The serum T4 concentration was 4.0 μg/dl, which was right at the upper reference range limit (0.8-4.0 μg/dl). So I recommended that we measure free T4 by dialysis, which came back high at 75 pmol/L (reference range, 15-55 pmol/L). Based on this high free T4 value, I diagnosed hyperthyroidism and started the cat on methimazole (1.25 mg, BID).
After 2 weeks of treatment with methimazole, a recheck T4 value was normal at 2.5 μg/dl, and her vomiting had also stopped completely. I then rechecked her 4 weeks later and her T4 remained normal at 2.9 μg/dl.
The owners then decided that they wanted to treat their cat with I-131 so they stopped the methimazole for 10 days and we repeated the serum total and free T4 values to confirm the diagnosis. Results revealed normal serum concentration of both T4 (2.8 μg/dl) as well as free T4 (15 pmol/L). So based on those thyroid results, she is not currently hyperthyroid.
- So did she have a transient hyperthyroid episode or was she not hyperthyroid in the first place?
- If her free T4s were elevated due to non-thyroidal illness, I wouldn't have expected the vomiting to stop so coincidentally with the start of the methimazole.
- And I would have thought that her T4s would have gone a good bit lower when she was started on the methimazole if she were not truly hyperthyroid.
To make the diagnosis of hyperthyroidism, we need more evidence than just a high free T4 value, or even a high T4. We see false-positive elevations in free T4 in up to 10-15% of euthyroid cats (1,2), so the finding of a high free T4 alone really isn't that helpful unless other historical and physical exam findings also support hyperthyroidism.
In addition, I've also seen a few euthyroid cats (and dogs) that have slightly high total T4 and free T4 concentrations (3,4). This is probably due to the fact that all of the labs are going away from the use of radioimmunoassay (RIA) and chemiluminescence to more automated techniques that just aren't as accurate so we are seeing more false-positive test results.
Normal cats do not necessarily fall into the hypothyroid range when treated with relatively low doses of methimazole, as you used in this cat. So that cannot be used to rel ably diagnose or exclude occult hyperthyroidism in cats.
So what do I look for in these cats with suspected hyperthyroidism?
- Well, it's not always so easy to confirm the diagnosis of early or "occult" hyperthyroidism, but the finding of a thyroid nodule is key (5-11). If we cannot palpate a thyroid nodule but hyperthyroidism is still suspected, use of thyroid scintigraphy can be extremely help in ruling out the diagnosis (12).
- Other findings I look for on my physical examination, include tachycardia, pounding heart on palpation of the thoracic wall, heart murmur, muscle wasting over the back, and increased nervousness (5-11).
- Historically, we would want to know if the appetite was increased or just normal or even decreased. Almost all hyperthyroid cats vomit because they eat too much too fast —if this cat's vomiting is not related to eating, it's less likely that the vomiting was related to hyperthyroidism.
- On your routine blood work, I look at the serum alanine aminotransference activity (ALT) because over 70% of hyperthyroid cats will have a high value (5-11). On the CBC, a low normal or low RBC count, hemoglobin or PCV goes against hyperthyroidism. Excess T4 stimulates the bone marrow so these cats are never anemic unless something else is going on.
- I know that this cat responded to methimazole, but that doesn't mean very much unless we stop the drug (and vomiting returns) and retrial the cat on the methimazole again to determine if a cause-and effect relationship really exists (in other words, the vomiting stops again on the drug).
- 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. Journal of the American Veterinary Medical Association 2001;218:529-536.
- Mooney CT, Little CJ, Macrae AW. Effect of illness not associated with the thyroid gland on serum total and free thyroxine concentrations in cats. Journal of the American Veterinary Medical Association 1996;208:2004-2008.
- Peterson ME: Diagnostic testing for feline hyper- and hypothyroidism. Proceedings of the 2011 American College of Veterinary Internal Medicine (ACVIM) Forum. 2011; 95-97.
- Peterson ME. Diagnostic tests for hyperthyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21:2-9.
- Peterson ME, Kintzer PP, Cavanagh PG, et al. Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. Journal of the American Veterinary Medical Association 1981;183:103-110.
- Broussard JD, Peterson ME, Fox PR. Changes in clinical and laboratory findings in cats with hyperthyroidism from 1983 to 1993. Journal of the American Veterinary Medical Association 1995;206:302-305.
- Baral R, Peterson ME. Thyroid gland disorders. In: Little, S.E. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012; 571-592.
- Peterson ME: Hyperthyroidism, In: Ettinger SJ, Feldman EC (eds): Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat (Fifth Edition). Philadelphia, WB Saunders Co. 2000; pp 1400-1419.
- Peterson ME: Hyperthyroidism in cats. In: Melian C (ed): Manual de Endocrinología en Pequeños Animales (Manual of Small Animal Endocrinology). Multimedica, Barcelona, Spain, 2008, pp 127-168.
- Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012:92-110.
- Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.
- Peterson ME, Broome MR. Thyroid scintigraphic findings in 917 cats with hyperthryoidism. Journal of Veterinary Internal Medicine 2012; in press.