Endocrinology Flashcards
What is the amount of water intake in ml/kg/day for cats and dogs that is considered as polydipsia?
Dogs > 100 ml/kg/day
Cats > 50 ml/kg/day
Normal 50-60 ml/kg/day
Amount is affected by: environmental temperature, exercise, diet type (dry vs wet, salt content, protein content)
How do we ensure that PU/PD is present, how do we measure it?
Measure:
- Water intake
- Urine specific gravity (serial)
- Urine output (weigh cat litter)
What are the 5 steps in diagnosing PU/PD in a dog or a cat?
Step 1: Ensure PU/PD is present
Step 2: Review signalment, history and physical examination
Step 3: CBC, serum biochemistry, urinalysis, urine culture
Step 4: Additional tests as dictated by differential list
Step 5: Modified water deprivation test or desmopressin trial
Primary polydipsia (psychogenic polydipsia) is secondary to?
- Hyperthyroidism
- Liver failure
What are the causes of primary polyuria?
-> Central diabetes insipidus - lack of vasopressin (ADH) production
- neoplasia, trauma, surgery, inflammation, infection, etc.
-> Nephrogenic diabetes insipidus - problem with response to vasopressin
- primary (mutation in V2 receptor gene) rare!
- secondary (group of diseases causing this: hyper and hypoadrenocorticism, hyperthyroidism, etc.)
-> Loss of osmotic gradient
- osmotic diuresis (diabetes mellitus, chronic renal failure, etc.)
- loss of medullary tonicity (medullary washout, low urea/sodium)
Investigation: Step 1 - ensure PU/PD is present, what is involved in this step and what are the DDx?
DDx: incontinence, pollakiuria
Objective assessment:
- quantify water intake
- weigh cat litter
- urine osmolality or USG measurement
Cats: USG values decrease with age, lower values can be associated with health or PU/PD especially in renal disease, hyperthyroidism, diabetes mellitus, USG is not affected by timing of samples
Dogs: USG is variable within and between individuals, >1.025 makes PU/PD unlikely, decreases with age, varies with time of the day (higher in the morning)
Generally: take serial samples for USG, low values can be normal, collet samples at home (morning in dogs), values usually higher in cats
What is involved in step 2 of the investigation of PU/PD in dogs and cats?
Signalment, history & physical exam
- History
- Breed
- Age
- Sex
- Drugs
- Physical examination findings
Examples: physical exam findings that would indicate Cushing’s (pot belly, hair loss), plantigrade stance in cats or cataracts in dogs (diabetes), cachexia, ascites, goitre
What is involved in step 3 of the investigation of PU/PD in dogs and cats?
CBC, Biochemistry, UA, +/- culture - may confirm:
- Diabetes mellitus
- Hypercalcaemia
- Kidney disease
- Hypokalaemia
May provide evidence of disease:
- Hyperadrenocorticism (dogs) = increased ALP, ALT, cholesterol
- Hypoadrenocorticism (dogs) = azotaemia, increased K, decreased Na
- Hyperthyroidism (cats) = increased ALT, ALP
- Liver failure = decreased liver products, +/- increased ALT,ALP
What is involved in step 4 of the investigation of PU/PD in dogs and cats?
Additional tests as dictated by differential list (as identified in step 2)
Common tests:
- Abdominal ultrasound (adrenal disease, renal disease, liver disease)
- Adrenal function test - ACTH response test (hyperadrenocorticism, hypoadrenocorticism)
- Total T4 (cats) (hyperthyroidism)
- Bile acid stimulation test or ammonia (liver failure)
- SDMA (early kidney disease)
- Advanced imaging (CT, MRI)
What is involved in step 5 of the investigation of PU/PD in dogs and cats?
Modified water deprivation test
- the MWDT and desmopressin trial are very rarely necessary
- the MWDT differentiates between central diabetes insipidus, primary (congenital), nephrogenic diabetes insipidus and primary polydipsia, but can only be interpreted when all other causes have been excluded.
What is the treatment for central diabetes insipidus?
Tx: Desmopressin
- intranasal (eye drops)
- oral (tablet)
- Subcutaneous (especially cats)
Tx at night alone an option
- less expensive
- controls nocturia
- ensure free access to water
What are the most common causes of canine hypothyroidism?
Primary (>95%)
- Lymphocytic thyroiditis (50%)
- Idiopathic atrophy (50%)
- less common congenital or neoplasia
Secondary (congenital or acquired) rare, Tertiary (lack of TRH = lack of TSH) exceptionally rare
Was is the typical signalment for canine hypothyroidism?
Middle aged dogs
- rare in dogs < 2 years of age
- mean age 6-8 years (younger with lymphocytic thyroiditis)
Either sex
Any breed
- Mid to large pure breeds
- Dobermans, Hovawarts, Gordon setter, Old English sheepdogs, Retrievers, Ridgebacks, Spaniels, Shelties, Setters, Giant schnauzer
What are the clinical features of canine hypothyroidism?
Insidious in onset
Variable - many clinical signs possible
Dermatological features in 70-80% of cases
- poor quality coat
- dry
- failure to regrow
- non-inflammatory alopecia
- hyperpigmentation
- tragic facial expression (myxoedema)
Metabolic features (signs may be subtle) in >80% of cases
- lethargy
- weight gain
- exercise intolerance
- cold intolerance
What are some of the less common features of canine hypothyroidism?
Reproductive
- infertility (bitch)
- prolonged parturition
- increased puppy mortality
- galactorrhoea
Neuromuscular
- myopathies
- neuropathies
Cardiovascular
- bradyarrhythmias
Ocular
- corneal lipidosis
- reduced tear production
Gastrointestinal
- constipation
- vomiting
- diarrhoea
- gall bladder mucocele
Neurological
- myxoedema coma
- central vestibualar disease
What are the indications for testing with a suspected canine hypothyroidism case?
Supportive:
- metabolic (lethargy, weight gain)
- dermatologic (non inflammatory alopecia)
- clinical pathology (increased cholesterol)
- specific conditions (vestibular, gall bladder mucocele)
Not supportive:
- Polyuria and polydipsia
Which diagnostic testing is recommended and in which order once we have supportive indication for testing in a dog with suspected hypothyroidism?
- Perform routine clinical pathology
- supportive: mild non-regenerative anaemia, increased cholesterol, increased triglycerides, mild increase in ALT/ALP - Avoid thyroid function testing if external factors will impair interpretation of results
- nonthyroidal illness syndrome
- thyroid suppressive medications (prednisolone, phenobarbitone)
- consider breed effects (sighthounds) - First line testing: Total T4 and circulating TSH
- Decreased total T4; increased TSH = primary hypothyroidism
- Total T4; TSH WRL = euthyroid
- Total T4 WRL; TSH increased = subclinical hypoT4, recovery from NTIS/drugs, hypothyroidism with autoantibody interference
- Decreased total T4; TSH WRL = hypothyroidism, NTIS/drug therapy, breed (e.g. sighthound)
Additional Diagnostic tests: (if first line non diagnostic)
- Second line testing: Free T4
- less affected by NTIS, but can be decreased with severe disease - Third line testing (rarely needed):
- scintigraphy (limited availability)
- ultrasound (operator dependant, does not directly assess function)
- TSH-ST (gold standard but very expensive unless performing frequently)
When NOT to test a dog for hypothyroidism?
- Severe non-thyroidal illness
- Thyroid suppressive medications
Wait until:
- recovery from illness
- removal from thyroid suppressive drugs
Variable time to recovery of function
- often 4-6 weeks
Why is endogenous TSH high in primary hypothyroidism?
Because the thyroid glands are unable to produce any T4 or T3 so the feedback loop keeps going and more and more TSH is being produced to try and make up for the loss of T4 and T3.
How do we treat hypothyroidism in a dog?
Levothyroxine
- physiological prohormone, normalises T4 and T3
Dosing regimen: once or twice daily, same time each day, absorption is decreased by food so either always with or without food, avoid administration with other meds
Monitoring therapy:
- clinical signs: metabolic days to weeks, derm weeks to months
- monitor TSH: high values may indicate poor control long-term
- body weight: will decrease but diet and exercise change needed
What is the most common endocrinopathy in cats?
Hyperthyroidism - excess production of T4 and T3
Would you expect feline hyperthyroidism in a cat <6 years of age?
Very unlikely! it is extremely rare in cats younger than 6.
Feline hyperthyroidism is extremely common in older cats, and signs of mild disease can be subtle (e.g. weight loss alone). Testing should be considered in older cats with non-specific signs.
What is usually the first general clinical sign of feline hyperthyroidism?
Weight loss
What are the common clinical signs of feline hyperthyroidism?
- Weight loss despite an increase in appetite
- PU/PD
- Vomiting/diarrhoea
- Tachypnoea, tachycardia
- Heart murmur or identification of HCM
- Goitre, hyperactivity
- Increased ALT/ALP, decreased K, USG <1.035
- Unkempt/overgroomed
- Increased temperature (from increased metabolic rate)
How do we investigate feline hyperthyroidism?
-> Routine clinicopathological tests
- support a diagnosis
- eliminate other diseases with similar signs
- important in treatment decisions
-> Confirmatory
- Hormone analysis
-> Diagnostic imaging
- if required
What might we see on clinpath with feline hyperthyroidism?
-> Increased liver enzyme activity
- ALT, AST, ALP, GGT in over 90% of cases (may not be increased in mild or early cases)
- Most common cause of increased liver enzyme activities in older cats
+/- hypokalaemia
+/- decreased USG
+/- proteinuria
If hyperthyroidism is present in a cat but total T4 is normal, what can be the reason? and How do we diagnose these cats?
Usually mildly affected
- early disease
- concurrent illness (non-thyroidal illness suppresses total T4)
If cat has significant non-thyroidal illness:
Total T4 in high reference interval is inappropriate - Hyperthyroidism likely
If cat does not have significant non thyroidal illness:
Early hyperthyroidism or euthyroidism likely - options: retest total T4 later (min 4-6 weeks) values may remain in grey range, free T4 (expensive), TSH (species specific assay not widely available), scintigraphy (gold standard)
Can free T4 be used to diagnose feline hyperthyroidism?
yes but no..
free T4 concentrations are increased in almost all hyperthyroid cats but increased can also be seen in non thyroidal illness (sensitive, but not specific if non thyroidal illness present)
We are doing a hormone assay on a cat where we suspect hyperthyroidism, the result comes back as increased total T4 what is the diagnosis? and what would the diagnosis be if the result came back with decreased total T4?
Increased total T4 = hyperthyroidism - treat
Decreased total T4 = Non thyroidal illness - investigate further
