Hypothyroidism
Hypothyroidism is the most common endocrine disorder in dogs, resulting from inadequate production of thyroid hormones (T3 and T4). The thyroid gland regulates metabolism, and deficiency affects virtually every organ system. Most cases (95%) result from immune-mediated thyroiditis or idiopathic thyroid gland atrophy.
Symptoms & Signs
- Weight gain without increased appetite
- Lethargy and mental dullness
- Cold intolerance
- Bilateral symmetrical hair loss
- Dry, dull coat
- Thickened skin (myxedema)
- Hyperpigmentation
- Recurrent skin infections
- Bradycardia
- Tragic facial expression
Causes & Risk Factors
Primary hypothyroidism accounts for 95% of cases: lymphocytic thyroiditis (autoimmune destruction, 50%) and idiopathic atrophy (50%). Secondary hypothyroidism (pituitary dysfunction) is rare. Certain breeds have genetic predisposition to autoimmune thyroiditis.
Diagnosis
Baseline total T4 and free T4 by equilibrium dialysis. TSH stimulation test for equivocal cases. Thyroglobulin autoantibodies (TgAA) detect early autoimmune thyroiditis. Complete blood count may show mild anemia and hypercholesterolemia.
Treatment
Lifelong oral levothyroxine (synthetic T4) supplementation, typically twice daily initially, potentially reduced to once daily. Dose adjustments based on post-pill T4 monitoring at 4-6 week intervals until stable.
Prevention
Screening breeding stock with thyroid panels and TgAA testing. OFA thyroid registry participation. Avoiding breeding of affected individuals or those with elevated autoantibodies.
Prognosis
Excellent with appropriate supplementation. Clinical improvement typically seen within 1-2 weeks for energy levels, 4-6 weeks for skin/coat changes. Lifelong medication required.
Affected Breeds (19)
| Breed | Species | Size |
|---|---|---|
| Airedale Terrier | Dog | Large |
| Akita | Dog | Large |
| American Staffordshire Terrier | Dog | Medium |
| Basenji | Dog | Small |
| Belgian Malinois | Dog | Large |
| Bouvier des Flandres | Dog | Large |
| Briard | Dog | Large |
| Brittany | Dog | Medium |
| Doberman Pinscher | Dog | Large |
| English Setter | Dog | Medium |
| Giant Schnauzer | Dog | Giant |
| Gordon Setter | Dog | Large |
| Irish Setter | Dog | Large |
| Irish Water Spaniel | Dog | Large |
| Italian Greyhound | Dog | Toy |
| Norwegian Elkhound | Dog | Medium |
| Saluki | Dog | Large |
| Tibetan Mastiff | Dog | Giant |
| Wirehaired Pointing Griffon | Dog | Medium |