Hypothyroidism

Question 1

Structure of the thyroid gland

Answer 1

Two glands are arranged in follicles, filled with ‘colloid’ containing thyroglobulin.

Question 2

Production of thyroid hormone

Answer 2

Synthesised from two connected tyrosine molecules, containing 3 or 4 iodine molecules.

Follicle cells ‘trap’ iodide by active transport, it is then incorporated into thyroglobulin uring thyroid peroxidase.

Question 3

Body systems affected by thyroid hormone

Answer 3

Metabolism
Growth, development, and osteogenesis
Skin and hair growth
Cardiovascular effects
Haematopoiesis

Question 4

Canine hypothyroidism

Answer 4

One of the most common canine endocrinopathies

Can be misdiagnosed since T4 levels fall during illness.

90% of cases are primary - acquired or congenital

10% are secondary

Question 5

SignalmentSignalment for canine hypothyroidism

Answer 5

Usually affects middle aged, pedigree dogs (mean age 7)

Rare in dogs <2years

Question 6

Clinical signs of canine hypothyroidism

Answer 6

Lethargy
Weight gain
Alopecia

Pyoderma and seborrhoea
Myxoedema
cold intolerance
Bradycardia
Rat tail

Question 7

Clinical signs of congential hypothyroidism

Answer 7

Mental retardation
Stunted growth key features
Disproportionate body size
Dull and lethargic
Persistence of puppy haircoat

Question 8

Biochemical findings in canine hypothyroidism

Answer 8

Increased cholesterol
Increased triglycerides
Increased glucose
Increased ALP +/- ALT (mild)

Question 9

Haematological changes in canine hypothyroidism

Answer 9

Normocytic normochromic non-regenerative anaemia
Presence of leptocytes

Question 10

Diagnostic tests for canine hypothyroidism

Answer 10

Serum T4 (sensitive but not specific)
Serum TSH - majority will ahve above reference range

Free T4
TRH and TSH stimulation
Anti-hormone antibodies
Anti-thyroglobulin antibodies

Question 11

Factors that can affect thryoid function

Answer 11

Concurrent illness
Medication
Anaesthesia
Age
Breed

Question 12

Interpretation of normal cTSH but total T4 decreased

Answer 12

Non-thyroidal illness
Drug therapy
About 20% of truly hypothyroid dogs

Treat non-thyroidal illness/withdraw drugs and re-test

Question 13

Interpretation of normal cTSH and Normal total T4

Answer 13

Euthyroid

Question 14

Interpretation of increased cTSH and total T4 decreased

Answer 14

Hypothyroid

Treat with thyroxine

Question 15

Interpretation of increased cTSH and normal total T4

Answer 15

Sulphonamide therapy
Recovery from non-thyroidal illness
Anti-T4 antibodies

Withdraw sulphonamides and re-test
Wait until recovery from other illness and retest
Measure anti-T4 antibodies

Question 16

Interpretation of T4 results

Answer 16

0-15nmol/l - Hypothyroid, sick euthyroid, or drug treatment

15-25nmol/l - Equivocal

> 25nmol/l - probably euthyroid

> 70nmol/l - T4 antibodies or overdosing with thyroxine

Question 17

Interpretation of TSH results

Answer 17

0-0.41ng/ml - normal, sick euthyroid (rarely hypothyroid)

0.41-0.6ng/ml - Equivocal

> 0.6ng/ml - hypothyroid (occasionally euthyroid)

Question 18

Treatment of canine hypothyroidism

Answer 18

Thyforon - tablets
Leventa - oral solution

Question 19

Monitoring response to hypothyroidism treatment

Answer 19

Assess reponse by clinical signs after 6-8 weeks
Measure total thyroxine concentration 4-6 hours post pill

Lethargy and mental demeanour tend to be first signs to improve.