Diagnosis

Feline hyperthyroidism is definitively diagnosed with the demonstration of an elevated serum T4 level.

a) Radiology:

• Radiology is useful to detect complications associated with thyrotoxicosis
• The pattern may be suggestive of hypertrophic cardiomyopathy
• Pulmonary venous congestion
• Pulmonary edema
• Pleural effusion
• Cardiomegaly
• There are no definitive findings indicative of hyperthyroidism

b) ECG – may reveal

• Normal ECG
• Increased R wave amplitude
• Sinus tachycardia
• Premature beats (atrial or ventricular)
• Widened QRS complexes
• No findings are definitive of hyperthyroidism

c) Blood Work – may reveal:

• Note that the serum T4 level can vary from sample to sample therefore if one is suspicious several T4 samples should be taken
• At times the total T4 may be normal but the free T4 will be elevated in hyperthyroidism
• A T3 suppression test demonstrating a flat response may provide a definitive diagnosis when the T4 level is normal
• A TSH stimulation test demonstrating a flat response supports a diagnosis of hyperthyroidism
• Elevated liver enzymes particularly ALT, AST, alkaline phosphatase
• In that these are old cats, other concurrent abnormalities may be present esp. renal disease
• Usually hypertensive

d) Echocardiography:

• Many cases of hyperthyroidism have findings typical of feline HCM
• A few cases have findings somewhat intermediate between those of feline HCM and feline DCM
• Infrequently one observes findings typical of feline DCM

Comment: The reports of hyperthyroidism that occurred in association with feline dilated cardiomyopathy were documented in the pre-taurine supplementation days. It is entirely possible that these cases also represented taurine deficiency.

e) Thyroid Imaging (radioisotope):

• 70% of hyperthyroid cats: both lobes appear enlarged
• 30% of hyperthyroid cats: only one lobe is enlarged and the contralateral lobe is suppressed and not visible
• Imaging can identify intrathoracic thyroid tissue in hyperthyroid cats
• Imaging can detect regional or distant metastasis of functional thyroid carcinoma