Saturday, January 29, 2011

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.

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