Buster is overweight at 17 pounds, but he has lost 3 pounds on a weight loss plan. Other than being overweight and increased bronchovesicular sounds on auscultation, his physical examination was unremarkable, with great skin and haircoat.
His overall blood work was totally normal, with the exception of a low serum T4 concentration (0.4 μg/dl; reference range = 0.8-4.0 μg/dl). I added on a free T4 value, expecting it to be normal; however, it was also low at 5 pmol/L (reference range = 10-50 pmol/L).
My question is this: Can the euthyroid sick syndrome can suppress serum T4 and T3 levels in cats, as drugs or disease can do in dogs? I do not feel treatment with levothyroxine (L-T4) is warranted in this cat, but I don't want to ignore low thyroid values either unless I can explain why they might be low.
I'd greatly appreciate your insight on this curious case.
I agree with you that Buster would not likely benefit from L-T4 supplementation.
Hypothyroidism is a clinical diagnosis -- if there aren't any clinical signs, the finding of low serum thyroid hormones values alone doesn't justify treatment. This is even more true in cat, where spontaneous hypothyroidism has only been documented in only a couple adults cats. Congenital hypothyroidism in kittens is much more common, but Buster certainly doesn't have congenital hypothyroidism since he is 10 years old!
Figure 1: Notice that the total T4 concentrations are low in about half of the cats with nonthyroidal disease (pink box plot on right).
Figure 2: Notice that the free T4 concentrations tend to stay within normal range in cats with nonthyroidal illness (pink box plot on right), but values are subnormal in about 20% of these cats. Also note the falsely "high" values in other sick cats, illustrating some of the problems with the free T4 assay in cats.
It certainly is possible that his asthma and steroid therapy has suppressed both his total and free T4 concentrations. In 2001, I wrote a research paper (1) where we measured total and free T4 in cat with hyperthyroidism and nonthyroidal illness. As you can see in Figures 1 and 2, both total and free T4 were suppressed to low levels in some of the cats who were ill.
1. 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;218:529-36.