Hypothyroidism in dogs (low thyroid hormone levels) is a common condition that can cause weight gain, lethargy, skin problems, and a variety of other symptoms. Integrative veterinarian Dr. Julie Buzby teaches dog parents about the causes, predisposed breeds, symptoms, diagnosis, treatment, monitoring, and prognosis for dogs with hypothyroidism.
Did you know that hypothyroidism is one of the most common endocrine diseases in dogs? If you’ve had the joy of having a lot of dogs in your life, there’s probably a pretty good chance that you or someone you know has had a hypothyroid dog. As a vet, some days it seems like it is everywhere!
What is hypothyroidism in dogs?
Hypothyroidism is an endocrine disorder in which the thyroid gland fails to produce adequate levels of thyroid hormones. This is problematic because having normal thyroid function is essential for every stage of a dog’s life, from puppyhood to the time he or she is a grey-muzzled senior.
Thyroid anatomy and physiology
When it comes to regulating thyroid hormones, your dog’s body relies on a carefully coordinated “messaging system” that starts in the brain. When thyroid hormone levels are too low, a small area of the brain called the hypothalamus secretes thyrotropin-releasing hormone (TRH). This signals the pituitary gland (also in the brain) to release thyroid-stimulating hormone (TSH) into the bloodstream.
TSH then travels to the thyroid gland in the neck to trigger T4 (thyroxine) and T3 (triiodothyronine) production. T4 is a precursor of T3, the form of thyroid hormone that exerts most of the effects in the body. T3 plays a role in growth, skeletal and neurological development, metabolism, heart function, cholesterol synthesis, regulation of some hormones, new red blood cell production, and many other body functions.
Most (>99%) of T4 stays bound to proteins in the bloodstream. But there is also an unbound portion called free T4 (fT4), which becomes relevant when discussing how hypothyroidism is diagnosed.
In summary, here are the main players:
- Thyrotropin-releasing hormone (TRH): Signals the pituitary to produce TSH when thyroid hormone levels are low
- Thyroid-stimulating hormone (TSH): Tells the thyroid to make T3 and T4 to increase thyroid hormone levels
- Thyroxine (T4): Less active form of thyroid hormone that is a precursor to T3
- Free T4 (fT4): Unbound form of T4 that represents <1% of T4 in the body
- Triiodothyronine (T3): The main biologically active form of thyroid hormone
When this system isn’t working properly, as is the case in hypothyroidism, your dog may not produce or activate enough thyroid hormones. This low level of thyroid hormones leads to many of the common symptoms that dog parents first notice, like low energy, weight gain, or changes in coat quality.
What causes hypothyroidism in dogs?
Decreases in thyroid hormone production most commonly result from destruction of the gland due to lymphocytic thyroiditis or replacement of functional cells with fat due to thyroid gland atrophy. Both lead to primary hypothyroidism, meaning that the problem lies within the thyroid gland itself.
Approximately 95% of cases of hypothyroidism are the primary form. And overall, lymphocytic thyroiditis is the most commonly recognized cause of decreased thyroid function in dogs.
It is, however, possible for a dog to develop secondary hypothyroidism. In this case, the low thyroid hormone levels result from the disruption of the signals from the brain.
Let’s take a closer look at the causes of hypothyroidism.
Lymphocytic thyroiditis
Lymphocytic thyroiditis is an autoimmune disorder that leads to destruction of the thyroid gland and a subsequent decrease in T3 and T4 production. In this condition, the immune system starts to see the glandular tissue of the thyroid as foreign. This leads to inflammation and an influx of immune system cells (lymphocytes, macrophages, and plasma cells). Over time, the immune system attack leads to scar tissue formation (fibrosis).
As a result, the thyroid is less responsive to TSH and produces lower amounts of thyroid hormone.
Idiopathic thyroid gland atrophy
With idiopathic thyroid gland atrophy, normal thyroid gland tissue is replaced by fatty tissue. Since there are fewer remaining functional thyroid cells, thyroid hormone production decreases.
Other causes of hypothyroidism
Some of the less common causes of hypothyroidism include:
Cancer
Depending on where it grows, cancer can cause both primary and secondary hypothyroidism in dogs. Tumors of the thyroid gland itself, including thyroid adenoma (benign) or carcinoma (malignant), cause primary hypothyroidism. This is the case because they damage the normal cellular structure of the gland.
However, tumors of the pituitary gland (adenoma or carcinoma) may cause secondary hypothyroidism by interfering with the production of TSH. Plus, they disrupt the other normal signaling pathways responsible for regulating thyroid hormone levels.
Trauma
Direct trauma to the thyroid gland, such as your dog pulling too tightly against his or her collar when walking on a leash, has been suggested as a possible cause of hypothyroidism. Additionally, head trauma could decrease TRH or TSH secretion, leading to hypothyroidism.
Medications
There are several prescription medications capable of decreasing thyroid hormone levels in dogs. While not an exhaustive list, some of the more common culprits include:
- Steroids: Prednisone for dogs, dexamethasone, or other steroids interfere with the release of TSH from the pituitary gland. This results in lower T4 levels in the bloodstream.
- Phenobarbital: This common anti-seizure medication increases the rate of thyroid hormone metabolism by the liver, leading to decreased hormone levels in circulation.
- Sulfa drugs: The antibiotic trimethoprim-sulfamethoxazole (SMZ-TMP) and other sulfa drugs directly interfere with thyroid hormone synthesis.
- Clomipramine (Clomicalm®): Typically used to manage separation anxiety and other behavioral issues, clomipramine also leads to decreased thyroid hormone levels. The proposed mechanism is suppression of TSH secretion.
Interestingly, this form of hypothyroidism is reversible once the course of medication is complete. Depending on the drug, dose, and how it was administered, it can take a varied amount of time for thyroid levels to normalize. Thus, your vet needs to know which medications your dog is taking when attempting to evaluate thyroid function.
For some medications, your vet may have you discontinue them for a period of time prior to performing blood tests for hypothyroidism. But if it is a medication that could be detrimental to stop (like an anticonvulsant), the vet may simply take the effects of the medication into account when interpreting the blood work.
(For more details, check out this 2023 Journal of Veterinary Internal Medicine article on the influence of medications on thyroid function in dogs.)
Congenital hypothyroidism
Congenital hypothyroidism is rare, but if it does occur, symptoms may be present in early puppyhood. The condition results from:
- Developmental failure of the gland during gestation
- A true iodine deficiency or an inability to convert iodine into thyroid hormones
- Defects in the production of thyroid hormones due to mutations in the thyroid peroxidase (TPO) gene
Pituitary dwarfism
Another type of congenital disease, pituitary dwarfism, may lead to secondary hypothyroidism due to the development of a goiter (nodular swelling on the thyroid gland). Since pituitary dwarfism is an autosomal recessive disorder, both parents must pass the defective gene to the puppy for symptoms to develop.
Which dogs tend to develop hypothyroidism?
Not counting congenital cases, hypothyroidism most commonly develops in middle–aged dogs. Typically, symptoms start between 4 and 10 years of age (average is 6-7 years).
Any dog can develop hypothyroidism, but there are certain breeds associated with a higher incidence of acquired (non-congenital) hypothyroidism. These include:
- Airedale Terrier
- Akita
- Basenji
- Beagle
- Border Collie
- Boxer
- Chesapeake Bay Retriever
- Cocker Spaniel
- Dachshund
- Dalmatian
- Doberman Pinscher
- Golden Retriever
- Great Dane
- Irish Setter
- Maltese
- Old English Sheepdog
- Rhodesian Ridgeback
- Schnauzer (Miniature)
- Shetland Sheepdog (Sheltie)
- Siberian Husky
What are the symptoms of hypothyroidism in dogs?
Because thyroid hormones influence so many tissues in the body, dogs with hypothyroidism can have a wide variety of clinical signs.
Systemic symptoms of hypothyroidism
Hypothyroidism affects the body as a whole, leading to:
- Lethargy: A lethargic dog may seem sluggish, sleep for longer, or be less interested in normal activities. In one study, 48% of hypothyroid dogs exhibited lethargy as a symptom.
- Exercise intolerance: The dog may tire quickly during activity or refuse to go on walks.
- Weakness: Since hypothyroidism can affect many nerves in the body, the dog may appear to be generally weak.
- Weight gain: Due to slower metabolism, hypothyroid dogs often gain weight without an increase in their dietary intake. In fact, up to 49% of hypothyroid dogs are obese.
- Mental dullness: Some dogs may seem like their brains are moving a bit slower than normal.
- Cold intolerance: The dog may burrow under blankets, try to lie on the heating vent or other warm areas, or not want to go out when it is cold outside.
Cardiac signs seen in hypothyroid dogs
Abnormal heart rhythms and/or bradycardia (an abnormally slow heart rate) are present in up to 26% of dogs with hypothyroidism. This is likely due to the fact that thyroid hormones play a role in heart rate and contraction strength.
Hypothyroidism in dogs can cause skin problems
Approximately 60-80% of hypothyroid dogs will experience skin and hair coat changes, including:
- Dull, dry, brittle hair: Coat changes typically develop because the normal hair regrowth cycle slows down from a lack of thyroid hormone.
- Generalized skin changes: This may include dry, scaly, or greasy skin (dog dandruff), which develops due to a disruption of the normal cycle of skin cell turnover.
- Hyperpigmentation of skin: Darkening of the skin develops due to plugging of the follicles with sebum (oily substance secreted by the skin) and dead skin cells. Or it may be secondary to chronic inflammation of the skin in high-friction areas like the armpits and groin.
- Skin and ear infections: Hypothyroid dogs are prone to recurrent skin infections (pyoderma in dogs) and ear infections (otitis in dogs).
- Hair loss: You may notice your dog losing hair. At first, it occurs over areas of friction like the outer thighs, lower portion of the chest, tail, and tail base. Hair loss on a dog’s tail can create a rat tail appearance. Later on in the disease, the hair loss tends to be bilaterally symmetrical (equal on both sides) and occurs on the trunk and tail. The head and lower portion of the limbs usually keep most of their hair.
- Slow regrowth of hair: The dog’s hair may take months to regrow or never grow back completely. And it may have a different texture in the areas that do regrow. This is especially noticeable after a dog is shaved for an IV catheter or surgical procedure.
- Thickened skin (myxedema): While rare, thickening of the skin around the face and eyes can give the dog a “tragic face” due to heavy, drooping upper eyelids.
- Delayed wound healing: Scrapes, cuts, or surgical incisions may not heal as quickly as expected.
Neurological changes in hypothyroid dogs
Dogs with hypothyroidism are at greater risk for focal or generalized neurologic changes. In fact, 6-29% of these dogs experienced nerve problems as part of their disease process. Some potential neurological symptoms are:
- Cranial nerve abnormalities such as facial nerve paralysis
- Vestibular disease in dogs leading to circling, head tilt, or ataxia (uncoordinated gait)
- Generalized weakness and lethargy due to dysfunction of many peripheral nerves (polyneuropathy)
- Hemiparesis (weakness on one side of the body)
- Hypermetric gait (exaggerated movement of the legs, almost like marching)
Additionally, Horner’s syndrome in dogs, laryngeal paralysis in dogs (lar par), and megaesophagus in dogs may also be associated with (but not necessarily caused by) hypothyroidism.
Uncommon symptoms of hypothyroidism in dogs
Hypothyroid dogs may also experience some less common symptoms such as:
- Infertility (both sexes): Low thyroid hormone leads to disruption of normal estrus cycles, spontaneous abortion, and poor viability of litters in breeding females. Likewise, testicular atrophy and low sperm count may develop in breeding males.
- Atherosclerosis: This condition happens when fats and cholesterol build up in the bloodstream and begin to stick to the inside of blood vessels. Over time, this buildup forms plaques that narrow the space inside the vessels. Plaque accumulation makes it harder for blood to flow, increasing the risk of blockages.
- Corneal deposits of lipid: Increased lipids (fats) in the bloodstream result in distinct areas of white deposits on the surface of the eye.
- Myxedema coma: This condition is an uncommon but life-threatening effect of unregulated hypothyroidism. Affected dogs may progress rapidly from lethargy to stupor or coma. Dogs with myxedema also experience dangerous decreases in heart and respiratory rates, blood pressure, and body temperature. If you see these signs, make an emergency vet visit immediately.
Head to the vet
If you suspect your dog could be hypothyroid, please schedule an appointment with your family vet. Hypothyroidism is almost never an emergency other than in the case of a myxedema coma. But it is still a good idea to get to the vet promptly so your dog can start feeling better.
How does the vet diagnose hypothyroidism in dogs?
To diagnose hypothyroidism, the vet will take the clinical signs, physical exam findings, your dog’s age and breed, and the blood test results into consideration.
Typically, the vet will begin by completing a history, including asking about any medications your dog is on. He or she will also perform a thorough physical examination, taking note of any symptoms described above.
Then the vet will recommend the diagnostics he or she thinks are best for your dog’s situation. Often, that involves blood tests for dogs that help evaluate thyroid function. Plus, he or she may suggest a blood chemistry, complete blood count (CBC), and a urinalysis to get a picture of your dog’s overall health.
Blood tests for hypothyroidism
Bloodwork levels that support a diagnosis of hypothyroidism include: Low total T4, high TSH, and low freeT4. Let’s take a closer look at these and other tests.
Total T4 (TT4 or T4)
The total T4 (written as T4 or TT4) represents the protein-bound T4 and the free T4. Hypothyroid dogs usually have a low total T4. However, there are many factors other than hypothyroidism that can cause a decreased TT4. They include:
- Concurrent illnesses (euthyroid sick syndrome)
- Medications (as discussed earlier)
- Reproductive status (progesterone levels in intact females can impact TT4)
- Age (TT4 decreases with age)
- Fluctuations through the day
- Alaskan sled dogs may have a low TT4, especially when training or racing
- Breed: Sighthounds (Afghans, Borzois, Irish Wolfhounds, Greyhounds, Whippets, Salukis, Italian Greyhounds, Ibizan Hounds, Pharaoh Hounds, Scottish Deerhounds) normally have lower TT4 levels (and fT4 levels, too).
Thus, total T4 (total thyroxine) is a screening test for hypothyroidism, but this level is not reliable enough to support a diagnosis on its own.
Free T4 (fT4)
Free T4, the measure of the portion of T4 that is free (unbound), is the definitive diagnostic tool for hypothyroidism. This is due in part to the fact that free T4 levels are less likely to fluctuate into falsely low ranges in response to non-thyroid disease, versus total T4 levels.
In other words, generally speaking, to make a diagnosis of hypothyroidism, the dog should have symptoms that fit, plus a low fT4, not just a low TT4. (However, due to financial constraints, there may be times when the vet makes a tentative diagnosis of hypothyroidism based on clinical signs and low TT4 alone).
TSH levels
A hypothyroid dog may have high levels of thyroid-stimulating hormone (TSH) since the body is trying to tell the thyroid gland to make more thyroid hormones. However, up to 42% of hypothyroid dogs actually have a normal TSH level. For this reason, the vet will consider the TSH levels, fT4/TT4 levels, and symptoms when making a diagnosis.
Considering the TSH levels is especially important in sighthounds, who normally have a lower T4 and free T4 than the reference range. According to the Michigan State Veterinary Diagnostic Laboratory (one of the foremost veterinary labs for thyroid diagnostics), sighthounds must have both high TSH levels and low T4/fT4 levels to be considered hypothyroid.
Additional thyroid tests
In some situations, the vet may also recommend the following tests to confirm the diagnosis:
- TSH and TRH response tests: These tests measure total T4 levels before and after injection of TSH or TRH. An elevated T4 post-injection is supportive of normal thyroid function, but little to no increase in T4 indicates hypothyroidism.
- Measurement of thyroglobulin antibodies (TgAA): The presence of antibodies against thyroglobulin (a glycoprotein released by thyroid follicular cells) helps diagnose immune-mediated thyroiditis.
- Nuclear scintigraphy: A valuable imaging tool for determining thyroid function in cases where both total T4 and free T4 levels are inconclusive.
Other non-specific findings that support the hypothyroidism diagnosis
Nonspecific bloodwork changes in hypothyroid dogs include elevations in cholesterol and triglyceride levels (up to 75% of cases). As we saw earlier, this may predispose the dog to atherosclerosis or lipid deposits in the corneas. Additionally, high liver enzymes in dogs are possible with hypothyroidism, and anemia in dogs is present in 30-40% of cases.
What is the approach for treating hypothyroidism in dogs?
Once your vet diagnoses your dog with hypothyroidism, he or she will generally recommend starting him or her on a regimen of thyroid hormone supplementation to compensate for the low levels in the body.
Levothyroxine, a medication for treating hypothyroidism in dogs
Levothyroxine (brand names Synthroid™, Thyro-Tabs™ Canine, ThyroKare™) is a synthetic thyroid hormone that is the standard for thyroid hormone replacement. Your dog’s vet will prescribe a starting dose based on your dog’s body weight. Typically, you will give the medication twice daily for the first 4-6 weeks.
After the initial dosing period, your vet will want to recheck your dog’s bloodwork to reassess his or her thyroid hormone levels. For these appointments, it’s important to let your vet know what time you gave the last dose of medication. Most diagnostic labs recommend scheduling blood collection 4-6 hours post-pill to accurately measure the peak concentrations of your dog’s medication in the bloodstream.
If your dog has a good response to treatment on bloodwork and his or her symptoms are well-controlled, the vet may decrease your dog’s dose to once daily. However, some dogs do best when staying on twice-daily dosing.
Side effects of levothyroxine
Side effects of levothyroxine are uncommon. But they may include:
- Weight loss
- Increased water intake and urine output
- Hyperactivity
- Loss of appetite
- Itching
- Vomiting
- Diarrhea
- Skin inflammation
If at any time your dog shows some of these side effects, please let your vet know. There is a lot of similarity between the side effects of levothyroxine and the signs of a chronic overdose (thyrotoxicosis), so the vet may want to recheck your dog’s bloodwork.
Are there natural remedies for hypothyroidism?
For dog parents who are looking for a natural or holistic way to address hypothyroidism, there are several options to consider which are used in addition to (but typically not in place of) thyroid hormone supplementation. These include:
- Herbal supplements like ashwagandha, turmeric, or functional mushrooms
- Nutraceuticals such as selenium, zinc or iodine
- Dietary modifications, including healthy foods that are high in omega-3 fatty acids for dogs. While there are currently no formulated prescription diets on the market for dogs with hypothyroidism, working with a veterinary nutritionist allows you to create a nutritionally complete diet for your dog using whole foods. These include fish, like sardines and salmon, eggs, and vegetables like spinach, green beans, or red bell peppers.
- TCVM (Traditional Chinese Veterinary Medicine) offers a holistic approach to managing your dog’s health after a diagnosis of hypothyroidism. TCVM veterinarians specialize in using multiple approaches to manage disease, including acupuncture, herbal supplements, and dietary support. The TCVM Practitioner Directory can be a great way to find veterinarians who specialize in TCVM in your area.
If you are interested in pursuing some of these natural ways to support thyroid function, please consult with a TCVM practitioner or vet who is comfortable with holistic treatments. He or she can help you figure out the products and doses that are safe and beneficial for your dog. Do not try to treat your hypothyroid dog yourself with herbs, nutraceuticals, or other natural approaches.
What monitoring will hypothyroid dogs need?
For newly diagnosed dogs, most veterinarians will recommend rechecks every 4-8 weeks until the dog’s thyroid levels and symptoms have stabilized. Then rechecks may space out to 1-2 times a year once symptoms are well controlled. If your dog starts developing new symptoms or you have concerns, your vet may recommend repeating bloodwork sooner rather than later. This can be helpful in case the dog needs a dose adjustment.
What is the prognosis?
In most cases of hypothyroidism, the prognosis is usually good once a therapeutic level of dosing is achieved (this will vary from dog to dog). Dogs often begin to lose weight and have more energy fairly soon after starting the levothyroxine. However, hair loss and skin abnormalities may take months to correct.
It’s also worth noting that the prognosis for both myxedema coma and congenital hypothyroidism in neonatal puppies is guarded to poor due to the severity of symptoms and systems affected.
Treating hypothyroidism can feel like a big commitment at first, especially with the need for ongoing medication and regular monitoring. The good news, however, is that most dogs respond very well to treatment. And it soon becomes a simple part of their routine.
What are the consequences of untreated hypothyroidism in dogs?
Without treatment, symptoms may gradually worsen over time and begin to affect your dog’s comfort and quality of life. In very rare and advanced cases, severe complications can develop, like myxedema coma, slow heart rate, or arrhythmias. Thus, early and consistent care is important.
Work with your vet to turn your dog’s hypothyroidism into “happythyroidism”
A diagnosis of hypothyroidism can feel overwhelming at first. The good news, though, is that it’s a highly manageable condition. And most dogs go on to live happy, active lives with the right care.
A consistent medication schedule, routine monitoring, and a little patience as your vet works to ensure your dog’s thyroid levels are balanced can go far. Soon you’ll likely start to see your dog’s energy return, along with improvements in his or her weight, coat, and overall well-being.
With the right support and a strong partnership with your veterinarian, managing hypothyroidism helps your dog stay happy, active, and feeling his or her best.
Is your dog hypothyroid?
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