Q & A: Hypothyroidism in a 17-year-old Chihuahua

My patient is an older 17-year old male Chihuahua. His weight is normal at 6.8 pounds. He is having episodes of pain at time of defecation and appears painful in his back.

We ran routine blood work which was all completely normal. However, a screening serum total T4 concentration was low at 0.4 μg/dl (reference range, 0.38-4.0 μg/dl). I added on a free T4 concentration and that was also low at  0.3 ng/ml (0.6 to 3.7 ng/ml). 

Would you recommend starting a thyroid supplementation in this dog?

My Response:

I doubt if this dog is truely hypothyroid for a number of reasons (1-3).

• First of all, the clinical signs the dog is showing are not at all characteristic of hypothyroidism.
• Secondly, the dog has neither anemia or hypercholesterolemia, which would make a diagnosis of hypothyroidism more likely.
• Finally, would be very unusual for a 17-year old dog to develop hypothyroidism; most dogs with hypothyroidism are young to middle age adult dogs, not geriatric animals.

In this dog, it's more likely that the serum thyroid concentrations are low either because of 1) a non-thyroidal illness, or 2) administration of a drug that is causing "suppression" of the circulating thyroid values. Such thyroid-lowering drugs include the following (3-5):

• Phenobarbital
• Sulfur antibiotics
• Clomipramine (Clomicalm) 
• Aspirin and other anti-inflammatory drugs
• Glucocorticoids

The steroids involved can be oral, parenteral, or topical, applied to the eyes, ears or skin. We need to get a good drug history in these dogs not only by reviewing the record to see what has been dispensed, but also by asking the owner what they're actually doing, as they may have bags of medications at home that we know nothing about.

Of course, if you do believe that the dog has hypothyroidism, then the next step would be to verify the low serum T4 concentration, repeat the serum free T4 using an equilibrium dialysis technique if possible, and to measure a serum cTSH concentration (2,3). If the total and free T4 remain low and the cTSH value is high, then treatment would be indicated.

The most accurate way to make a diagnosis (or rule out) a diagnosis of hypothyroidism is by use of thyroid scintigraphy (6,7). Although not widely used or available as a diagnostic test for dogs with suspected hypothyroidism, the diagnosis of hypothyroidism is easily made by the lack of finding any visible cervial thyroid tissue with thyroid imaging (see Figure below).

Thyroid Image (Scintigraphy) in a Boston Terrier with Hypothyroidism.
Notice the uptake of the radioactive tracer by the salivary glands, but the complete lack of uptake by the thyroid gland.  The normal location for the 2 thyroid lobes is indicated by yellow ovals.

Thyroid imaging typically reveals decreased or even absent radionuclide uptake in dogs with hypothyroidism (thyroid gland is not visible on the scan). In contrast, dogs that have falsely low serum thyroid hormone concentrations secondary to illness or drug therapy have a normal thyroid image. In one study of comparing thyroid imaging to other diagnostic tests for hypothyroidism in dogs, there was no overlap between dogs with primary hypothyroidism and dogs with nonthyroidal illness when thyroid scintigraphy was employed (6).   Of all of the current thyroid imaging techniques (CT, ultrasound), nuclear imaging is considered to be the best test for dogs with suspected hypothyroidism.

References:

1. Chastain CB. Canine pseudohypothyroidism and covert hypothyroidism. Problems in Veterinary Medicine 1990;2:693-716.  
2. Mooney, CT. Canine hypothyroidism: A review of aetiology and diagnosis. New Zealand Veterinary Journal 2011;59:105-114.
3. Peterson ME, Melián C, Nichols R. Measurement of serum total thyroxine, triiodothyronine, free thyroxine, and thyrotropin concentrations for diagnosis of hypothyroidism in dogs. Journal of the American Veterinary Medical Association 1997;211:1396-1402. 
4. Kantrowitz LB, Peterson ME, Trepanier LA, et al. Serum total thyroxine, total triiodothyronine, free thyroxine, and thyrotropin concentrations in epileptic dogs treated with anticonvulsants. Journal of the American Veterinary Medical Association 1999;214:1804-1808. 
5. Williamson NL, Frank LA, Hnilica KA. Effects of short-term trimethoprim-sulfamethoxazole administration on thyroid function in dogs. Journal of the American Veterinary Medical Association 2002;221:802-806. 
6. Diaz Espineira MM, Mol JA, Peeters ME, et al. Assessment of thyroid function in dogs with low plasma thyroxine concentration. J Vet Intern Med 2007;21:25-32. http://www.ncbi.nlm.nih.gov/pubmed/17338146
7. Taeymans O, Peremans K, Saunders JH. Thyroid imaging in the dog: current status and future directions.J Vet Intern Med 2007;21:673-684. http://www.ncbi.nlm.nih.gov/pubmed/17708386