Endocrinology: Thyroid

Question 1

Hormonal cascade to produce thyroid hormones

Answer 1

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

Question 2

What are T4 and T3 and which one is made more

Answer 2

T4 is made in preference
T3 = triiodothyronine
T4 = thyroxine

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

Question 3

Functions of thyroid hormones

Answer 3

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

Question 4

Types of primary hypothyroidism

Answer 4

Acquired causes 95% of cases
Congenital

Question 5

Types of acquired primary hypothyroid disease

Answer 5

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

+ rarely neoplasia, medication e.g potentiated sulphonamides

Question 6

What medication might cause primary acquired hypothyroidism

Answer 6

Potentiated sulphonamides

Question 7

What would a congenital hypothyroid dog look like

Answer 7

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

Question 8

What causes secondary/central hypothyroidism

Answer 8

Pituitary dysfunction

Question 9

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

Answer 9

Signalment - middle aged pedigree dogs

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

Question 10

Why do we see seborrhoea and pyoderma in some hypothyroid cases

Answer 10

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

Question 11

What is myxoedema

Answer 11

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

Question 12

Biochemistry changes in hypothyroidism

Answer 12

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

Question 13

Haematological changes in hypothyroidism

Answer 13

Mild non-regenerative anaemia, may see target cells

Question 14

What are the mainstay tests for hypothyroidism

Answer 14

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)

Question 15

What might cause a decrease in serum T4 other than hypothyroidism

Answer 15

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

Question 16

What might cause increase in TSH that isn’t hypothyroidism

Answer 16

Sulphonamides
Anti-T4 antibodies
Non-thyroidal illness recovery

Question 17

Why might measuring free T4 be a good idea

Answer 17

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

Question 18

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

Answer 18

Not really
Some think it is an early marker of hypothyroidism

Question 19

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

Answer 19

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

Question 20

What is goiter

Answer 20

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

Question 21

How can dietary iodide excess cause goitre

Answer 21

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

Question 22

How does thyroid hormone production work within the gland

Answer 22

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

Question 23

What is the histopathological changes seen in lymphocytic thyroiditis

Answer 23

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

Question 24

Thyroid atrophy histopath

Answer 24

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

Question 25

What signs might longstnading hypercholesterolaemia as part of hypothyroidism lead to

Answer 25

Atherosclerosis Hepatomegaly Renal glomerular lipidosis Corneal lipidosis

Question 26

What is myxoedema is a specific sign of

Answer 26

Hypothyroidism; specific but rare

Question 27

What type of thyroid neoplasms are seen in dogs vs cats

Answer 27

In dogs mostly carcinoma In cats mostly adenoma

Question 28

What are causes of hyperthyroidism

Answer 28

Main ones = bilateral thyroid hyperplasia or adenomas Rarely <5%: thyroid carcinomas

Question 29

What are the risk factors of hyperthyroidism in cats

Answer 29

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

Question 30

Clinical signs of hyperthyroidism

Answer 30

Main one = weight loss despite ravenous appetite (polyphagia) Tachycardia, cardiac abnormalities, hyperactivitiy Can feel a palpable goitre in almost all affected cats

Question 31

Blood work changes in a hyperthyroid cat

Answer 31

- 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

Question 32

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

Answer 32

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

Question 33

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

Answer 33

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

Question 34

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

Answer 34

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

Question 35

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

Answer 35

Due to non-thyroidal illness suppressing the T4 level

Question 36

How does a T3 suppression test work

Answer 36

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

Question 37

What can thyroid scintigraphy be useful for

Answer 37

Detecting ectopic thyroid tissue

Question 38

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

Answer 38

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

Question 39

What cardiac effect might hyperthyroidism be associated with

Answer 39

Reversible hypertrophic cardiomyopathy

Question 40

How do we monitor medical management of hyperthyroidism

Answer 40

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

Question 41

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

Answer 41

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

Question 42

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

Answer 42

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

Question 43

What drugs are used to medically manage hyperthyroidism

Answer 43

Carbimazole (gets metabolised to methimazole) Methimazole

Question 44

What can we use to perform radioablation of thyroid tissue

Answer 44

Iodine 131`

Question 45

How do hyperthyroid diets help

Answer 45

Contain very low levels of iodine

Question 46

Where do we look for metastasis from thyroid carcinomas

Answer 46

Cranial cervical LNs then retropharyngeal LNs then lungs BECAUSE the drainage is cranial

Question 47

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

Answer 47

Palpable goitre (otherwise might be ectopic tissue)

Question 48

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

Answer 48

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

Question 49

Where do we autotransplant the parathyroid to if accidentally removed

Answer 49

Sternohyoid muscle

Question 50

What ion is especially important to monitor after thyroid removal

Answer 50

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

Question 51

How does canine hyperthyroidism present

Answer 51

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

Question 52

Why wouldnt we do an incisional biopsy on a thyroid mass

Answer 52

RIsk of uncontrollable bleeding + spreading tumour via tract

Question 53

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

Answer 53

Can't see parathyroid to avoid removing it

Question 54

What is the prognosis like for carcinomas of the thyroid

Answer 54

Good since slow growing and slow to metastasise

Question 55

What is used to treat hypothyroidism mediaclly

Answer 55

Levothyroxine sodium (in form of thyforon, leventa)