Feline Hyperthyroidism
Feline hyperthyroidism is found mainly
in elderly cats, 9-22 years old, of both sexes. Clinical signs
usually develop gradually and include loss of weight despite
hyperphagia, frequent bowel movements with abundant soft or
liquid stool, polydypsia, polyuria, nervousness, and poor
haircoat. Findings of physical examination may include tachycardia
murmurs (200-300 dpm), gallop murmurs, atrial premature contractions,
and occasionally, a mass in the area of thyroid gland may
be palpated. Clinical diagnosis may be confirmed by serum
T3 and/or T4 levels which are elevated.
A variety of proliferative thyroid
lesions have been associated with pathology of feline hyperthyroidism.
Most lesions are benign, including thyroid nodular hyperplasia
and adenomas. Nodular hyperplasia can be manifested as adenomatous
hyperplasia or multinodular goiter. These lesions are usually
bilateral and only slightly enlarge the gland. Adenomas are
usually unilateral and markedly enlarge and distort the glandular
structure. Thyroid carcinoma (follicular or papillary) can
occur and also cause hyperthyroidism.
Nevertheless, discovery of a thyroid
mass does not necessarily indicate that the cat is hyperthyroid
because many of these lesions are non-functional. The other
important point is that cardiomyopathy is often a serious
complication of feline hyperthyroidism. The heart is enlarged
with left ventricular hypertrophy that may progress to dilatation
of ventricles.
- by Tsang Long Lin, DVM, PhD
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