📉 Hypothyroidism in Dogs – The Most Common Hormone Imbalance: A 2025 Vet’s Guide

By Dr Duncan Houston, BVSc

🔬 What Is Hypothyroidism?

Hypothyroidism is a metabolic disorder caused by reduced production of thyroid hormone. It most commonly affects middle-aged to older dogs and leads to a generalized slowing down of body functions. It’s considered the most common hormonal imbalance in dogs. 🧠⚙️

🌡️ What Does the Thyroid Gland Do?

The thyroid gland, located near the throat, produces T4 (thyroxine), an inactive hormone. T4 is converted into T3, the active form, within tissues. T3 regulates energy metabolism in almost every cell. 🧬

• Total T4 – Most circulating T4, largely bound to proteins
• Free T4 – The unbound, biologically active form
• TSH (thyroid-stimulating hormone) – Produced by the pituitary to signal the thyroid to produce more T4

When the thyroid fails to respond properly, symptoms develop across multiple systems. 📉

📋 Causes of Hypothyroidism

• 🛡️ Immune-mediated destruction (most common)
• 🧬 Thyroid atrophy (end result of immune destruction)
• 🍽️ Iodine deficiency (rare in dogs)
• 🐶 Congenital hypothyroidism (very rare)

🐾 Breeds at Higher Risk

• Golden Retriever
• Doberman Pinscher
• Irish Setter
• Great Dane
• Boxer
• Dachshund

📉 Symptoms of Hypothyroidism

Hypothyroidism affects metabolism, skin, hair, nerves, heart, and more. Common signs include:

• 😴 Fatigue or lethargy
• ⚖️ Weight gain without increased food intake
• 🧊 Cold intolerance
• 🐕 Dry, thinning coat (especially tail and neck)
• 🌑 Darkened, thickened skin or “tragic facial expression”
• 🧪 Chronic skin infections
• 📉 Anemia and high cholesterol
• 🦴 Poor wound healing

One large study found:

• 88% had skin abnormalities
• 49% were obese
• 48% were lethargic
• 36% were anemic

💓 Cardiovascular & Neurologic Signs

• ❤️ Slow heart rate (in up to 26%)
• 🧠 Weakness, facial paralysis, or balance issues (polyneuropathy, vestibular signs)
• 🌀 Ataxia, circling, cranial nerve deficits

👁️ Eye Signs

Corneal dystrophy (white opacity) and fatty eye deposits may indicate hypothyroidism—especially when paired with systemic signs. 👁️

🧪 Testing for Hypothyroidism

1. 🧪 Total T4 (screening)

• Low value may suggest hypothyroidism—but other illnesses and drugs can also lower it
• Normal T4 rules out hypothyroidism in most dogs

2. 🧪 Free T4 (by equilibrium dialysis)

• Less influenced by non-thyroidal illness
• More reliable than total T4 alone

3. 🧠 TSH (canine thyroid-stimulating hormone)

• Helps identify hypothyroid dogs, but 25–40% of confirmed cases have normal TSH

4. 🔬 TGAA (Thyroglobulin Autoantibodies)

• Indicates immune thyroiditis
• Useful for predicting future hypothyroidism in dogs with borderline test results

5. 💊 Medication Trial

Sometimes, the best test is a treatment trial. If energy, coat, and weight improve, the diagnosis is likely correct. 🐾

🧪 Factors That Interfere with Testing

• 💊 Medications (e.g., prednisone, phenobarbital, NSAIDs, clomipramine)
• 😷 Concurrent illnesses
• 🐕 Breed variation (e.g., Greyhounds naturally have low T4)

💊 Treatment of Hypothyroidism

Medication

• 💊 Levothyroxine (T4) supplementation is the standard
• 🕐 Typically dosed twice daily, may reduce to once daily when stable

Avoid “Natural” Supplements

Do not use raw thyroid gland supplements from pigs or cows—inconsistent hormone levels make them unreliable and unsafe. ❌

🩺 Monitoring Therapy

• 🩸 Blood tests to confirm appropriate dosing
• 📉 Symptoms should improve within weeks
• 🐾 Hair regrowth may take several months

Monitoring with T4

• Take blood 4–6 hours after dosing (peak)
• Or test just before next dose (trough)

Monitoring with TSH

• Helpful for identifying underdosing (high TSH levels)
• Does not indicate overdosing

📲 Ask A Vet: Help With Hypothyroidism

Confused by thyroid test results? Need help adjusting dose or choosing a diet? The Ask A Vet App gives you direct access to expert guidance 24/7. Visit AskAVet.com or download the app today. 🐕📱

✅ Summary: Key Takeaways

• 🧬 Hypothyroidism is the most common endocrine disorder in dogs
• ⚠️ Affects skin, weight, energy, nerves, and heart
• 🧪 Diagnosis requires careful interpretation of T4, Free T4, and TSH
• 💊 Treated with daily levothyroxine (T4)
• 📈 Regular monitoring ensures long-term success

💬 Final Words from Dr Duncan Houston

Hypothyroidism may be common, but it’s not always easy to spot or diagnose. Once identified, however, it’s one of the most rewarding conditions to treat—your dog can bounce back with energy, a shiny coat, and a new lease on life. 🐶💙

For testing support, medication reviews, or behavior concerns, tap into the Ask A Vet App or visit AskAVet.com. Let’s get your dog feeling their best again. 🩺🐾