Lesson 10, Topic 3
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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