🦋 Hypothyroidism in Senior Dogs: Symptoms, Diagnosis & Treatment

What Causes Hypothyroidism in Dogs?

The vast majority of canine hypothyroidism cases (95%+) are caused by one of two immune-mediated processes:

Lymphocytic thyroiditis — The immune system attacks and destroys thyroid tissue. This is believed to have a genetic component and is more common in certain breeds.

Idiopathic atrophy — Thyroid tissue is replaced by fat for unknown reasons, resulting in a shrunken, non-functional gland.

Breeds with a known predisposition include Golden Retrievers, Doberman Pinschers, Irish Setters, Boxers, Cocker Spaniels, Airedale Terriers, and Dachshunds. Most affected dogs are between 4 and 10 years old at diagnosis.

Signs and Symptoms

Because thyroid hormone affects every cell in the body, the symptoms of hypothyroidism are wide-ranging and can mimic many other conditions — including "just getting old." Common signs include:

• Weight gain without increased appetite — Slowed metabolism causes weight gain even with normal food intake
• Lethargy and exercise intolerance — Dogs become sluggish, sleep more, and tire quickly
• Cold intolerance — Seeking warm spots, reluctance to go outside in cool weather
• Coat and skin changes — Dull, dry, brittle coat; hair loss (often symmetrical on the trunk); thickened, darkened skin
• "Tragic face" — Thickening of facial skin gives some dogs a sad, droopy appearance
• Slow heart rate (bradycardia)
• Neurological signs — In severe cases: weakness, incoordination, facial nerve paralysis, head tilting
• Reproductive issues — Irregular heat cycles in intact females, infertility

Diagnosing Hypothyroidism

Diagnosis is based on clinical signs combined with blood tests. The standard diagnostic workup includes:

Total T4 (Thyroxine) — A basic screening test. Low T4 supports hypothyroidism, but results can be suppressed by other illnesses or medications (a phenomenon called "euthyroid sick syndrome").

Free T4 by Equilibrium Dialysis (fT4 ED) — More specific and not as affected by non-thyroidal illness. Considered the most reliable single test.

Canine TSH (Thyroid-Stimulating Hormone) — When combined with T4 or fT4, significantly improves diagnostic accuracy. Elevated TSH with low T4/fT4 is highly supportive of hypothyroidism.

Your vet may also check a complete blood panel (often showing elevated cholesterol and triglycerides) and urinalysis before reaching a diagnosis.

Treatment: Levothyroxine

The great news about hypothyroidism: it's highly treatable and the medication is inexpensive. Levothyroxine (synthetic T4) is given orally once or twice daily. Brand names include Soloxine and Thyro-Tabs.

Most dogs show improvement within 4–8 weeks of starting treatment:

• Energy levels return to normal
• Coat begins to regrow and improve in quality
• Weight starts to normalize (in combination with appropriate diet)
• Neurological signs, if present, often improve significantly

Initial follow-up testing is done 4–8 weeks after starting medication to ensure T4 levels are in the optimal range. Once stable, monitoring every 6–12 months is typical. Dogs generally require lifelong treatment.

Important: Avoid giving levothyroxine with food — it's best absorbed on an empty stomach. Some vets recommend giving it 1 hour before or 3 hours after meals. Always follow your vet's instructions.