Hypothyroidism

An underactive thyroid slows everything down. It is the most common hormone disorder of dogs, usually appearing between four and ten years, mostly in mid-size and large breeds and rarely in toys¹. In a UK study of more than 900,000 dogs, the Standard Doberman Pinscher, Tibetan Terrier and Boxer had the highest odds, and dogs aged eleven to thirteen had over four times the odds of dogs aged five to seven². The Golden Retriever, Irish Setter, Miniature Schnauzer, Dachshund, Cocker Spaniel and Airedale Terrier are also commonly affected¹.

The signs are easy to write off as age: dullness, tiredness, weight gain without extra food, feeling the cold, a thin or dull coat, hair loss on the flanks and tail, and recurring skin and ear infections¹. Diagnosis takes blood tests, interpreted with care because other illnesses lower thyroid hormone too; AAHA's 2023 endocrine guidelines give the algorithm³.

Treatment is a daily thyroid tablet for life, with periodic blood checks to adjust the dose. Most dogs brighten within weeks¹³.

This is general information, not a diagnosis. Your vet knows your dog.

Sources

1. Hypothyroidism in Animals. Merck Veterinary Manual. Accessed 2026-09-22.
2. Frequency, breed predispositions and other demographic risk factors for diagnosis of hypothyroidism in dogs under primary veterinary care in the UK (O'Neill DG et al.). Canine Medicine and Genetics 9:11, 2022 (peer reviewed). Accessed 2026-09-22.
3. 2023 AAHA Selected Endocrinopathies of Dogs and Cats Guidelines. American Animal Hospital Association, Journal of the American Animal Hospital Association 59(3):113-135. Accessed 2026-09-22.