Q & A: Huge Thyroid Mass In Cat Treated With Long-Term Methimazole

Bailey is a 14-year-old male neutered domestic short hair who was diagnosed with hyperthyroidism by another veterinarian about 3 years ago and has been on methimazole since then. He also has a history of chronic constipation that has been well controlled in the last year on enulose (Lactulose) only.

I last saw Bailey a year ago, at which point he was doing well clinically but had lost about 1 lb. since his last check. At that time, he was on 5 mg of methimazole BID. A geriatric profile at that time showed a total T4 of 3.3 μg/dl, a normal CBC and serum chemistry profile (creatinine = 1.5 mg/dl, BUN = 23 mg/dl), and a urine specific gravity of 1.065. I recommended increasing his dose of methimazole to 7.5 mg in the morning and 5 mg in the evening and rechecking in 4-6 weeks.

The owner did increase the daily methimazole dose but did not follow up with us until now. Bailey has lost an additional 3 lbs. since his last exam and has developed polyphagia, polyuria, and polydipsia over the last month.

On exam yesterday, the cat now has a fairly large mass in his left ventral neck area (about 3-4 cm, slightly firm and irregular), which I had not noted on the last exam. His blood work from yesterday showed a T4 of 1.9 μg/dl, and very normal renal function (serum creatinine = 1.1 mg/dl; BUN = 22 mg/dl, USG =1.030).

So, I'm suspicious that the thyroid mass is or has become a carcinoma at this point. But would you expect the cat to lose weight secondary to a thyroid carcinoma even if the T4 is within normal range? Could it be secreting another hormone to cause the weight loss?

I thought I should recommend full-body radiographs to try to rule out any metastasis or other obvious neoplasia, and then consider surgical removal and biopsy of the mass. I've read that it can be helpful to follow-up with I-131 treatment after excision of a thyroid carcinoma, so I would speak to the owner about referral for that.

Does that sound like a reasonable course of action to you or are there other things you would recommend doing first? Thank you very much for any help or advice you can give me on this perplexing case.

My Response:

There are many ways you could go in your workup of this cat. Most cats that I see who are loosing weight while on treatment with methimazole have high serum T4 concentrations, which can explain the weight loss. Obviously, that's not the case here, which makes this cat more interesting!

Many cats (in fact, probably nearly all hyperthyroid cats) will have an increase in goiter size with time — that makes sense since we aren't inhibiting thyroid tumor growth with the methimazole. We are only blocking thyroid hormone secretion with the drug.

Recently, a paper was published showing that on thyroid biopsy, some cats with long-standing hyperthyroidism had evidence of transformation of thyroid adenoma to carcinoma (1). I do believe that this happens more than we realize, and I now see almost a cat a month with thyroid carcinoma. Almost all cats with thyroid carcinoma that I see have been on methimazole for longer than 2 years.

So in your cat, the enlargement of the thyroid mass could indicate that the tumor has simply grown larger with time, or it could indicate malignant transformation. As you indicated, thyroid biopsy would be helpful in making that diagnosis. Many of these cats have extension or metastasis into the thoracic cavity so you might not be able to cure that cat with surgical thyroidectomy if that is the case. What I like to do in that situation is to perform a thyroid scan (thyroid scintigraphy) prior to surgery, which we certainly could do in this cat. This will tell us where the thyroid tumor tissue is located and help direct what needs to be removed or biopsied, if the cat does go to surgery.

Thyroid Scan of cat with large thyroid carcinoma (left) compared to normal cat (right)
Notice the extension & invasion of the thyroid tumor into the thoracic cavity
(horizontal line indicates the region of the thoracic inlet)

That all said, why is your cat loosing weight despite a normal serum T4? With the polyphagia, the cat should bee taking in enough calories. If there vomiting or diarrhea? (I assume that diarrhea isn't present because of the constipation.)

Cats with weight loss that are eating normally must have either increased loss of glucose in the urine or impaired absorption of nutrients from the GIT. To that end, I would recommend that you perform an abdominal ultrasound, in addition to your full-body x-rays, prior to either thyroid biopsy or thyroid scintigraphy. Urine culture should also be considered to exclude pyelonephritis.

As far as other hormones being secreted, I'd suggest that you also measure a serum free T4 and T3 concentration on this cat. It's possible that the free T4 or T3 concentrations are still high and that could explain some of weight loss in this cat.

Reference:
1. Hibbert A, Gruffydd-Jones T, Barrett EL, Day MJ, Harvey AM. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. J Feline Med Surg. 2009 11:116-24.