Hypothyroidism — deficiency of thyroid hormones (T3 and T4) — is the most common endocrine disorder in dogs. It affects middle-aged to older dogs and is almost invariably caused by immune-mediated destruction of the thyroid gland (lymphocytic thyroiditis) or idiopathic thyroid atrophy. With appropriate treatment, prognosis is excellent.
Pathophysiology
The thyroid gland produces T3 and T4, which regulate metabolic rate across virtually all tissues. In hypothyroidism, reduced thyroid hormone levels slow metabolism, affecting energy balance, thermoregulation, cardiovascular function, reproductive performance, and neurological function. Large and giant breeds (Golden Retrievers, Dobermanns, Irish Setters, Labrador Retrievers, Great Danes) are predisposed.
Clinical Signs
- Classic signs: Weight gain without increased appetite, lethargy and exercise intolerance, cold intolerance (seeking warm places)
- Dermatological: Symmetrical alopecia (often affecting trunk and tail), dry and brittle coat, thickened skin (myxoedema), hyperpigmentation
- Reproductive: Infertility, anovulation, galactorrhoea in females
- Neurological: Peripheral neuropathy, rarely myxoedema coma (a life-threatening emergency)
- Cardiovascular: Bradycardia, reduced cardiac output
Diagnosis
Total T4 (TT4) is reduced in most hypothyroid dogs, but false positives occur (euthyroid sick syndrome — non-thyroidal illness suppressing T4). Free T4 by equilibrium dialysis (fT4-ED) is more specific. TSH (canine TSH, cTSH) is elevated in primary hypothyroidism — elevated cTSH with low fT4-ED is highly specific. Cholesterol is commonly elevated. Thyroglobulin autoantibodies support a diagnosis of lymphocytic thyroiditis.
Treatment
Levothyroxine (synthetic T4) is administered orally once or twice daily. Initial dose: 0.02 mg/kg once daily. Post-pill T4 monitoring at 4–6 hours after pill, aiming for high-normal T4 range. Clinical improvement typically evident within 4–8 weeks — coat regrowth is the most visible indicator. Treatment is lifelong; rechecks every 6 months once stable.
Welfare Impact
Untreated hypothyroidism causes chronic low-grade suffering — persistent lethargy, skin discomfort, cold intolerance, and weight-related joint stress are all welfare concerns. Owners often attribute signs to "normal aging" — promoting testing in middle-aged large-breed dogs with compatible signs enables earlier diagnosis. Well-treated hypothyroid dogs have excellent quality of life comparable to unaffected dogs.