Hypothyroidism (low thyroid hormone level) is the most common endocrine disease in dogs but is rare in cats. The usual onset is middle age and there are some breed predispositions—Golden Retriever, Doberman Pinscher, Irish Setter, Miniature Schnauzer, Dachshund, Cocker Spaniel, Airedale Terrier, Great Dane and Old English Sheepdog.

Hypothyroidism affects practically every organ of the body. Common signs are failure of hair growth, skin pigmentation, lethargy, mental dullness, dry flaky haircoat, slow heart rate, infertility, constipation, and weight gain. It can also be related to neurological and bleeding disorders. Often signs are subtle and thyroid testing is often done just because the patient is not responding to treatment as expected.

Hypothyroidism patients usually respond well to thyroid hormone replacement. Treatment should be considered disease management, not a cure, meaning that patients are treated for life. Testing is done every 6 – 12 months to monitor progress.