Hypothyroidism is a deficiency of thyroid hormone caused by underactive thyroid glands. It is the most common endocrine (hormonal) disorder in dogs, typically caused by immune-mediated destruction or atrophy of the thyroid gland. The classic signs are weight gain, lethargy, cold intolerance, hair loss, and a dull coat — symptoms that develop so slowly they are frequently mistaken for normal aging.
The thyroid gland produces hormones that regulate metabolic rate across virtually every body system. When production falls below normal, the metabolic rate slows: energy expenditure decreases, leading to weight gain even without increased food intake; heart rate slows; cold intolerance develops; the coat becomes dull, dry, and brittle; and mental dullness and lethargy become apparent. In severe cases, a tragic facial expression described as 'tragic face' may develop from skin thickening and myxoedema.
Hypothyroidism is confirmed through blood testing. Total T4 (thyroxine) is the initial screening test — a low result is consistent with hypothyroidism, though false positives occur from non-thyroidal illness. Free T4 measured by equilibrium dialysis is more accurate. TSH (thyroid-stimulating hormone) levels are evaluated alongside T4 — an elevated TSH with low T4 is the most definitive combination. Full thyroid panels are needed for borderline cases.
Treatment is straightforward and highly effective: once daily oral levothyroxine (synthetic T4). Most dogs show noticeable improvement in energy levels, coat quality, and weight management within 4–8 weeks of starting treatment. Lifelong medication is required. Dose adjustments based on repeat thyroid testing (initially after 4–8 weeks, then every 6–12 months once stable) ensure the dog remains in the appropriate therapeutic range.
📌 Key Facts
Most common endocrine disorder in dogs: Hypothyroidism
Classic presentation: Weight gain + lethargy + dull coat
Most common cause: Immune-mediated thyroid gland destruction
Treatment: Daily oral levothyroxine (lifelong) — very effective
Frequently Asked Questions about Hypothyroidism
Q: Can hypothyroidism be confused with old age?
Yes, very commonly. The gradual onset of lethargy, weight gain, reduced activity, and coat changes closely mimics what many owners attribute to 'normal aging' in their dog. This is why veterinary bloodwork — including thyroid testing — in middle-aged dogs showing these signs is important. Treating the underlying hypothyroidism often restores significant energy and wellbeing.
Q: Do cats get hypothyroidism?
Spontaneous hypothyroidism is very rare in cats. Cats far more commonly develop hyperthyroidism (overactive thyroid). Hypothyroidism in cats is almost exclusively seen as a complication of radioactive iodine treatment or surgical removal for hyperthyroidism, where too much thyroid tissue was destroyed or removed.
Q: Is hypothyroidism in dogs the same as in humans?
The condition is similar, but caused slightly differently. In humans, autoimmune thyroiditis (Hashimoto's disease) is most common. In dogs, both immune-mediated destruction and idiopathic atrophy of the thyroid gland occur. The treatment (levothyroxine) and the monitoring approach are very similar between species.
⚠ Medical Disclaimer: This glossary entry is for educational purposes only
and does not constitute veterinary advice. Always consult a licensed veterinarian for diagnosis
and treatment of your pet's health conditions.