Hypothyroidism in Dogs: Is An Underactive Thyroid to Blame?

Hypothyroidism symptoms, diagnosis and treatments
By Sara Greenslit DVM, April 2018, Updated September 2021
hyperthyroidism in dogs

My first dog, Ouzel, a quiet and gentle Lab mix, seemed to have an old soul from the start; a toddler could walk him on a leash. When he was two and staying at a friend’s while I was out of town, he tolerated being sat upon by her Aussie, Okra, pretty much the whole weekend. My friend suggested that he might be hypothyroid. And indeed he was.

What is hypothyroidism in Dogs?

Fluctuations in hormones caused by inflammation or shrinkage of the thyroid gland cause hypothyroidism in dogs. The classic signs of hyperthyroidism in dogs include weight gain, lethargy, cold intolerance, skin and coat changes (yeasty-smelling fur; dark, lacy patches in the groin; excess shedding), vague and subtle symptoms that had snuck in slowly. Basically, a dog’s metabolism is working at a crawl.

Primary hypothyroidism is the most common hormonal disorder in dogs. The average onset of hypothyroidism in dogs is at seven years old. It is often found in large breed dogs—Doberman Pinschers, Golden Retrievers, Irish Setters, Great Danes and Boxers—and some smaller types, including Cocker Spaniels, Dachshunds, Poodles and Miniature Schnauzers. These dog breeds probably inherit a genetic disposition for hypothyroidism. To date, there is no evidence that vaccines induce the disorder.

Hypothyroid dogs tend to have elevated fasting cholesterol and triglycerides, elevated liver values, and mild anemia. Less often, they exhibit neurological weakness, poor appetite, corneal deposits, slow heart rate, heart arrhythmia, or infertility.


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While overproduction of thyroid hormones is often closely associated with cancer, it is rare in dogs. Rather, more than 95 percent of hypothyroid canine cases are caused by unknown gland atrophy or immune destruction of the thyroid. When damaged, the gland cannot respond as well to the TSH (thyroid-stimulating hormone) secreted from the pituitary gland to make T4 (levothyroxine) and T3 (triidothyronine), currency used by the body for the metabolism of energy.

Once diagnosed, hypothyroid dogs tend to respond well to a long-term, twice-daily dosage protocol of oral levothyroxine. Typically, within four to six weeks, they have more energy, they lose weight and their coats begin to shine. After T4 levels normalize, some dogs can be transitioned to a once-a-day medication schedule.

Diagnosing Hypothyroidism

Sometimes, however, a dog’s thyroid disorder is atypical—their blood work doesn’t match the signs, or vice versa. A dog may be suffering from other illnesses that lower T4 (called euthyroid sick syndrome), and certain medications can decrease thyroid levels as well. Adding to the confusion, for some breed types—among them, Sight Hounds, Basenjis and athletic dogs—thyroid levels are normally low. So how do we know if a dog is truly hypothyroid?

To sort things out, we turn to what is currently the most thorough thyroid panel, offered by the Michigan State University Diagnostic Center for Population and Animal Health (DCPAH). Your vet may call this the “Michigan panel.” (See the box below for definitions of thyroid terms.) Still, even with the panel, the results are often less than crystal clear.

The Michigan panel evaluates TgAA (antithyroidglobulin antibodies), autoantibodies to the thyroid. A TgAA that is greater than 35 percent of the positive control value is positive for the presence of lymphocytic thyroiditis, but might not indicate full-blown hypothyroidism. According to DCPAH, “Positive values indicate thyroid gland pathology, and also tell us about the validity of the thyroid hormone results.” A caveat: TgAA can be elevated even when the dog is not yet truly hypothyroid. Because the gland must lose at least 60 percent of its function before clinical signs can be observed—which can take years, or may never happen— retesting a dog every six to twelve months is recommended.

To further define the results, DCPAH recommends testing fT4ED (free T4 by equilibrium dialysis) when “T4AA [a subset of TgAA] have been documented, non-thyroidal illness is known to be present, or the dog has received interfering substances such as steroids or phenobarbital.”

Some medications lower T4 levels; phenobarbital, trimethoprim-sulfonamide, zonisamide, clomipramine and any glucocorticoid (drugs that stop inflammation, such as oral or topicals for ears, eyes and skin) are on this list. So, if your dog is being given one of these medications, one way to clarify blood results is to work with your vet to safely wean him off the medication. After halting the medication for the prescribed period, blood work can be rechecked. If it’s not possible to stop a medication, a six-to-eight-week levothyroxine trial is sometimes employed.

In the end, after all the grey areas and questions, hypothyroidism comes down to this: treat the dog, not the blood work. Your dog's vet can help you sort through the diagnostics and set up an appropriate monitoring schedule.

How did Ouzel do once he was diagnosed? Wonderfully! He perked up; started to play; and developed a luxuriously thick, golden coat. He remained easygoing and prone to naps, but he also loved to swim in the very cold Lake Superior, something that would have been excruciating if he had still been hypothyroid. In fact, he spent so much time in the water on vacations that I started to call him a hippo!

Sara Greenslit, DVM, CVA, is a small-animal  veterinarian and writer who lives and practices  in Madison, Wisc.