Hypothyroidism is one of the most frequently diagnosed endocrinopathies (hormonal diseases) in dogs.

It disrupts the entire metabolism, can take many different forms, and remains both underdiagnosed in some cases and overdiagnosed in others when tests are misinterpreted.

Understanding its mechanisms, clinical signs, and diagnostic limitations is useful for any dog owner, especially for predisposed medium to large breeds.

Mechanisms and Causes

The thyroid is a small endocrine gland located on either side of the trachea in a dog's neck.

It primarily produces T4 (thyroxine) and T3 (triiodothyronine) hormones, which play an essential role in regulating metabolism, body temperature, energy, weight, skin, and coat. When it malfunctions, the entire body can be affected, especially in cases of hypothyroidism.

In the vast majority of cases, canine hypothyroidism is primary, meaning it is related to a direct impairment of the thyroid gland.

Two main mechanisms are described in veterinary literature:

- lymphocytic thyroiditis (of autoimmune origin)

- idiopathic follicular atrophy (the exact cause of which remains less clear, but which could, in some cases, correspond to the terminal evolution of an unnoticed autoimmune thyroiditis.)

Lymphocytic thyroiditis has a documented hereditary component.

The risk varies greatly among breeds, suggesting an underlying genetic predisposition, the molecular basis of which is the subject of active research.

Clinical Signs

Hypothyroidism slows down the general metabolism, which explains the diversity of its manifestations. Among the most commonly reported signs are weight gain, lethargy, decreased exercise tolerance, generalized weakness, bradycardia, and various skin or coat abnormalities.

Dermatological signs are particularly frequent: dull, dry coat that regrows slowly, alopecia (partial or total hair loss in one area of the body, or sometimes over the entire coat), skin thickening, hyperpigmentation (abnormally darker skin coloration due to increased skin pigment), seborrhea (a skin disorder characterized by excessive sebum production and/or abnormal scaling; the skin can then become oily, dry, flaky, and sometimes malodorous).

Neurological manifestations are possible, but less common. Biologically, hypercholesterolemia (too much cholesterol) and mild anemia (low red blood cell count) can also be observed.

The problem is that this picture is not specific.

Many of these signs can be seen in other diseases. In practice, therefore, clinical examination alone is never enough to make a reliable diagnosis.

Diagnosis: Caution and Rigor

Diagnosis relies on the combined interpretation of the clinical context and thyroid panel, including total T4, free T4, and canine TSH.

The difficulty is that a low T4 does not automatically mean hypothyroidism.

It can also be observed in other systemic diseases or under the effect of certain medications: this is the euthyroid sick syndrome.

This point is fundamental. A study conducted in general practice in the UK showed that a significant proportion of dogs treated for hypothyroidism were likely not actually affected and had received levothyroxine without sufficiently rigorous diagnostic confirmation.

In short: a dog that is not actually hypothyroid can be treated incorrectly.

The most robust panel ideally combines: total T4, free T4 by equilibrium dialysis, canine TSH, and, depending on the case, anti-thyroglobulin antibodies.

This is significantly more reliable than an isolated T4 measurement.

At-Risk Breeds

Several breeds show a documented predisposition, including Dobermans, Golden Retrievers, Labrador Retrievers, Beagles, Irish Setters, and Boxers.

The Briard is also mentioned in some veterinary sources as a predisposed breed, without being among the most massively affected in large epidemiological studies.

Neutering has also been identified as an additional risk factor in some analyses.

Treatment and Prognosis

The standard treatment is daily administration of levothyroxine, which is synthetic T4. When properly indicated and adjusted, it generally yields good to very good results, with progressive clinical improvement within a few weeks.

Dose adjustment requires regular biological monitoring, and treatment is usually lifelong. The prognosis is generally favorable in the majority of correctly diagnosed and well-managed dogs.

In Summary

Main Cause: autoimmune lymphocytic thyroiditis or idiopathic follicular atrophy

Frequent Signs: weight gain, lethargy, dull coat, alopecia, decreased exercise tolerance

Diagnosis: complete thyroid panel (never an isolated T4 alone)

Treatment: daily oral levothyroxine with regular monitoring

Prognosis: generally favorable if diagnosis is correct

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Sources

1. Panciera DL. Hypothyroidism in dogs: 66 cases (1987-1992). J Am Vet Med Assoc, 1994. PubMed 8175472

2. Mooney CT. Canine hypothyroidism: a review of aetiology and diagnosis. N Z Vet J, 2011. PubMed 21541883

3. Scott-Moncrieff JC. Clinical signs and concurrent diseases of hypothyroidism in dogs and cats. Vet Clin North Am Small Anim Pract, 2007. PubMed 17619007

4. O’Neill DG et al. Frequency, breed predispositions and demographic risk factors for diagnosis of hypothyroidism in dogs in the UK. Canine Med Genet, 2022. PubMed 36217196

5. Graham PA et al. Lymphocytic thyroiditis. Vet Clin North Am Small Anim Pract, 2001. PubMed 11570132