Choosing the best tests to diagnose feline hyperthyroidism
In this article, I describe the most common clinical signs and physical examination findings in hyperthyroid cats. I also review the available diagnostic methods, including their advantages and disadvantages.
CLINICAL SIGNS AND PHYSICAL EXAMINATION FINDINGS
In a study of 202 cats with hyperthyroidism at the Animal Medical Center in New York City, the clinical sign most commonly reported by owners was weight loss.1 Polyphagia, vomiting, polyuria and polydipsia, increased activity, a decreased appetite or anorexia, diarrhea, weakness, dyspnea, panting, and a large fecal volume were also common owner complaints. Hyperthyroidism has been reported in cats ranging in age from 4 to 22 years, but 95% of cases occur in cats older than 10 years.1On physical examination, a palpable enlargement of one or both thyroid glands is likely. Enlargement of the glands may precede a rise in T4 concentrations, emphasizing the importance of a thorough physical examination.2 A cat with advanced hyperthyroidism often has a low body condition score with muscle atrophy. A host of other physical examination findings may also be present, such as an unkempt coat, a heart murmur, tachycardia, a gallop rhythm, restlessness, or aggression. Some cats with pronounced cardiac effects may be in congestive heart failure.
The clinical signs associated with hyperthyroidism have been declining in severity over the years.1 In one review, the authors compared the electrocardiographic and radiographic changes seen in two populations of cats confirmed to have hyperthyroidism—one from 1979 to 1982 and one from 1992 to 1993.3 Sinus tachycardia, which is the most commonly recognized cardiac manifestation of feline thyrotoxicosis, was not as prevalent in the 1993 group as it was in the 1982 group. That was also true of an increased R-wave amplitude on lead II electrocardiography. Both groups had similar low incidences of atrial and ventricular dysrhythmias, but the 1993 group had a significantly higher occurrence of right bundle branch block. Thoracic radiographs were deemed necessary in a larger proportion of the 1982 group than in the 1993 group. Although no significant differences in cardiac size were noted on radiographs between the two groups, a larger number of cats in the 1982 group had evidence of congestive heart failure. These findings suggest that feline hyperthyroidism is being diagnosed earlier today and that newly diagnosed cats have less severe cardiovascular clinical signs than a decade ago.
Hyperthyroidism can be definitively diagnosed by measuring increased serum thyroid hormone concentrations or by performing scintigraphy. Both tests have advantages and disadvantages.
The advantages of using serum tests to diagnose hyperthyroidism include their low cost, ease (no need for anesthesia), and availability (standard laboratory tests). The main disadvantage of serum testing is that the results may not be easy to interpret if a cat has other illnesses or if you are trying to diagnose hyperthyroidism in its early stages. Other tests (e.g. T3 suppression, thyroid-stimulating hormone and thyrotropin-releasing hormone stimulation testing, scintigraphy) are available if the suspicion is high for hyperthyroidism in cats with appropriate clinical signs but normal thyroid hormone concentrations.