Read the French translation: Diagnostic et Traitement de l'Hypothyroidie Canine

Introduction

The image we have of canine hypothyroidism has changed during the last decade. We use to consider it as the most common endocrinopathy in dogs. Most endocrinologists will agree that nowadays, other endocrine diseases such as hypercortisolism, are more frequently observed. In the past, many dogs have been erroneously diagnosed with hypothyroidism. Indeed, evaluation of thyroid function in dogs is not always straightforward. The vague and non-specific clinical signs of hypothyroidism and the fact that numerous factors can influence thyroid function test results are major contributors to the difficulty in diagnosing this disease. During this presentation, emphasis will be placed on some new developments regarding diagnostic tests and factors influencing the results of these tests.

Diagnosis of Canine Hypothyroidism

Because of the vague clinical signs and the absence of specific abnormalities on a routine blood test, the diagnosis should be confirmed through a specific evaluation of the thyroid gland. As always, laboratory results should be interpreted in the light of history and physical examination findings. A thorough clinical examination of the patient, knowledge of the advantages and disadvantages of all available tests and knowledge of the factors that can influence the results, will allow the veterinarian to correctly diagnose the disease. The table below contains a summary of the advantages and disadvantages of the most commonly used thyroid hormone tests in dogs.

Table. Advantages and disadvantages of the most commonly used thyroid tests.

*rhTSH: recombinant human TSH: Dosages ranging from 50-100 µg have been used, however, in the presence of concurrent disease or drug administration, a dosage of 150 µg is recommended.

Thyroid hormone concentrations vary opposite to age. Half of the dogs older than 6 years of age have lower TT4 serum values. Thyrotropin stimulation test results are also less pronounced in older dogs.

Some dog breeds clearly have T4 serum concentrations lower (approximately half) than values observed in most breeds. This has been documented in Greyhounds, Sloughis, Basenji and Whippet. Diagnosis of hypothyroidism can be particularly difficult in those breeds and further diagnostic tests, such as scintigraphy or rhTSH stimulation test, are therefore recommended.

Summary of the effects of some drugs on canine thyroid function test results.

*Sulfonamide-induced hypothyroid crisis has been reported.

Treatment of Canine Hypothyroidism

Treatment of hypothyroidism consists in lifelong administration of synthetic levothyroxine (L-T4). Dosages used to treat dogs are higher than in human medicine. Initial treatment dosages vary from 10 to 22 µg/kg q 12 or q 24 hours according to the author and the formulation used, with a maximum of 0,8 mg of L-thyroxine q 12 hour. The patient is revaluated 1 to 2 months after initiating therapy and dosage is adjusted based on clinical response, and results of the TT4. When interpreting the result of TT4, time of sampling compared to the administration of the medication, should be taken into consideration. Most commonly blood is taken 3 to 6 hours after the last medication is administered (post-tablet test) and peak concentrations are measured. In this case, TT4 is expected to be within the reference range (upper half limit), and a TT4 value just above the reference range is accepted. In most patients, follow-up of TSH does not offer a significant advantage over a measurement of T4 solely.

References

1.  Boretti FS, et al. J Vet Intern Med, 23: 856-861, 2009.

2.  Daminet S, Ferguson DC. J Vet Intern Med, 17: 663-472, 2003.

3.  De Roover K, et al. J Vet Intern Med, 20: 812-817, 2006.

4.  Daminet S, et al. Can Vet J, 48: 1273-1279, 2007.

5.  Pinilla M, et al. Vet Radiol & Ultrasound, 50: 224-229, 2009.