeding + spreading tumour via tract

53
Q

Why do we need lifelong calcium supplementation when removing bilateral carcinoma thyroid (dog)

A

Can’t see parathyroid to avoid removing it

54
Q

What is the prognosis like for carcinomas of the thyroid

A

Good since slow growing and slow to metastasise

55
Q

What is used to treat hypothyroidism mediaclly

A

Levothyroxine sodium (in form of thyforon, leventa)

56
Q
A

Michaelmas 5th year (41 decks)

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