21. Hyperthyroidism

Question 1

Signalment?

Answer 1

Signalment:
Hyperthyroidism In dogs ls very rare, but common In cats

Question 2

Feline Hyperthyroidism?

Answer 2

Feline hyperthyroidism;
Pathophysiology:
Elevated circulating levels of thyroxine (T4) and triiodothyronine (T3)
Most common cause Is functional adenomatous hyperplasia which causes gland enlargement (goitre),
can be bilateral (80%) or unilateral (20%)
Mainly older cats (average 9-12 years)
Affect multiple organs systems:
> Renal effects: Hyperthyroidism can lead to CRF due to Increased GRF and proteinuria, they can have either overt CRF or seem normal (“masked”) only to have It become overt once the
hyperthyroidism Is treated
> Cardiovascular effects: Can lead to the development of hypertrophic cardiomyopathy, systemic
hypertension,

Question 3

Clinical signs?

Answer 3

Clinical Signs:
-Weight loss with normal to Increased appetite, vomiting +/- diarrhoea or Increased faecal volume,
polyuria and polydipsia, anxiety, restlessness and excitability, poor unkempt hair coat
-Tachycardia(> 240bpm), systolic murmurs, gallop rhythms and arrhythmias. Secondary hypertropic: cardiomyopathy may lead to congestive heart failure
- Palpable thyroid mass (goitre) anywhere between larynx and thoracic Inlet, more commonly bilateral but can be unilateral.

Question 4

Diagnosis?

Answer 4

Diagnosis:
-Biochemistry: Elevated ALP and ALT without primary hepatic disease, occasionally azotaemia,
- hyperglycaemia
- Urinalysis: Variable USG and proteinuria
- Serum total T4:
- > Elevated levels are diagnostic if >50nmol/l. but In young cats >70nmol/l can be normal
» If not elevated and clinical signs suggestive of hyperthyroidism repeat test at a later date or do a free T4 If elevated, then diagnostic

Question 5

Treatment?

Answer 5

Treatment:
- Must assess renal function before and after treatment as the hyperthyroidism may be masking CRF -:-.
- if CRF is evident prior to treatment, then treatment of the hyperthyroidism may not be warranted
-Blocking thyroid hormone synthesis:
» Oral carbimazole:
5 mg/cat PO BID if <100nmoVL, TID if >100nmoVL.
Carbimazole Is metabolised Into methimazole
* Side effects:
o Short term: In 20% of patients but only short term, vomiting, anorexia, and lethargy
o Long term: Skin rashes and pruritus, blood dyscrasias (reduced platelets and granulocytes)
- Transdermal methimazole:
0.05-0.1 ml on skin BID-SID (5mg methimazole/0.1ml)
If Applied to the inside of the ear, takes longer to reduce thyroid levels
Surgery:
> Thyroidectomy
) If underlying chronic renal failure ls unmasked by a trial on medical therapy, this may not be
Indicated
> Potential complication is hypoparathyroidism 2-3 days post-operatively
> Treated with IV calcium gluconate then long term with oral calcium and vitamin D
Radioactive iodine therapy
» Considered gold standard
> Relatively low risk, can provide lifelong euthyroidism in 80-90% of patients
), If underlying chronic renal failure is unmasked by a trial of medical therapy this may not be Indicated

Question 6

Monitoring?

Answer 6

Monitoring:
-Poor owner compliance is the main cause of failure
- Ideally repeat serum total T4, haematology and biochemistry every 2 weeks for 12 weeks but otherwise at least every 4 weeks:
- Serum total T4:
Aim for mid normal serum total T4
If massive drop from > 1OOnmol/L to normal, then trial! Then reduce frequency
If no decrease In serum total T 4, then Increase dose by 5 mg SID
- Renal enzymes and urinalysis:
- If mild azotaemia then can monitor and treat,
- If overt azotaemia then may need to stop hyperthyroid therapy and monitor
- Blood dyscrasias: Reduced platelets and granulocytes
- Once stable, then repeat every 3-6 months