Hypothyroidism in dogs
Also called Underactive thyroid
The commonest hormone disease of dogs, usually in middle-aged and larger dogs. The thyroid gland makes too little hormone, slowing the metabolism, and many signs can mimic other diseases.
Cause and spread
- Cause
- Over 95% of cases come from destruction of the thyroid gland itself, most often by immune-mediated thyroiditis or by shrinkage of the gland.
- How it spreads
- It is not infectious. It is most common at 4 to 10 years, in mid- to large-sized breeds, and especially in Dobermans, Beagles and Golden Retrievers.
Signs to watch for
- Lethargy, dullness and unwillingness to exercise
- Weight gain without eating more
- Dry, thin coat with excessive shedding, slow regrowth and hair loss
- Skin infections and dark skin
- Feeling cold and a slow heart rate
Many diseases look alike. Only a laboratory test can tell them apart, so please see a veterinarian for any sick animal.
How it is diagnosed
Hypothyroidism is probably overdiagnosed, because many other illnesses and some drugs can lower thyroid hormone in healthy dogs. A total T4 is a good first screening test (sensitivity about 90%); free T4 and TSH add accuracy, and raised cholesterol (in about 80% of affected dogs) supports it. Results must be read together with the signs by your veterinarian.
- Usually the first testTotal T4A good screening test for an underactive thyroid.
- Confirms the diseaseThyroid Profile (T4, T3, Free T4, TSH)The whole thyroid picture in one test.
- Supports the diagnosisFree T4Adds accuracy when total T4 is unclear.
- Supports the diagnosisThyroid Stimulating Hormone (TSH)Helps confirm a thyroid problem.
- Supports the diagnosisCholesterolRaised in about 80% of dogs with the disease.
- Supports the diagnosisComplete Blood Count (CBC)May show a mild anaemia.
Prevention and control
- It usually results from destruction of the thyroid gland, so it is not generally preventable; early recognition and treatment by your veterinarian are what matter
- Do not start thyroid medicine without a proper diagnosis
Where this information comes from
- Hypothyroidism in Animals · Merck Veterinary Manual
General information for animal owners and veterinarians, last checked October 2026. It does not replace the advice of your veterinarian, who should decide which tests and treatment your animal needs.