Hypothyroidism in Dogs
The most common endocrine disease in dogs, caused by insufficient thyroid hormone production. Leads to a slowed metabolism affecting nearly every body system. Easily diagnosed and treated with daily medication.
Last updated: 2026-05-06
Severity
mild
When to Act
See Vet Soon
Symptoms & Signs
Weight gain without increased appetite
Dog gains weight despite eating the same or less — classic metabolic slowdown.
Lethargy and mental dullness
Dog sleeps more, is less responsive, and seems "depressed."
Hair coat changes
Dry, brittle coat; symmetrical hair loss on the flanks and tail ("rat tail"); hyperpigmentation.
Cold intolerance
Seeks heat — lies near radiators or under blankets, reluctant to go out in cold weather.
Recurrent skin and ear infections
Impaired skin barrier and immune function from low thyroid.
Slow heart rate
Bradycardia on physical examination.
Behavioral Changes to Watch For
Pets can't tell us what's wrong. These behavioral changes are often the first clues that something is wrong.
🐾 Lethargy and "slowing down"
Dog seems older than their years — sleeps constantly, doesn't greet visitors, no interest in walks.
What You May Notice:
People ask if your young dog is a senior — they move slowly and seem disengaged.
Causes & Risk Factors
Causes
- •Primary hypothyroidism: autoimmune (lymphocytic thyroiditis) or idiopathic atrophy of the thyroid gland — >95% of cases
- •Secondary hypothyroidism: pituitary dysfunction — rare
- •Iatrogenic: after treatment for hyperthyroidism or thyroid surgery
Risk Factors
- ⚠Middle-aged dogs (4-10 years), typically medium to large breed
- ⚠Certain breeds: Golden Retriever, Doberman Pinscher, Labrador, Cocker Spaniel, Boxer, Great Dane
- ⚠No sex predilection
How It's Diagnosed
- 1Total T4 (thyroxine) — low; but can also be low from non-thyroidal illness (sick euthyroid syndrome)
- 2Free T4 by equilibrium dialysis — more accurate, less affected by concurrent illness
- 3TSH — elevated (lack of negative feedback from low thyroid hormone)
- 4Thyroglobulin autoantibodies — supports autoimmune etiology
- 5Complete blood count — mild normocytic normochromic anemia
- 6Cholesterol and triglycerides — often elevated
Treatment Options
Levothyroxine (Thyroid Hormone Replacement)
Synthetic T4 — exactly replaces what the thyroid should be producing.
Steps
- 1.Levothyroxine (Soloxine, Thyro-Tabs) 0.02 mg/kg PO twice daily (or total daily dose given once daily — twice daily preferred)
- 2.Recheck T4 and TSH 4-6 weeks after starting or adjusting dose
- 3.Timing: give on an empty stomach for best absorption
- 4.Lifelong daily medication
Expected Outcome
Dramatic improvement: increased energy within 1-2 weeks; coat improvement within 4-8 weeks; weight loss over 2-3 months.
Precautions
- !Over-supplementation causes thyrotoxicosis — restlessness, panting, weight loss, tachycardia
- !Regular monitoring is essential — dose often needs adjustment over time
Common Medications Used
| Medication | Usage | Important Notes |
|---|---|---|
| Levothyroxine (Soloxine) | Synthetic T4 — lifelong thyroid hormone replacement | Excellent prognosis with treatment. Dramatic before/after improvement in most dogs. |
Prevention
- ✓No prevention for autoimmune thyroiditis
- ✓Do not breed hypothyroid dogs
- ✓Annual wellness blood work for early detection in predisposed breeds
When to See a Veterinarian
- ⚠️Unexplained weight gain + lethargy
- ⚠️Symmetrical hair loss
- ⚠️Recurrent infections in a middle-aged dog
- ⚠️"Old dog" behavior in a young/middle-aged dog
Frequently Asked Questions
Will my dog need treatment for life?
Prognosis
Excellent with treatment. Most dogs return to completely normal life on daily levothyroxine. Occasional dose adjustments are needed. Without treatment, quality of life is poor but the condition is not directly fatal.
References
- [1] ACVIM — Canine Hypothyroidism Guidelines
- [2] MSD Veterinary Manual
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