Hypothyroidism means the thyroid gland, in the neck, does not make enough of its hormone. The University of Minnesota dermatology handbook calls it perhaps the most common hormone (endocrine) disease of dogs, though it is also diagnosed more often than it really occurs [1]. Signs usually appear only once most of the gland has been lost, and most affected dogs show both skin changes and signs of a slowed body [1].
If your dog’s coat is thinning or the skin is changing, see your vet. Other diseases can cause the same signs.
For other reasons dogs lose hair, see hair loss in dogs.
Coat changes
The typical pattern is hair loss that is even on both sides of the trunk, with skin that may turn darker (hyperpigmentation) [1]. The hair loss is usually not itchy and the skin is not red or inflamed [1].
The lower half of the tail can go bald, a look vets nickname “rat tail” [1]. The face and lower legs are usually spared, but some dogs lose hair and darken on the top of the muzzle [1].
Skin changes
Scaling or greasy skin (seborrhea) and blackheads (comedones) often develop as a knock-on effect of the hormone shortage [1].
In some dogs with long-standing disease, material builds up in the deeper skin layer and thickens it. This is called myxedema, and on the face it can give a puffy, sad look that vets describe as a “tragic” expression [1].
Skin infection
Surface bacterial skin infection (superficial pyoderma) is common in dogs with hypothyroidism [1]. When an infection like this is present, the skin can become inflamed and the dog may start to itch [1].
Signs beyond the skin
A slower metabolism can cause moderate weight gain without a bigger appetite, low energy, weakness, tiring easily on walks and trouble coping with cold [1]. Owners often do not notice how dull their dog had become until it brightens up on treatment [1]. Many dogs, though, do not show the classic picture [1].
Which dogs get it
It is most common in middle-aged and older dogs, roughly 4 to 8 years old [1]. Any breed can be affected. Breeds reported to be at higher risk include the Giant Schnauzer, Borzoi, Cocker Spaniel, Great Dane, Irish Wolfhound, Boxer, Rhodesian Ridgeback, Irish Setter, Golden Retriever, Doberman Pinscher and English Setter [1].
How vets diagnose it
There is no single simple test. Vets put together the history, the signs, routine blood work and thyroid hormone tests [1]. Other illnesses and some medicines can lower thyroid test results in a healthy thyroid, so a low number on its own is not enough [1]. Sighthounds also tend to have lower thyroid levels than other breeds [1].
Treatment
Your vet will advise on treatment for your dog.
At home, as directed by a vet:
- do not give your dog human or leftover medicines or supplements unless your vet says so;
- give any treatment exactly as your vet prescribes;
- go back for any recheck or blood test your vet books.
When to see your vet
See your vet if your dog has:
- hair loss on both sides of the body or a bald tail [1];
- darker, scaly, greasy or thickened skin, or a puffy face [1];
- low energy, weight gain without a bigger appetite, or trouble coping with cold [1];
- any coat or skin change you are unsure about.
The bottom line
An underactive thyroid can change a dog’s coat and skin as well as energy and weight. Only a vet can tell it apart from other causes of hair loss and skin change.
This article is for general information only and does not replace advice from your veterinarian. If your dog’s coat or skin is changing, see your vet.
Sources
- Torres SMF. 1.5 Hypothyroidism: Canine. Small and Large Animal Dermatology Handbook, Vol. 2. University of Minnesota, 2025. CC BY 4.0. https://open.lib.umn.edu/animaldermatology2/chapter/hypothyroidism-canine/