Feline hyperthyroidism

Question 1

Draw a exaggerated picture of a cat with hyperthyroidism?

Answer 1

Question 2

How does feline hyperthyroidism develop?

Answer 2

Feline hyperthyroidism occurs due to the development of hyperfunctional thyroid nodules, which are benign. Most (>98%) are a functional adenoma of the thyroid gland and many may well be palpable. Rarely they are associated with malignant thyroid neoplasia.

Question 3

What causes hyperthyroidism?

Answer 3

Multinodular adenomatous hyperplasia (95%) (benign)

• Bilateral 70%
• Unilateral 30%- the contralateral gland atrophies
• Ectopic thyroid tissue is seen in 3-5% cats (~20%?)
  
  • Cranial mediastinum
  • Neck

Thyroid carcinoma (malignancy)

• ~3% of all hyperthyroid cats
• Malignant transformation in cats treated for benign disease?

Question 4

What is the signalment for hyperthyroidism?

Answer 4

Nearly always older cats

• Average age 13 yrs (range 4-20yrs)

Dsh and dlh most common (moggy cat type problem)

• Maybe less common in siamese, himalayans

Males and females equally affected

Question 5

What are the function of thyroid hormones?

Answer 5

• Multiple actions throughout the body
  
  • Thermoregulation
  • Carbohydrate, protein, and lipid metabolism
  • Interaction with cns (­ sympathetic drive) Why we see some of the cardiovascular effects
• Hyperthyroidism ->multi systemic signs
• Significant individual variation in the ability to cope with hormone excesses

Question 6

Typical presenting signs of hyperthryoidism include?

Answer 6

• Weight loss (94%) despite a good appetite (78%)
• Behaviour change
  
  • Restless/hyperactive
  • “short fuse”
  • Vocalisation
• Mouth breathing
• Polydipsia/polyuria (72%)
• Hair coat changes
• Gi signs
  
  • Vomiting (30%)
  • Diarrhoea (51%)

Question 7

What may be observed on physical exam of a hyperthyroid cat?

Answer 7

• Poor bcs
• Palpable thyroid mass
  
  • Varies with experience- practise is crucial!
  • Can be very mobile, check thoracic inlet
  • Not all masses are functional thyroid adenomas
• Cardiovascular
  
  • Tachycardia often >240 bpm
  • Murmur
  • Gallop rhythm or premature beats
• Tachypnoea +/- mouth breathing

Question 8

Use a drawing to illustrate where a thyroid mass can be palpated?

Answer 8

Question 9

Cats may be hyperthyroid and have other clinical problems contributing to clinical signs, like what?

Answer 9

• Polydipsia, polyphagia, wt loss
  
  • Diabetes
• Polyphagia, wt loss, v/d
  
  • Ibd
  • Gi lymphoma
• Polydipsia, wt loss, vomiting
  
  • Chronic kidney disease
• Wt loss, v/d
  
  • 1ry liver disease
• Tachycardia, murmur, arrhythmia
  
  • Hypertrophic cardiomyopathy

Question 10

What is the path to diagnosis of hyperthyroidism?

Answer 10

• Suspicion based on signalment and clinical history
• Appropriate findings on physical examination
• Haematology and biochemistry
  
  • help rule out or rule in some of the other differentials
• Urinalysis: stix, SG, sediment +/- culture
  
  • rule out diabetes
  • 12-22% of hyperthyroid cats have UTI & often asymptomatic
  • what is the best way to collect a urine sample? Cystocentisis if stress can be prevented.

Question 11

How can haemotology appear in a cat with hyperthyroid?

Answer 11

• Often normal
• Mild increase in PCV not uncommon
• <20% of cats may have
• neutrophilia
• lymphopenia
• eosinopenia
• monocytopenia

Why would we request haematology when investigating a possible hyperthyroid cat? Because our treatment at some point might cause problems on haematology so good to have a baseline before treatment.

Question 12

How may biochem results appear with a cat with hyperthyroid?

Answer 12

• Mild to moderate increase in one or more liver enzymes (seen in ~90% of hyperthyroid cats)
  
  • ALT usually less than 500 IU/l
  • look at ALT, ALP, AST
  • normalise with effective hyperthyroid treatment
• Urea and creatinine
  
  • low muscle mass can ↓ creatinine in to the normal range
  • hyperthyroid cats have increased GFR
  • azotaemia at diagnosis of hyperthyroidism is worrying as has a negative impact on survival
  • check urine SG: usually >1.035 if “only” hyperthyroid
• Phosphate increased in ~20% cases
• Stress hyperglycaemia?
• Investigate persistent changes.
• Renal disease and hyperthyroidism are common in older cats therefore a proportion of older cats will have both conditions- this can be a diagnostic challenge.

Question 13

With regards to total T4 test and when the history and clinical signs are suggestive a result that is .. means?

Answer 13

• > ref range= hyperthyroid
• low end of the ref range= not hyperthyroid
• high end of ref range= “possible”
• tT4 has good sensitivity and specificity…. but no test is perfect

Question 14

How are false negative tT4 caused?

Answer 14

False negative tT4

• Daily fluctuations
• Early in the disease course
• Abnormal tT4 suppressed in to the ref range by non thyroidal illness (sick euthyroid)
  
  • e.g IBD
  • In these euthyroid cases review the cat again and choose diff time of day

Question 15

What are the diagnostic dilemmas in using tT4 and free T4?

Answer 15

• Low tT4 and high fT4: “is my case hyperthyroid?”
• tT4 is consistent with non thyroidal illness.
• Some cats with concurrent illness have a high fT4

Question 16

Is there a role for TSH testing in a cat suspected of hyperthroidism?

Answer 16

• No cat specific assay available yet
• The canine assay can be used
• Hyperthyroid cats should have very low ie undetectable TSH
• No value on its own but in difficult situations a measurable serum TSH rules out hyperthyroidism

TSH is a test used in combination with T4

TSH in an untreated hyperthyroid cat should always be very low due to negative feedback effects

Question 17

What is euthyroid sick syndrome?

Answer 17

Euthyroid sick syndrome: concurrent disease such as starvation, sepsis, trauma or stress can cause a depression in basal thyroid levels as a normal response, to minimise the catabolic effects of thyroid hormones.

This occurs in many species and may lead to false diagnoses of hypothyroidism or a missed diagnosis of hyperthyroidism.

Question 18

What is the T3 suppression test and why is it used?

Answer 18

The T3 suppression test: rarely needed

• For differentiating mild hyperthyroid cats from euthyroid cats
• Oral T3 given for 3 days
  
  • Normal cats:
    
    • -ve feedback suppresses TRH & TSH resulting in ↓T4
    • >50% drop from basal = normal
  • Hyperthyroid cat:
    
    • loss of –ve feedback loop means no suppression of T4 compared with basal
    • ie T4 production is not subject to normal control systems
• This test is very unlikely to be done these days- it used to be more popular before fT4 assays and before we had an understanding of the role of TSH.

Question 19

If you are struggling to diagnose hyperthyroidism what can you do?

Answer 19

• Review the patient
  
  • history
  • physical exam
  • all results
• Consider additional tests to look for non thyroidal illness
  
  • diagnostic imaging?
  • urine culture?
• Speak to a clin path lab about dynamic testing
  
  • TRH stim test?
• Refer for possible scintigraphy?

Question 20

What reasons are there to be cautious when treating hyperthyroidism?

Answer 20

Hyperthyroid cats and CKD

• 10-14% cats are azotaemic at diagnosis
  
  • shorter survival times than non azotaemic cats
• increased catecholamines, metabolic rate and cardiac output increase GFR and can hide CKD
• successful treatment of hyperthyroidism can reveal occult renal disease
• sudden drop in GFR with treatment can cause or worsen azotaemia
• a reversible treatment might be preferred?
  
  • Mild azotaemia can develop in 10-50% cats but survival may not be adversely affected unless cats become hypothyroid
• Azotaemia= high urea and creatinine suggesting renal disease if urine SG is not showing adequate concentrating ability

Question 21

Discuss hyperthyroid cats and hypertension?

Answer 21

• 10-23% of hyperT4 cats are hypertensive at the time of diagnosis
• Monitor closely: treat hypertension if necessary
  
  • amlodipine
• Hypertension has been shown to develop in 20% of well managed hyperthyroid cats
  
  • No clear reason for this but need to protect target organs:
    
    • kidneys
    • heart
    • eyes

Question 22

Discuss ocular changes in hyperthyroidism in cats?

Answer 22

Question 23

Discuss hyperthyroid cats and heart disease?

Answer 23

• Metabolic effects of hyperthyroid state are usually reversible
• Thyrotoxic cardiomyopathy is best managed by treating the hyperthyroid state
• Minimise stress
• Uncontrolled hyperthyroidism can increase anaesthetic risks

Question 24

What are the treatment options for hyperthryoidism?

Answer 24

Options for life long control:

Medical management

• oral (tablets? liquid?)
• transdermal?

Dietary management

Options for a cure:

• Surgical thyroidectomy
• Radioactive iodine (I131)

Decisions are made on an individual case basis

Question 25

Name anti-thyroid medication:

Answer 25

**Methimazole (Felimazole®)** * 2.5mg/cat bid * tablet sizes 1.25mg, 2.5mg, 5mg **Carbimazole (Vidalta®)** * metabolised to methimazole * sustained release * 15mg/cat/sid * tablet sizes 10mg and 15mg Mechanism: interferes with synthesis of thyroid hormones (T3 and T4)

Question 26

What should you aim to achieve with anti-thyroid medication?

Answer 26

* Aim to achieve tT4 in the lower half of the reference range * Avoid causing hypothyroidism especially if azotaemic * Dose titration according to clinical response to treatment and tT4 results

Question 27

What are the advantages and disadvantages of anti-thyroid medication?

Answer 27

**Advantages:** * Quick * Licensed products * Reversible * enables assessment for occult renal disease * Useful to stabilise pre op * Dose titration is possible * Cheap…for a while * Safe? **Disadvantages:** * Life long * treatment * monitoring * Compliance “issues” * stressful * effective dosing? * Side effects

Question 28

What are some mild and severe side effects of anti-thyroid medication?

Answer 28

**Mild** * Self limiting…. start with a low dose? * \<10% cats may show * lethargy * vomiting * anorexia * eosinophilia * leucopenia * lymphocytosis **Severe: some can be life threatening** * Facial pruritus (\<5%) * Severe blood dyscrasia (\<5%) Why running haematology before treatment is important * neutropenia * thrombocytopenia * IMHA * Hepatic necrosis (\<2%) * Myasthenia gravis Usually resolve within 5-7 days but must withdraw treatment permanently \*Can occur at any stage on treatment\*

Question 29

Discuss dietary management of hyperthyroidsim?

Answer 29

* Iodine restricted diet: Hills y/d * Must have no access to other food * indoor only cats * problem in multi cat household * Possibly need to be given bottled or filtered water * Aim: Normalises T4 in 8 weeks * Concerns: low protein diet not always ideal for older cats * Large studies with good long term follow up are still lacking

Question 30

Discuss thyroidectomy?

Answer 30

* Stabilise prior to GA/surgery with medical management * May not be the ideal treatment for cats with pre-existing azotaemia (azotaemia * is a reversible treatment preferred? * Unilateral or bilateral? * Careful surgical technique is critical to avoid damage to the parathyroid glands (removal of the parathyroid glands will turn cat hypocalcaemia)

Question 31

Discuss the advantages and disadvantages of thyroidectomy?

Answer 31

**Advantages:** * Curative * Cheap? * Quick and effective **Disadvantages:** * Anaesthetic risk? * Irreversible * Hypoparathyroidism? * Hypothyroidism? * Ectopic tissue can be missed

Question 32

Discuss the signalment of iatrogenic hypoparathyroidism?

Answer 32

* Hypocalcaemia usually within 72 hours of surgery but can be up to 7 days * Signs of hypocalcaemia: * weakness * anorexia * muscle tremors and hyperaesthesia * twitching and seizures * Severity varies…. * mild, subclinical, self limiting * life threatening

Question 33

How should iatrogenic hypocalcaemia be managed?

Answer 33

* 10% calcium gluconate IV 0.5-1ml/kg to effect * Best given as part of IVFT * Vitamin D helps promote calcium uptake and should be used in conjunction with oral/injectable calcium * Use the 1,25-(OH)2 Vit D form * Rocaltrol®, AT10® * Reduce dose gradually over weeks to months if possible

Question 34

Discuss radioactive iodine treatment?

Answer 34

* “Treatment of choice”? * I131 by s/c injection * Taken up and concentrated by active adenomatous thyroid tissue * Emits radiation to local tissue causing necrosis * Quiescent atrophied thyroid tissue is spared and can therefore recover

Question 35

Discuss the advantages and disadvantages of Radioactive iodine (I131) treatment?

Answer 35

**Advantages:** * Curative * Cheap? * Low morbidity and mortality * Only effective treatment for thyroid carcinoma **Disadvantages:** * Irreversible * High initial cost * Prolonged hospitalisation? * unsuitable if on other medication * May be advised to stop anti thyroid medication and then need to travel for treatment!

Question 36

How should anti thyroid medication be monitored?

Answer 36

Blood tests every 2-3 weeks after start of treatment or change in doset * tT4 * avoid hypothyroid state especially if azotaemic * aim for lower half of reference range * +/- TSH * biochemistry and haematology * monitors for side effects * How are the kidneys responding to drop in GFR * Longer term, less frequent blood tests but will continue life long * Urinalysis especially if any suspicion of UTI