What Is Hypothyroidism?
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.
Understanding Hypothyroidism
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.