Hypothyroidism in Dogs: Symptoms, Signs, and How It's Treated

When a dog starts gaining weight despite eating the same amount, loses interest in walks, and develops a dull, thinning coat — the thyroid gland deserves a close look. Hypothyroidism is one of the most commonly diagnosed hormonal conditions in dogs in 2026, and the good news is that once diagnosed, it's highly treatable.

What Is Hypothyroidism?

The thyroid gland, located in the neck, produces hormones that regulate metabolism throughout the body. When the thyroid underproduces these hormones, metabolism slows — affecting nearly every organ system. According to the Merck Veterinary Manual, the most common cause in dogs is autoimmune thyroiditis, where the dog's own immune system gradually destroys thyroid tissue. A second cause, idiopathic thyroid atrophy, occurs when thyroid tissue is replaced by fat for unknown reasons.

Middle-aged dogs (4-10 years) are most commonly affected. Breeds with higher risk include Golden Retrievers, Doberman Pinschers, Irish Setters, Cocker Spaniels, Dachshunds, and Boxers.

Signs and Symptoms of Hypothyroidism in Dogs

Classic Signs

• Weight gain without increased appetite — metabolism slows and fat accumulates despite normal food intake
• Lethargy and exercise intolerance — your dog seems tired, reluctant to exercise, and sleeps more than usual
• Hair loss and poor coat quality — patchy hair loss, especially on the trunk and tail ("rat tail"), with a dull, dry coat
• Cold intolerance — seeking warm spots, shivering in temperatures that never bothered them before
• Thickened or scaly skin — skin may feel rough or flaky, with excessive dandruff

Less Common Signs

• Slow heart rate (bradycardia)
• Chronic ear infections and skin infections
• Muscle weakness or stiffness
• Neurological signs in severe cases — head tilt, difficulty walking, or facial nerve paralysis
• High cholesterol (detected on bloodwork, not visible to owners)

The combination of unexplained weight gain, coat loss, and lethargy should prompt a thyroid test. These symptoms overlap with Cushing's disease, so your vet will want to run a panel.

When to Worry: Urgent Signs

Call your vet promptly if your dog develops:

• Facial nerve paralysis (drooping face, inability to close one eye)
• Difficulty swallowing
• Sudden loss of coordination or balance
• Signs of severe weakness in any limb
• Myxedema coma (extremely rare — profound lethargy, swollen face, very low body temperature)

What to Do at Home

There is no safe home treatment for hypothyroidism. But here's how to prepare:

• Schedule bloodwork. A complete blood panel including a T4 (thyroxine) level is the first step. If the T4 is low, a free T4 and TSH may be ordered to confirm.
• Track your observations. How long has the weight gain been happening? Has the coat changed? This timeline helps your vet.
• Don't start supplements. Iodine supplements can disrupt thyroid tests and worsen the condition.
• Note any skin or ear infections. These are common secondary issues in hypothyroid dogs.

Treatment: Levothyroxine for Life

Hypothyroidism in dogs is treatable but not curable. According to PetMD, treatment involves oral levothyroxine (synthetic thyroid hormone) given daily — typically twice a day, on an empty stomach.

Most owners see improvement within 4-6 weeks: more energy, better coat, and weight normalization. Skin and coat changes take several months to fully resolve.

Dogs need thyroid level monitoring (blood tests) at 1 month, 3 months, 6 months, and then annually. The dose may need adjusting over time. Dogs stay on levothyroxine for life.

How Voyage Can Help

Weight gain and lethargy have many causes — from thyroid problems to joint pain and arthritis to dental disease. Voyage AI Vet can help you sort through your dog's specific combination of symptoms and determine the most likely culprit — starting at $4.99/month. No appointment needed.

Frequently Asked Questions

Q: How is hypothyroidism diagnosed in dogs? A: A blood test measuring total T4 levels is the starting point. If levels are borderline, free T4 by equilibrium dialysis and TSH levels provide more accuracy. Many cases require more than one test to confirm.

Q: Will my dog lose the weight after starting treatment? A: Most dogs lose the excess weight within 3-6 months of starting levothyroxine, especially if their exercise routine also improves as their energy returns.

Q: Is hypothyroidism painful for dogs? A: It's not painful in itself, but the fatigue, cold intolerance, and skin issues can significantly affect quality of life. Dogs on proper treatment typically feel dramatically better.

Q: Can hypothyroidism cause aggression in dogs? A: Yes — behavioral changes including aggression, anxiety, and unpredictability have been documented in hypothyroid dogs. These usually resolve with proper treatment, though this is less commonly recognized.

This article is for informational purposes only and is not a substitute for professional veterinary advice.