Feline hyperthyroidism Flashcards
Draw a exaggerated picture of a cat with hyperthyroidism?
How does feline hyperthyroidism develop?
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.
What causes hyperthyroidism?
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?
What is the signalment for hyperthyroidism?
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
What are the function of thyroid hormones?
- 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
Typical presenting signs of hyperthryoidism include?
- 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%)
What may be observed on physical exam of a hyperthyroid cat?
- 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
Use a drawing to illustrate where a thyroid mass can be palpated?
Cats may be hyperthyroid and have other clinical problems contributing to clinical signs, like what?
- 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
What is the path to diagnosis of hyperthyroidism?
- 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.
How can haemotology appear in a cat with hyperthyroid?
- 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.
How may biochem results appear with a cat with hyperthyroid?
- 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.
With regards to total T4 test and when the history and clinical signs are suggestive a result that is .. means?
- > 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
How are false negative tT4 caused?
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
What are the diagnostic dilemmas in using tT4 and free T4?
- 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
Is there a role for TSH testing in a cat suspected of hyperthroidism?
- 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
What is euthyroid sick syndrome?
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.
What is the T3 suppression test and why is it used?
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
- Normal cats:
- 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.
If you are struggling to diagnose hyperthyroidism what can you do?
- 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?
What reasons are there to be cautious when treating hyperthyroidism?
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
Discuss hyperthyroid cats and hypertension?
- 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
- No clear reason for this but need to protect target organs:
Discuss ocular changes in hyperthyroidism in cats?
Discuss hyperthyroid cats and heart disease?
- 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
What are the treatment options for hyperthryoidism?
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
