Thursday, March 3, 2011

Q & A: Free T4 vs. Thyroid Scintigraphy for Diagnosis of Occult Hyperthyroidism in Cats?

One of my patients is a 10-year-old, FS, DSH cat with a history of vomiting and recent weight loss.  I'm at a loss about where to go next with this cat and I would deeply appreciate any help you can provide.  

Routine CBC, serum chemistry panel, and complete urinalysis have been either completely normal or  unremarkable. Results of both chest radiographs and an abdominal ultrasound were unremarkable. 

Serum T4 values have been normal but appear to be slowly creeping up into the high-normal range (normal < 4.0 μg/dl).  In addition, on my last examination, I palpated a small cervical nodule that might be thyroid tumor.  Here's some cats' serial T4 values and body weight:

Date:                   T4 (ug/dl)             Weight (pounds)              
12/14/08                  2.0                           12.3         
11/20/09                  1.8                           11.7      
11/19/10                  2.3                           10.9      
02/13/11                  2.8                           10.2

I believed that this cat was probably early hyperthyroid and placed her on methimazole 2.5 mg once daily.  My intention was to recheck the T4 after a couple of weeks.  However, the cat developed SEVERE gastroenteritis within 2 days of starting the medication. It took about 5 days of supportive care to get her back to normal.   

Where should I go from here? Would running a free T4 on the cat be helpful? My inclination is to try the cat on transdermal methimazole to see if the cat would regain some of the lost weight, but I'm worried that the cat might severe another severe reaction to the medication.  

Would a thyroid scan be a helpful diagnostic aid in this cat?

My Response:

Cats like this can be problematic and sometimes difficult to diagnose, even if truly hyperthyroid. Running a free T4 may be helpful, since the sensitivity of that test is higher than a total T4 concentration. In other words, this cat may have a mid- to high-normal total T4 but the free T4 could be clearly high. That would be consistent with hyperthyroidism.

The problem is that a high free T4 is not 100% diagnostic for hyperthyroidism since the specificity of the assay is rather poor. In other words, it's quite common to see 'falsely' high free T4 concentrations in cats that have nonthyroidal illness and are not  hyperthyroid at all (see figure on left). 

So the finding of a high free T4 by itself can never be considered diagnostic for feline hyperthyroidism.

A diagnostic imaging procedure called thyroid scintigraphy or thyroid scanning has long been considered the 'gold standard' for diagnosing feline hyperthyroidism. After administrating a short-acting radionuclide that concentrates in thyroid tissue, thyroid imaging directly visualizes the  normal thyroid gland, as well as the small tumor(s) responsible for hyperthyroidism in cats.
Thyroid scintigraphy of a normal cat. Notice the similar uptake of the radionuclide in the 2 normal thyroid lobes and salivary glands

Thyroid scintigraphy of a mildly hyperthyroid cat with small bilateral thyroid adenomas. Notice the increased uptake (brightness) of the radionuclide in both thyroid lobes.
The advantage of thyroid imaging is that it's the most sensitive test we have to diagnose early hyperthyroidism in cats. With the scan, we can calculate the size and the brightness of the thyroid gland. Because this procedure uses the physiology of the thyroid gland to create an image, thyroid imaging is so sensitive that it can actually demonstrate the presence of a hyperfunctioning thyroid tumor long before they cause the serum T4 to rise to above normal.


Anonymous said...

Great post! If you were going to do scintigraphy on a cat that has been on methimazole, would you withdraw it, and if so, for how long (to avoid the rebound phenomenon)?

Dr. Mark E. Peterson said...

If the cat is obviously hyperthyroid (serum T4 clearly high), and you are doing the thyroid scan prior to radioiodine or thyroidectomy, stopping the methimazole is not really necessary.

On the other hand, if you were doing the scan on a cat in order to confirm the diagnosis (normal pretreatment serum T4 values), then you should stop the methimazole for at least a week (and 2 weeks is better). If you overdose a euthyroid cat with methimazole and create iatrogenic hypothyroidism, the resulting high TSH can produce thyroid hyperplasia and make the scan look falsely positive for hyperthyroidism (ie, enlargement of both thyroid lobes and increased salivary:thyroid ratio).