Diagnosing Subclinical Hyperthyroidism in Cats

I have a quick question about a "problem" case, a 9-year old M/C DSH. This cat has lost about 3 pounds over the last year, but weighed 22 pounds last year so the owner has been trying hard to get the cat to lose weight.

The cat came in for his recheck last week. His physical examination was normal and we ran a routine blood panel (CBC, serum chemistry panel, and total T4). His serum T4 was high-normal at 3.7 µg/dl (reference range, 0.8-4.0 µg/dl). I then added on a free T4 by dialysis, which came back as slightly high at 54 pmol/L (10-50 pmol/L). Looking back in his records, his free T4 concentrations were also slightly high when checked a year ago (58 pmol/L).

This cat is not clinical at all otherwise for hyperthyroidism. He has strange seizure-like episodes, about once every 3 months, but these have been happening since he was a young cat. There has been no change in these episodes.

My gut is telling me that because this cat is not clinical and his thyroid values have remained stable over the last year, we probably should not treat hyperthyroidism at this time. However, I wanted to get your opinion and see if you think that this cat is indeed hyperthyroid, and if so, if he may be a candidate for I-131 at this time.

My Response:

In our studies, we have found that up to 30% of cats that present like this (asymptomatic, high-normal T4, slightly high free T4 concentration) will be normal when evaluated by thyroid scintigraphy (1-3). So basically, this cat certainly could be in the preclincal stages of becoming hyperthyroid, but it's also very possible that the free T4 is falsely high and he is euthyroid.

Determination of free T4 is far from being a perfect test, and a diagnosis of hyperthyroidism should NEVER be based on the finding of a high free T4 alone (1,2,4). This is especially true in a cat like this one, that presents without a palpable thyroid nodule or clinical features of thyroid disease.

So you and the owners have 2 options at this time:

1. Do a thyroid scan (scintigraphy) to better define the cat's thyroid function (1-3,5,6). If the thyroid lobes are of normal size and display normal thyroid uptake of the radionuclide, we can definitely rule out hyperthyroidism. If, on the other hand, a small hyperfunctional thyroid adenoma is found, then we know that the cat has early hyperthyroidism and could be treated.
2. The second option is to continue to monitor the cat every 3-6 months (body weight, heart rate, neck palpation, and complete thyroid profile, including serum concentrations of total T4, free T4, and TSH). If hyperthyroid, the serum T4 and free T4 should continue to go higher, where as the TSH should be undetectable (1,2,7,8). If the serum TSH is found to be measurable, that goes against hyperthyroidism since even mild increases in circulating T4 and T3 should feedback to the pituitary to shut off TSH secretion.

Thyroid scintigraphy in a normal cat (on right) and cat with preclinical hyperthyroidism (on left).
Notice that the hyperthyroid cat has a small left thyroid adenoma with complete suppression of the right thyroid lobe. In cats with unilateral hyperthyroid disease, this lack of radionuclide uptake by the normal thyroid lobe is due to the fact that circulating TSH becomes suppressed in almost all hyperthyroid cats, even those with mild disease.

Either option is acceptable - just depends on what the owner wants to do. The cat is not going to be hurt by waiting, as long as he is closely monitored. At 9-years of age, he is still on the young side for being hyperthyroid, but about 5-10% of my cats are less than 10-years, so it's very possible that he is just preclinical at this time and will develop overt signs of hyperthyroidism within the next year.

References:

1. Peterson ME. More than just T4: Diagnostic testing for hyperthyroidism in cats. J Fel Med Surg 2013;15:765-777. 
2. Peterson ME. Feline focus: Diagnostic testing for feline thyroid disease: hyperthyroidism. Compend Contin Educ Vet 2013;35:E3.
3. Peterson ME, Broome MR. Radioiodine for feline hyperthyroidism In: Bonagura JD, Twedt DC, eds. Kirk's Current Veterinary Therapy, Volume XV. Philadelphia: Saunders Elsevier, 2014.
4. Peterson ME, Melian C, Nichols R. Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. J Am Vet Med Assoc 2001;218:529-536. 
5. Broome MR. Thyroid scintigraphy in hyperthyroidism. Clin Tech Small Anim Pract 2006;21:10-16. 
6. Peterson ME, Broome MR. Thyroid scintigraphic findings in 917 cats with hyperthyroidism. J Vet Intern Med 2012;26:754.
7. Wakeling J, Elliott J, Syme H. Evaluation of predictors for the diagnosis of hyperthyroidism in cats. J Vet Intern Med 2011;25:1057-1065. 
8. Wakeling J. Use of thyroid stimulating hormone (TSH) in cats. Can Vet J 2010;51:33-34.