Hyperthyroidism

Question 1

clinical signs

Answer 1

mainly older cats but average age has come down to ~ 10.2 years
weight loss with variable appetite
polydipsia
hyperactive but they can also be lethargic
cardiac changes – tachycardia +/- gallop rhythms, systolic murmurs
thyroid ‘nodule’ in ~ 70% of cases

Question 2

cardiovascular disease

Answer 2

hyperthyroidism results in incr catecholamine sensitivity
develop tachycardia and tachydysrhythmias
diastolic gallop rhythms – impaired ventricular relaxation
heart failure
tachypnea and panting
decreased systemic vascular resistance

Question 3

hypertension

Answer 3

a higher prevalence of hypertension in hyperthyroid cats
however retinopathy is uncommon in hyperthyroidism
many cats develop hypertension due to treatment of hyperthyroidism

Question 4

clinical pathology

Answer 4

elevated alanine aminotransferase (ALT) in 88% of cases
elevated alkaline phosphatase (ALP) in 45% of cases
elevated bile acids in 30 – 65% of cases
stress leukogram - common
physiological neutrophilia – very common if cat is a jerk
azotaemia

Question 5

confirmation of diagnosis

Answer 5

basal total T4 
basal free T4 - by equilibrium dialysis
basal cTSH levels might be helpful 
T3 suppression test 
thyroid scintigraphy

Question 6

follow-up test options

Answer 6

elevated free T4 equilibrium dialysis

can repeat basal total T4

Question 7

renal function

Answer 7

urea is generally higher than creatinine
hyperthyroidism increase GFR
correcting hyperthyroidism - increase in creatinine
elevated creatinine is NOT clinically significant renal failure
hyperfiltration results in glomerular sclerosis – progressive glomerular sclerosis will result in decreasing GFR

Question 8

medical treatment

Answer 8

carbimazole + methimazole - carbimazole is converted to methimazole
“induction” lasts between 2 - 3 weeks
if no effect then consider increasing the frequency or individual dose
once “controlled” use a maintenance dose
side effects in 5-20%

Question 9

medical treatment adverse effects

Answer 9

inappetance, vomiting and diarrhoea
depression, lethargy and general listlessness
facial pruritis
blood dyscrasias; including anemia, granulocytopenia, thromboplasias
minimal benefit in changing the thiolurea
invariably means medical management can’t be used, regardless of route of administration

Question 10

surgical treatment

Answer 10

modified intracapsulartechnique
unilateral thyroidectomy & bilateral hyroidectomy - ectopic thyroid tissue
• 5/86 bilateral thyroidectomy patients developed post-operative hypocalcaemia
5% had recurrence of hyperthyroidism (all with evidence of ectopic thyroid tissue)

Question 11

I131 treatment

Answer 11

dose is between 110 & 183 MBq 
> 97% efficacy 
2 week stay in hospital 
median survival time is > 6.5 yrs 
off medications for at least 2 weeks prior to treatment 
£1700-1900