Diagnosing Feline Hyperthyroidism: Not Always as Simple as One Might Believe

Earlier this year, Dr. Mark Peterson participated in an Endocrinology course organized by the American College of Veterinary Internal Medicine (ACVIM). An overview of his lecture on "Diagnosing feline hyperthyroidism" was summarized by Dr. Jennifer Garcia and published in the July 2015 issue of Veterinary Medicine. To access this article online, click here.

Diagnosing feline hyperthyroidism: It's not always as simple as it seems

Don't rely too heavily on T4 concentrations since cats can have a false elevation.

In his presentation, “Diagnosis of hyperthyroidism: A critical evaluation of our current available tests,” Mark Peterson, DVM, DACVIM, discussed some of the pitfalls in relying too heavily on thyroid (thyroxine, or T4) testing alone. While a total T4 concentration will be enough to make an accurate diagnosis of hyperthyroidism in more than 90% of cases, he warned to always pay attention to the clinical signs and physical examination findings. There are cats that can have a false elevation in their T4 concentration, so supportive clinical signs as well as a palpable thyroid nodule will help rule in or rule out the diagnosis.

When it comes to successfully palpating for evidence of a thyroid nodule, Peterson detailed a few of his favorite techniques:

• Stand behind the cat with the cat facing away from you—the cat feels less stressed if it can’t see you. Peterson also puts the cat in a basket with a towel so the cat feels more secure and is less squirmy. Use your thumb and index finger to gently run the length of the trachea from the larynx to the thoracic inlet.
• Alternatively, with the cat in the same position, turn its head to the left and palpate. Repeat with the cat’s head turned in the other direction.

Examine the cat from behind, with the cat facing the owner.

For patients in which a thyroid nodule can be palpated but there are no clinical signs and there is no elevation in T4 concentration, he recommends monitoring signs at home and rechecking the level in six to 12 months.

Peterson also noted that there are different cut-off values from laboratory to laboratory. This means that a T4 concentration that is normal at one laboratory, may actually be elevated at another. This serves as another reminder of the importance of the physical examination and clinical signs when trying to diagnose hyperthyroidism.

Q & A: Bilateral Thyroid Nodules in a Cat, But Normal T4 and Free T4?

Agatha is an 11-year-old F/S DSH first seen 3 months ago. The owners complained that she had beeen moping around for the past week and was more "clingy" than normal. She had vomited twice and urinated outside of the box on a few ocassions.  My physical examination at that time was completely normal. Her heart rate was 168 bpm, there were no palpable thyroid nodules, and her weight was 11.3 pounds.

I saw the cat again last week, with the  new complaint of sounding hoarse for 3 weeks. The cat was also eating and drinking less, and she was lethargy and less active. My physical examination revealed that  her body weight was down to 10.2 pounds (i.e., a 1 pound weight loss). The cat also had mild anisocoria, and now I could palpate bilateral thyroid nodules!

I performed routine blood work (CBC, serum chemistry panel) which was completely normal. Her serum T4 value was also normal at 1.5 μg/dl (reference range, 0.8 - 4.0 μg/dl), and the free T4 by dialysis was normal at 23 nmol/L (reference range, 10-50 nmol/L).

It's amazing how much this cat has deteriorated in just 3 months.  How can you explain the cat's thyroid goiter with the normal T4 and free T4 results? Do my results rule out thyroid disease? 

My Response:

Both the serum thyroid tests and history rule out hyperthyroidism, but this cat could still have thyroid hyperplasia or neoplasia. The cervical masses you palpate certainly may be thyroid nodules but could also be lymph nodes, parathyroid tumors, or another type of subcutaneous tumor.

I'd start with fine needle aspiration of the cervical masses. Those results will dictate where our workup should go next. If those results are consistent with thyroid tissue, the next step would be to do thyroid scan (scintigraphy) in order to document that the cervical tumors are indeed thyroid in origin and to rule out thyroid carcinoma.