Hyperthyroidism Flashcards
What is the most common endocrinopathy in cats?
Hyperthyroidism
Most common causes of hyperthyroidism
Bilateral thyroid hyperplasia
Adenoma
Sequence of release of thyroid hormones
TRH (hypothalamus) -> TSH (anterior pituitary) -> T4 (95%) and T3 (5%) (thyroid)
Negative feedback of T4 on anterior pituitary.
How common is bilateral lobe involvement in hyperthyroidism?
> 70% of cases
Clinical signs of hyperthyroidism in cats
Insidious onset
Weight loss (with increased appetite)
PU/PD
Hyperactivity
GI signs
Tachycardia (+/- heart murmur or gallop rhythm)
Palpable goitre in 98%
Hypertension may be seen
Horner’s syndrome may be seen
Haematological changes for diagnosis of hyperthyroidism
Limited value
Mild-moderate increase in PCV, red blood cell count, and haemoglobin.
Rarely anaemia.
Stress leucogram.
Serum biochemistry changes in diagnosis of hyperthyroidism
Mild-moderate elevations of liver enzymes.
Mild-moderate increases in urea and creatinine (less common).
Increase in phopshate (less common).
Possibly decreased ioinsed calcium and increased PTH.
Elevated GFR masks renal failure so must assess renal function before and during therapy.
Radiography and echocardiography in diagnosis of hyperthyroidism
Reversible hypertrophic cardiomyopathy, so evidence of cardiac enlargement.
Most commonly on echocardiograph:
- Left ventricular hypertrophy
- Left atrial dilation
- Interventricular septum hypertrophy
Myocardial hypercontractility.
Total T4 concentrations in diagnosis of hyperthyroidism
Elevated circulating T4 (10% in normal range).
However non-thyroidal illness can supress total T4 to mid-high end of reference range - retest in 4-6 weeks.
Free T4 concentrations in diagnosis of hyperthyroidism
Measured by equilibrium dialysis.
Elevation seen in 98%, icluding 95% of those within normal range for total T4.
Always measure alongside total T4 as 12% of eurythroid sick cats have an elevation in free T4.
T3 suppression test in diagnosis of hyperthyroidism
Suppressive effects of T3 are lost.
Therefore, the concentration of serum total T4 shows no or minimal decrease in hyperthyroid cats following the administration of oral T3.
TSH in diagnosis of hyperthyroidism
Detection is poor and the test may be inappropriate for distinguishing normal from hyperthyroid cats.
Thyroid scintigraphy in diagnosis of hyperthyroidism
Rseerved for patients suspected to have a thyroid carcinoma or in the detection of thyroid tissue prior to surgery.
Blood pressure in diagnosis of hyperthyroidism
Hypertension very common.
Medical management of hyperthyroidism
Not curative
Relies on owner and client compliance.
Widely available.
Necessary prior to surgical thyroidectomy.
Reversibly block iodination of thyroglobulin.
Usually oral.
Monitor serum total T4 10 days to 3 weeks afterstartting therapy.
Aim is to maintain total T4 within lower half of reference interrval.
