Feline hyperthyroidism

Question 1

Clinical signs

Answer 1

• goitre (98% of cats)
• aggressiveness
• reduced tolerance to stress
• increased appetitie
• v+
• PUPD
• hyperactivity
• weight loss
• d+ / increased faecal volume
• muscle wasting
• thin body condition
• tachycardia >240
• cardiac murmur
• hypertension

Question 2

CS of apathetic hyperthyroidism

Answer 2

• poor appetite
• apathetic thyrotoxicosis

Question 3

Thyroid storm

Answer 3

• rare but life-threatening complication of hyperthyroidism described in people
• exaggerated manifestation of thyrotoxicosis (hyperthermia, CNS signs, cardiovascular abnormalities)
• in cats: thyroid crisis -> severe tachycardia (>300bpm), tachypnoea, panting, resp distress, profound weakness, ventroflexion, sudden blindness

Question 4

Diagnosis - non-specific tests

Answer 4

Haematology
- not very rewarding
- can show comorbidities

Biochemistry
- secondary hepatopathy (ALT, ALP)
- kidney parameters

Urinalysis
- SC (as hyperthyroidism increases GFR)

Question 5

Diagnosis - specific tests

Answer 5

Total T4
- increased in 90-95% of hyperthyroid cats
- day to day fluctuations
- suppresses by non-thyroidal dz
- immunoassays may underestimate the level

Free T4
- unbound
- can diffuse into the cell
- more sensitive, but increases chances of false positives
- not affected by non-thyroidal illness

T3
- active form of thyroid hormone
- derived from T4 in extra thyroidal tissues

TSH (canine)
- non-thyroidal illness affects it

Scintigraphy

Question 6

Diagnosis - which test to use?

Answer 6

No gold standard test, interpret along CS required.

Total T4 best initial test = appropriate CS + high TT4 = hyperthyroidism.

High TT4 with a few signs only = retest.

Normal TT4 and supportive CS = doesn’t rule out hyperthyroidism -> you need another test.

Free T4 and TSH are secondary test and shouldn’t be used alone to diagnose hyperthyroidism

Question 7

What is euthyroid sick syndrome?

Answer 7

• non-thyroidal dz affecting thyroid hormone levels

Question 8

What are the tx options?

Answer 8

• radioactive iodine
• surgery
• anti-thyroid meds
• dietary management

Question 9

Radioactive iodine

Answer 9

• administration of 131I (radioactive iodine) - SC injection
• simple & effective but requires licensing and hospitalisation facilities
• cats become ‘radioactive’ so have to be isolated
• curative
• target euthyroid not hypothyroid

Question 10

Specific indications for radioactive indications

Answer 10

• ectopic tumours
• locally invasive carcinomas

Question 11

Advantages of radioactive iodine

Answer 11

• single injection
• no side effects
• no anaesthesia

Question 12

Reasons to perform scintigraphy

Answer 12

• confirms hyperthyroid diagnosis
• excludes diagnosis in euthyroid cats with falsely high thyroid concentration (free T4 can be elevated in 10-30% of euthyroid cats due to non-thyroidal illness therefore high free T4 doesn’t confirm hyperthyroidism on its own)
• identifies the pattern of the dz
• identifies ectopic tissue
• aids diagnosis of carcinoma
• provides proof of success of tx

Question 13

Surgical therapy

Answer 13

• 65% of cats have bilateral dz
• ‘mincing’ a parathyroid gland and implantation into muscle
  – trying to maintain the presence of the parathyroid gland, otherwise will induce hypocalcaemia
• bilateral procedure can be staged
• bilateral thyroidectomy achieves 90% euthyroid state
• consider GA

Question 14

Medical therapy

Answer 14

Anti-thyroid drugs
- carbimazole (‘pro drug’)
- methimazole
- oral / transdermal ointment / liquid

Not getting rid of the adenoma therefore thyroid adenomas can undergo malignant transformation.

Question 15

Adverse reactions to medical therapy

Answer 15

Non-life threatening adverse reactions:
- anorexia, v+, lethargy
- face & neck excoriation

Life threatening adverse reactions:
- dyscrasia (leukaemia, anaemia, thrombocytopenia)
- hepatopathy

Question 16

Dietary management - things to consider

Answer 16

• general nutritional assessment (BCS & MCS)
• weight loss (mainly muscle mass) = protein calories malnutrition secondary to increased energy requirements and excessive protein catabolism
• sarcopaenia of aging
• comorbidities

Question 17

Ultra-low iodine diet

Answer 17

• role of iodine in thyroid hormone production
• the concept of no iodine = no thyroid hormone production
• restricted-iodine diet (Hill’s prescription diet y/d feline)
• it manages but doesn’t cure the dz or reverses the adenomatous changes in the thyroid

Question 18

Indications for ultra-low iodine diet

Answer 18

• when definitive tx option is not possible
• when cats can’t be medicated by the owners
• when cats develop side effects of anti-thyroid meds

Question 19

Issues with ultra-low iodine diet

Answer 19

• must be fed exclusively
• questionable palatability
• low protein content

Question 20

Monitoring of a hyperthyroid cat

Answer 20

• funds-dependent
• highly dependent on tx choice
• radioactive iodine: unmasking kidney dz and becoming azotaemic, potential for iatrogenic hypothyroidism, T4
• anti-thyroid meds: T4, biochem/haem
• low-iodine diet: T4 only?

Non-negotiables:
- CE
- weight check
- nutritional assessment
- BCS/MCS
- fundic exam
- BP

Question 21

Monitoring of a hyperthyroid cat - anti-thyroid meds

Answer 21

• CE
• T4 levels
• biochem/haem/urinalysis
• blood pressure
• side effects of drugs

Question 22

Monitoring of a hyperthyroid cat - low-iodine diet

Answer 22

• dietary management (y/d): reassess at 4w (approx 50% cats T4 within ref range), monthly check till euthyroid, after?
• physical exam: including BCS/MCS, T4 levels, BUN, creatinine)
• if still hyperthyroid after 1-2m reconsider diet and think about other options