If your dog is rapidly gaining weight, even though you haven’t changed their diet or portion size, their coat is brittle or thinning, and they don’t have their usual get-up-and-go energy, they could have an underactive thyroid.
Hypothyroidism in dogs is one of the most common endocrine (hormonal) diseases. Although it can be tricky to diagnose and usually needs lifelong management, this disease is relatively easy to treat.
What Is Hypothyroidism in Dogs?
The thyroid glands are on either side of the windpipe. They produce important hormones that help regulate a dog’s metabolism. Hypothyroidism in dogs (also known as an underactive thyroid) results from the body’s inability to produce and release sufficient amounts of thyroid hormones.
If this progressive disorder is left untreated, it can be life-threatening, potentially affecting your dog’s heart rate, respiration, core temperature, nervous system, muscle tone, and more.
It’s also possible for your dog’s thyroid glands to produce excessive amounts of thyroid hormones. Known as hyperthyroidism, this thyroid disease is a much rarer condition in dogs.
What Are the Causes of Hypothyroidism in Dogs?
Dr. Audrey K. Cook, BVM&S, M.Sc. Vet Ed, FRCVS, Dip ACVIM, Dip ECVIM, Dip ABVP (Feline Practice), is a Professor in the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, Texas A&M University. She says that, in the majority of patients, the underlying cause of hypothyroidism is thought to be an immune-mediated attack on the thyroid glands. “In some dogs, the glands atrophy and lose function without an obvious cause,” she says.
These cases commonly develop in middle-aged and senior dogs and are classed as primary hypothyroidism. “We also occasionally see congenital hypothyroidism [present from birth],” Dr. Cook says. She explains that high-risk breeds include Golden Retrievers, Doberman Pinschers, Beagles, Borzois, Great Danes, Irish Setters, and Old English Sheepdogs. It’s less common in small dogs.
“Issues with the function of the pituitary gland can also result in hypothyroidism,” Dr. Cook says. Known as secondary hypothyroidism, this is rare.
What Are the Symptoms of Hypothyroidism in Dogs?
Dr. Cook lists the following as the most common symptoms to look out for:
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Weight gain despite no change in appetite
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General lethargy and disinterest
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Symmetrical hair loss on the torso and tail tip
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Infections (especially skin and ears)
However, clinical signs are wide-ranging and variable because thyroid hormones can affect most organs. Other possible symptoms and signs include:
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A dull and brittle coat
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High cholesterol
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Less cold tolerance
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Skin hyperpigmentation (darker areas) and thickening
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Bradyarrhythmia (an abnormally slow resting heart rate)
Some rare potential symptoms include neurological difficulties (leading to balance problems, numbness, and weakness), megaesophagus (dilation of the esophagus, which makes it difficult for food to reach the stomach), and changes in the eyes due to corneal dystrophy.
How Is Canine Hypothyroidism Diagnosed?
Your veterinarian will perform various blood tests when they suspect hypothyroidism. Be aware that it is not always a simple one-and-done process.
“We start by measuring the total thyroxine (T4) in blood,” Dr. Cook says. “If this is normal, then we can pretty much discount hypothyroidism.”
When T4 levels are low, this can indicate hypothyroidism, but, to complicate things, drugs (including anti-seizure meds, NSAIDs, and heart meds) and other diseases can also cause low values. Also, the T4 levels of Sighthounds, such as Greyhounds, are normally significantly lower than standard.
“So, we then measure free thyroxine (fT4). If this is low, the dog is very likely to be hypothyroid (unless it is dying of some other condition),” Dr. Cook says. “We also measure thyroid stimulating hormone (TSH). If this is high in a dog with low T4 and fT4, we can be 100% certain the dog is hypothyroid,” she says. “However, if this is not high, we may still conclude that the dog is hypothyroid if the balance of evidence supports this diagnosis.”
Misdiagnosis is rare but can happen. Typically, the symptoms won’t improve once on medication for these dogs, indicating that retesting and further diagnostic work are required.
Treatment of Hypothyroidism in Dogs
Fortunately, treatment is generally highly effective for dogs with hypothyroidism. “Inexpensive oral meds are given, usually twice daily; sometimes just once daily,” Dr. Cook says.
The drugs provide synthetic hormone replacement, and your dog will usually remain on them for the rest of their life to prevent the recurrence of symptoms of this incurable condition. Twice-daily medication is sometimes a permanent requirement, but often, once the symptoms are under control, a single daily dose is sufficient.
“We do need to measure T4 every now and then to be sure the dose is on target,” Dr. Cook says. Typically, tests will happen every eight weeks during the first six months to a year and then once or twice a year after that. While the drug is safe to administer long-term, if the dose is too low, symptoms can persist. If it’s too high, dogs can lose too much weight and become restless.
Your veterinarian may also have to treat the symptoms of any problems caused by hypothyroidism, such as skin or ear infections.
Prognosis of Hypothyroidism in Dogs
While treatment for hypothyroidism is lifelong, once your dog is on medication, the outlook is excellent.
Within a few weeks, you should see a noticeable improvement in your dog’s energy levels and overall well-being. It can take a few months to see major skin, coat, and weight gain improvements.
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