Diagnosing Naturally Occurring Hypothyroidism in Cats

I have a 5-year-old female DSH cat that has recently been diagnosed with hypothyroidism.  This diagnosis was based on a low serum T4 value that was detected during a pre-anesthetic workup for a preventative dental procedure. She had been looking unthrifty and "off" to me at home for a while, but has had no other signs of illness. Her appetite remains normal with no signs of vomiting or diarrhea.

We sent out a complete serum thyroid panel two weeks after the initial low T4 value and the results were as follows:

• Total T4: 0.3 µg/dL (reference range, 0.8-4.7 µg/dL)
• Free T4 - 0.35 ng/dL (reference range, 0.7-2.6 ng/dL)
• Total T3: <35 ng/dL (52-182 ng/dL)
• TSH 0.01 ng/ml (reference range, 0.05-0.42 ng/ml)

How would you interpret this panel?  Should we treat her as being hypothyroid? She has never been hyperthyroid and has never been treated with methimazole or radioiodine.

Is there value in trying to use one of the herbal remedies  for thyroid support (e.g., Thytrophin PMG) or should we just start her on thyroid hormone? If you think we should use thyroid hormone, which product is best to use (L-T4, L-T3, or a combination)?

My Response:

Based on your cat's serum thyroid panel, the results are not supportive of primary hypothyroidism. As the thyroid gland fails in hypothyroidism, the pituitary would respond by increasing the secretion of thyroid stimulating hormone (TSH) (1-4). In your cat, all of the serum thyroid hormone values (T4, T3, and free T4) are very low. However, the serum TSH level remains well within the reference range limits, not high as would be expected with primary hypothyroidism.

Again, the result of this thyroid panel goes against a diagnosis of primary hypothyroidism. Spontaneous hypothyroidism (i.e., not secondary to treatment of hyperthyroidism) is extremely rare in the adult cat, with only a few cases reported (4,5), so I'm not surprised that the serum TSH value is normal in your cat.

The most likely explanation for these serum thyroid results is that your cat is likely suffering from another illness. It's well known that any nonthyroidal disease can act to lower circulating T4 and T3 concentrations in cats (6-10); thyroid hormone replacement in those cats would unlikely be of any benefit.

I'm assuming that you did routine blood work (CBC, serum chemistry panel, urinalysis) in addition to the T4 for the pre-dental panel. Were there any abnormalities on this blood-work or urine?  If not, a chest radiography or abdominal ultrasound may be indicated as the next step to search for the underlying problem. Once we determine what's wrong with your cat and treat the primary problem, the serum thyroid values will likely normalize.

In any case, supplementation with thyroid hormone or any one of the herbal remedies are unlikely to be of any use and have the potential to do harm (11,12), depending on the underlying cause of your cats problems.  If your cat was indeed hypothyroid (which she isn't!), we generally use twice daily L-T4 replacement, but some cats do appear to respond better to combination L-T4/L-T3 therapy.

References:

1. Baral R, Peterson ME. Thyroid gland disorders In: Little SE, ed. The Cat: Clinical Medicine and Management. Philadelphia: Elsevier Saunders, 2012;571-592.
2. Peterson ME. Feline focus: Diagnostic testing for feline thyroid disease: Hypothyroidism. Compend Contin Educ Vet 2013;35:E1-E6. 
3. Greco DS. Diagnosis of congenital and adult-onset hypothyroidism in cats. Clin Tech Small Anim Pract 2006;21:40-44. 
4. Daminet S. Feline hypothyroidism In: Mooney CT, Peterson ME, eds. Manual of Canine and Feline Endocrinology Fourth ed. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;1-5.
5. Rand JS, Levine J, Best SJ, et al. Spontaneous adult-onset hypothyroidism in a cat. J Vet Intern Med 1993;7:272-276. 
6. 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. 
7. Peterson ME, Gamble DA. Effect of nonthyroidal illness on serum thyroxine concentrations in cats: 494 cases (1988). J Am Vet Med Assoc 1990;197:1203-1208. 
8. Mooney CT, Little CJ, Macrae AW. Effect of illness not associated with the thyroid gland on serum total and free thyroxine concentrations in cats. J Am Vet Med Assoc 1996;208:2004-2008. 
9. McLoughlin MA, DiBartola SP, Birchard SJ, et al. Influence of systemic nonthyroidal illness on serum concentrations of thyroxine in hyperthyroid cats. J Am Anim Hosp Assoc 1993;29:227-234. 
10. Mooney CT. The effects of nonthyroidal factors on tests of thyroid function. Australian College of Veterinary Scientists, Science Week 2010;28-30.  
11. Bello G, Paliani G, Annetta MG, et al. Treating nonthyroidal illness syndrome in the critically ill patient: still a matter of controversy.  Curr Drug Targets 2009;10:778-787. 
12. Adler SM, Wartofsky L. The nonthyroidal illness syndrome. Endocrinol Metab Clin North Am 2007;36:657-672, vi.

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