Q & A: High Serum ALT Values in a Cat with a Normal T4

I have a quick question about liver function tests in hyperthyroid cats. Is there a serum thyroid concentration at which you begin to believe elevations in the serum alanine aminotransferase (ALT) activity may be due to subtle (occult) feline hyperthyroidism?

I have a cat with mildly high ALT activity with a serum T4 value of 3.8 μg/dl (reference range, 0.8-4.0 μg/dl). Thanks!

My Response:

As you know, mild to marked increases in the serum activities of many liver enzymes, including alanine aminotransferase (ALT) and alkaline phosphatase (ALP) are the most common and striking biochemical abnormalities of feline hyperthyroidism (1,2). These liver enzymes changes and T4 concentrations are related, with liver enzyme abnormalities being more common in cats with severe hyperthyroidism. That said high liver enzymes can be an early indicator of hyperthyroidism in cats. Although how thyroid hormone excess stimulates the high ALT and ALP activity is not completely understood, it is clear that these high liver enzymes return to normal upon successful treatment of hyperthyroidism (3).

If a cat with a high ALT also has mild weight loss, a good appetite, and a serum T4 in the upper half of the reference range (or higher), I would certainly consider hyperthyroidism as a differential diagnosis.

How do we best diagnose early hyperthyroidism when the serum T4 concentrations are normal?  Your options would be to follow the serum concentrations of total T4 and free T4 at monthly intervals for awhile to see if they increase into the hyperthyroid range (4). Or you could do a T3 suppression test or do a thyroid scan (if available) if you and the owner are in a hurry to make a diagnosis of mild hyperthyroidism.

Of course, as you are monitoring the serum T4 concentration, you also want to follow the serum ALT values. An abdominal ultrasound to look at the liver is never a bad idea either, especially if the liver function tests continue to rise but the serum thyroid function tests remain normal.

References:

1. Thoday KL, Mooney CT. Historical, clinical and laboratory features of 126 hyperthyroid cats. Veterinary Record 1992;1:257-64.
2. 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;206:302-5.
3. Berent AC, Drobatz KJ, Ziemer L, Johnson VS, Ward CR. Liver function in cats with hyperthyroidism before and after 131-I therapy. Journal of Veterinary Internal Medicine 2007;2:1217-23.
4. Peterson ME. Diagnostic tests for hyperthyroidism in cats. Clinical Techniques in Small Animal Practice 2006;21: 2-9.