Hypothyroidism in Dogs: Symptoms, Diagnosis and Treatment

Hypothyroidism is the most common endocrine disorder in dogs and yet one of the most underdiagnosed. It occurs when the thyroid gland does not produce enough thyroid hormone, which slows down metabolism across the board. It mainly affects adult and middle-aged dogs, with higher incidence in large and medium breeds. With the correct diagnosis and appropriate treatment, the dog's quality of life returns to full.

Causes: Why the Thyroid Fails

In 95% of cases, canine hypothyroidism is primary, meaning the problem lies in the thyroid gland itself. The two most common causes are lymphocytic thyroiditis (autoimmune attack on thyroid tissue) and idiopathic atrophy (gradual replacement of thyroid tissue with fat).

Predisposed Breeds

Although any dog can develop hypothyroidism, certain breeds have greater genetic predisposition: Golden Retriever, Labrador Retriever, Doberman Pinscher, Boxer, Cocker Spaniel, Great Dane, Schnauzer, and Irish Setter. The disease usually manifests between 4 and 10 years of age.

Symptoms: Metabolism in Slow Motion

Since thyroid hormone regulates the metabolism of virtually all organs, symptoms are varied and nonspecific:

• Weight gain without dietary changes, sometimes without increased appetite.
• Lethargy and exercise intolerance: the dog tires easily, prefers to rest.
• Cold intolerance: seeks heat sources, shivers.
• Coat changes: dull, dry, brittle coat; bilateral symmetric hair loss without itching, especially on the trunk and tail ("rat tail").
• Thickened skin with a myxedematous appearance.
• Bradycardia (low heart rate).
• Neurological problems in advanced cases: paresis, peripheral neuropathy.

Diagnosis

Diagnosis is based on a combination of clinical signs and blood tests: total T4 (TT4) as initial screening, free T4 (fT4) which is more specific, canine TSH (cTSH) which is elevated in primary hypothyroidism, and complete blood work which may show mild anemia and hyperlipidemia.

Treatment and Prognosis

Treatment is straightforward, effective and lifelong: oral sodium levothyroxine (synthetic thyroid hormone), usually once or twice daily. Dose is adjusted through blood tests at 4-8 weeks, then every 6-12 months. Response is very good: energy and activity increase within weeks. Weight and coat normalization may take 3-6 months. Most dogs with well-controlled hypothyroidism have a completely normal life expectancy and quality of life.