Results of the dog's complete blood count and serum chemistry profile was completely normal. However, the serum T4 concentration was low at 0.5 μg/dl (reference range, 1.0-4.0 μg/dl), so I added on a serum cTSH and free T4 concentrations.
The serum free T4 concentration was also low at 0.3 ng/dl (reference range, 0.6-3.7 ng/dl); the cTSH value was low at 0.26 ng/ml (reference range, 0.5-0.42 ng/ml).
The dog is clinically very healthy, and not on any treatments. I wasn't really looking for hypothyroidism but the T4 was included as part of the senior profile I ran.
With the low total and free T4 and low TSH, I am considering that the dog has secondary (pituitary) hypothyroidism (TSH deficiency). I'm planning to start L-T4 replacement. Do you agree? Could the dog have euthyroid sick syndrome and not be hypothyroid?
So your question is really this: What do we do when the serum T4 is low, but the dog is apparently healthy and not showing any clinical features of hypothyroidism? And the short answer is simply this—don't measure serum T4 in dogs that don't have any clinical and biochemical evidence of hypothyroidism.
Thyroid function testing is best done when there is a clinical suspicion of thyroid disease (e.g., lethargy, weight gain despite an indifferent appetite, coat change), ideally combined with other lab evidence of hypothyroidism (e.g., hypercholesterolemia, mild anemia) (1-3).
- Determination of serum T4 (or free T4) has no place as a screening test in apparently healthy dogs. False positive results are extremely common, even in dogs that aren't acting ill. Too often, you will end up with a “low-ish” T4 and then be obliged to chase it when in fact there is nothing wrong with the patient.
- There is no need to supplement a dog with L-thyroxine on the basis of an incidental low T4. Instead, the owners should be advised as to possible causes of low T4 other than thyroid disease, and at best, coached on how to watch for clinical signs of hypothyroidism.
That all makes sense. So now I know: if a low total T4 is detected on routine bloodwork, don't bother with the cTSH if there aren't signs of hypothyroidism. No thyroxine for this dog then. The owner will be very happy!
- Mooney CT. Canine hypothyroidism: a review of aetiology and diagnosis. New Zealand Veterinary Journal. 2011;59:105-114.
- Scott-Moncrieff JC. Clinical signs and concurrent diseases of hypothyroidism in dogs and cats.
- Veterinary Clinics of North America Small Animal Practice 2007;37:709-722,
- Ferguson DC. Testing for hypothyroidism in dogs. Veterinary Clinics of North America Small Animal Practice 2007;37:647-669.