Hypothyroidism is the most common endocrine (hormonal) disorder in dogs, yet it often goes undiagnosed for months to years because its symptoms mimic "normal aging." The thyroid gland produces hormones that regulate metabolism in every cell of the body. When production drops, everything slows down — weight gain, lethargy, skin problems, and cognitive changes that owners attribute to getting older.
Cause
95% of canine hypothyroidism is caused by lymphocytic thyroiditis (autoimmune destruction of the thyroid gland) or idiopathic atrophy (gradual replacement of thyroid tissue with fat). Both processes are irreversible and progressive.
Symptoms
| System | Signs |
|---|---|
| Metabolic | Weight gain without increased food intake, lethargy, exercise intolerance, cold intolerance |
| Skin/Coat | Bilateral symmetric hair loss, "rat tail" (tail hair loss), dry/dull coat, thickened skin, recurrent skin infections, hyperpigmentation |
| Neurological | Facial nerve paralysis, vestibular disease, cognitive changes, peripheral neuropathy |
| Cardiovascular | Slow heart rate (bradycardia) |
| Reproductive | Infertility, irregular cycles, failure to conceive |
The "tragic triad": Lethargy + weight gain + skin problems in a middle-aged dog = suspect hypothyroidism until proven otherwise.
Commonly Affected Breeds
Golden Retriever, Labrador Retriever, Doberman Pinscher, Irish Setter, Dachshund, Cocker Spaniel, Boxer, Great Dane. Typically diagnosed between ages 4-10.
Diagnosis
- Total T4 (thyroxine): Screening test. Low T4 suggests hypothyroidism but has false positives (non-thyroidal illness, medications).
- Free T4 by equilibrium dialysis: More accurate than total T4. Less affected by non-thyroidal illness.
- TSH (thyroid-stimulating hormone): Elevated TSH + low T4/fT4 = strong diagnostic evidence.
- Thyroglobulin autoantibodies (TgAA): Detects autoimmune thyroiditis. Can identify disease before clinical signs appear.
Treatment
- Levothyroxine (Soloxine, Thyro-Tabs): Synthetic thyroid hormone. Oral, twice daily initially.
- Starting dose: 0.02 mg/kg twice daily
- Response time: Energy improves within 1-2 weeks. Skin/coat improvements take 4-12 weeks. Weight loss takes 1-3 months.
- Monitoring: T4 levels checked 4-6 weeks after starting, then every 6 months. Blood drawn 4-6 hours post-pill (peak level).
- Cost: $15-$40/month depending on dog size. One of the cheapest chronic medications.
Frequently Asked Questions
Is hypothyroidism curable?
No — but it's perfectly manageable. The thyroid gland is permanently damaged, so supplementation is lifelong. The good news: levothyroxine is inexpensive, safe, and dogs return to completely normal function on appropriate doses. They don't "feel" hypothyroid once treated.
My dog gained weight. Could it be thyroid?
Possibly — but obesity from overfeeding is far more common than thyroid-related weight gain. Hypothyroid weight gain is typically 10-20% of body weight, accompanied by lethargy and coat changes. Simple overeating causes weight gain without coat or energy changes. Your vet can test thyroid function with a blood panel.
