Last updated: May 16, 2026

Hypothyroidism in Dogs: Signs & Treatment

Hypothyroidism is the most common hormonal disorder in dogs, yet it often goes undiagnosed for months because its symptoms develop so gradually. If your dog has been gaining weight despite a normal diet, losing hair in unusual patterns, or seeming more sluggish than usual, hypothyroidism could be the explanation. The good news is that once diagnosed, hypothyroidism is one of the most manageable endocrine disorders — treatment is effective, affordable, and lifelong quality of life is excellent.

This guide covers the causes, symptoms, diagnostic process, treatment, and long-term management of hypothyroidism in dogs so you can recognize the condition and support your dog through treatment.

What Is Hypothyroidism in Dogs?

Hypothyroidism occurs when the thyroid gland — a small, butterfly-shaped gland located in the neck — fails to produce adequate amounts of thyroid hormones. The two primary thyroid hormones, thyroxine (T4) and triiodothyronine (T3), play a crucial role in regulating metabolism throughout the body. They influence virtually every organ system, affecting energy levels, body temperature, heart rate, skin health, weight management, and brain function.

When thyroid hormone levels drop below normal, the entire body slows down. Metabolism decreases, leading to weight gain. The skin and coat deteriorate because cell turnover slows. Energy levels fall because the cells are not receiving the metabolic signal to be active. This system-wide slowdown produces the characteristic clinical picture of hypothyroidism.

Hypothyroidism in dogs occurs through two primary mechanisms:

• Lymphocytic thyroiditis — This is the most common cause, accounting for approximately 50% of cases. It is an autoimmune condition in which the dog’s immune system mistakenly attacks and destroys the thyroid gland tissue. The destruction happens gradually over months to years, and symptoms appear only after a significant portion of the gland has been damaged.
• Idiopathic thyroid atrophy — In this condition, the functional thyroid tissue is gradually replaced by fat tissue for unknown reasons. This accounts for approximately 30% to 40% of cases. The result is the same — insufficient thyroid hormone production — but the mechanism is different from autoimmune destruction.

Less common causes include thyroid tumors, congenital thyroid abnormalities, and secondary hypothyroidism caused by pituitary gland dysfunction. These account for a small percentage of cases.

Which Dogs Are at Risk?

Hypothyroidism can affect any dog, but certain factors increase the likelihood of developing the condition.

Breed predisposition. Medium to large breeds are more commonly affected than small breeds. Breeds with a higher incidence include Golden Retrievers, Doberman Pinschers, Irish Setters, Dachshunds, Cocker Spaniels, Airedale Terriers, Boxers, Miniature Schnauzers, and Great Danes. The autoimmune form shows particularly strong breed predispositions, suggesting a significant genetic component.

Age. Hypothyroidism is typically diagnosed in middle-aged dogs, most commonly between four and ten years of age. The condition is rare in dogs under two years old because the thyroid gland destruction or atrophy takes time to progress to the point where symptoms become apparent.

Spaying and neutering. Some studies suggest that spayed and neutered dogs may have a slightly higher risk of developing hypothyroidism compared to intact dogs, though the relationship is not entirely clear and may be influenced by other factors.

Obesity. While obesity is more commonly a consequence of hypothyroidism rather than a cause, dogs that are already overweight may be more likely to show clinical signs earlier in the disease process because the metabolic demands of the extra weight compound the effects of reduced thyroid function.

Signs and Symptoms of Hypothyroidism

The symptoms of hypothyroidism in dogs are varied and often develop so gradually that owners may attribute them to normal aging. The classic presentation involves a combination of metabolic, dermatologic, and behavioral changes.

Metabolic Signs

• Weight gain — Perhaps the most commonly recognized symptom. Dogs gain weight despite maintaining a normal or even reduced caloric intake. The weight gain is due to a slowed metabolic rate rather than overeating.
• Lethargy and exercise intolerance — Affected dogs become less active, tire more quickly during walks or play, and may seem more interested in sleeping than participating in activities they once enjoyed.
• Cold intolerance — Because thyroid hormones help regulate body temperature, hypothyroid dogs may seek warm places, shiver more easily, and show reluctance to go outside in cold weather.
• Slowed heart rate (bradycardia) — The heart rate may be lower than expected, though this is not always clinically obvious.

Skin and Coat Changes

• Bilateral symmetrical hair loss — Hair loss that appears equally on both sides of the body is a classic sign. It commonly affects the trunk, flanks, and tail (sometimes called “rat tail” when the tail loses its fur). The head and legs are usually spared.
• Dry, dull coat — The coat loses its normal luster and may become brittle. Hair that is shaved or trimmed may be very slow to regrow.
• Thickened, darkened skin — The skin may become thicker, develop dark pigmentation (hyperpigmentation), and feel doughy or puffy.
• Recurrent skin infections — Hypothyroid dogs are prone to recurring bacterial skin infections (pyoderma) and yeast infections because the impaired immune function and altered skin environment make them susceptible.
• Excessive shedding — Some dogs shed more than usual before the hair loss pattern becomes apparent.

Other Signs

• “Tragic facial expression” — Facial puffiness caused by the accumulation of mucopolysaccharides in the skin gives some hypothyroid dogs a sad or droopy appearance. The eyelids may droop, and the face may appear swollen.
• Reproductive problems — Intact females may have irregular heat cycles, infertility, or failure to conceive. Male dogs may show reduced libido and fertility.
• Neurological signs — In some cases, hypothyroidism can cause peripheral neuropathy (nerve damage), resulting in weakness, dragging of the feet, facial nerve paralysis, or vestibular signs like head tilting and loss of balance.
• Elevated cholesterol — High cholesterol levels found on routine blood work may be the first clue that points toward hypothyroidism.

Diagnosing Hypothyroidism in Dogs

Diagnosing hypothyroidism requires blood testing, but interpreting the results can be more complex than it might seem. Several factors can affect thyroid hormone levels, making accurate diagnosis important.

Initial Screening: Total T4

The total T4 (thyroxine) test is usually the first screening test performed. A normal T4 level essentially rules out hypothyroidism. However, a low T4 does not confirm hypothyroidism on its own because many other conditions and medications can lower T4 levels — a phenomenon known as “euthyroid sick syndrome” or “non-thyroidal illness.” Illness, certain medications (including corticosteroids and anti-seizure drugs), and even normal individual variation can suppress T4 without the dog actually being hypothyroid.

Confirmatory Testing: Free T4 and TSH

When a low T4 is found, additional testing is needed to confirm the diagnosis:

• Free T4 by equilibrium dialysis (fT4ED) — This measures the unbound, biologically active form of T4 and is less affected by non-thyroidal illness than total T4. It is considered the most accurate single test for diagnosing hypothyroidism.
• TSH (thyroid-stimulating hormone) — In hypothyroidism, the pituitary gland produces more TSH in an attempt to stimulate the underperforming thyroid. An elevated TSH in combination with a low free T4 strongly supports the diagnosis. However, TSH levels can be normal in some hypothyroid dogs, so a normal TSH does not rule out the condition.
• Thyroglobulin autoantibodies (TgAA) — This test detects antibodies against thyroglobulin, a protein involved in thyroid hormone production. Positive results indicate lymphocytic thyroiditis (the autoimmune form) and can help confirm the diagnosis and identify the cause.

Additional Blood Work

A complete blood count and chemistry panel are typically performed alongside thyroid testing. Common findings in hypothyroid dogs include mild non-regenerative anemia, elevated cholesterol and triglycerides, and sometimes elevated liver enzymes. These secondary findings support the diagnosis.

Treatment of Hypothyroidism in Dogs

Treatment for hypothyroidism is straightforward, effective, and lifelong. It involves daily oral supplementation with synthetic thyroid hormone to replace what the thyroid gland can no longer produce.

Levothyroxine (Synthetic T4)

Levothyroxine sodium is the standard treatment for canine hypothyroidism. It is given orally, typically twice daily, though some dogs may eventually be maintained on once-daily dosing. The starting dose is calculated based on the dog’s body weight, and adjustments are made based on follow-up blood tests and clinical response.

Most dogs begin to show improvement within the first few weeks of treatment. Energy levels and activity often improve first — many owners report that their dog seems like a “different animal” within one to two weeks. Weight loss begins gradually over the next few weeks to months with appropriate diet and exercise. Skin and coat improvements take the longest, often requiring four to six months for full hair regrowth and normalization of skin quality.

Monitoring and Dose Adjustments

Regular monitoring is essential to ensure the thyroid hormone levels are in the optimal range. The first recheck is typically scheduled four to six weeks after starting treatment. Blood is drawn four to six hours after the morning dose to measure peak T4 levels. The goal is to achieve a T4 level in the upper half of the normal range at this post-pill sample time.

Once the dose is stabilized, monitoring blood work is recommended every six to twelve months. Dose adjustments may be needed over time as the remaining thyroid tissue continues to deteriorate or as the dog’s weight changes.

What to Expect During Treatment

Response to treatment follows a general timeline:

• Week 1-2 — Improved energy and mental alertness. The dog may seem more engaged and interested in activities.
• Week 2-4 — Appetite regulation begins. Some dogs that were eating less due to lethargy may initially eat more as their energy returns.
• Month 1-3 — Gradual weight loss begins (with appropriate diet). Skin infections start to resolve. The coat may go through a heavy shedding phase as old, damaged hair is replaced.
• Month 3-6 — Full coat regrowth. Skin texture and pigmentation normalize. Cholesterol levels return to normal. Overall body condition continues to improve.

Long-Term Management

Hypothyroidism requires lifelong management, but the commitment is relatively simple — daily medication and periodic blood tests. Here are key aspects of long-term care:

Medication consistency. Give the medication at the same time each day for consistent hormone levels. Levothyroxine is best absorbed on an empty stomach, so giving it 30 to 60 minutes before meals is ideal. If this is not practical, giving it with food is acceptable, but be consistent — always with food or always without — so that blood test results are comparable.

Weight management. Once thyroid levels are normalized, your dog’s metabolism will be restored, but weight loss still requires attention to diet and exercise. Work with your veterinarian to establish an appropriate caloric intake and exercise program. Most hypothyroid dogs reach their ideal weight within six to twelve months of starting treatment.

Regular veterinary check-ups. In addition to thyroid monitoring, regular comprehensive examinations help detect any changes in your dog’s health. Hypothyroid dogs may be at increased risk for other conditions, and early detection allows prompt management.

Watch for signs of overdosage. Too much thyroid supplementation can cause signs similar to hyperthyroidism, including increased thirst and urination, weight loss, restlessness, rapid heart rate, and diarrhea. If you notice these symptoms, contact your veterinarian to have the dose checked and adjusted.

Communicate changes to your vet. If you notice any changes in your dog’s weight, energy level, coat quality, or behavior between scheduled check-ups, let your veterinarian know. These changes may indicate a need for dose adjustment.

Frequently Asked Questions About Hypothyroidism in Dogs

Can Hypothyroidism in Dogs Be Cured?

Hypothyroidism in dogs cannot be cured because the thyroid gland damage is permanent. However, the condition is very effectively managed with daily thyroid hormone supplementation. Dogs on appropriate treatment have an excellent prognosis and can live completely normal lives. The medication replaces what the thyroid gland would normally produce, effectively normalizing all body functions affected by the hormone deficiency.

How Much Does Hypothyroidism Treatment Cost?

Levothyroxine is one of the most affordable long-term medications in veterinary medicine. The monthly cost for the medication itself is typically modest, varying by the dog’s size and dose. The primary ongoing cost is periodic blood testing to monitor thyroid levels, which is usually recommended every six to twelve months. Compared to many other chronic conditions, hypothyroidism is very economical to manage.

Can Hypothyroidism Cause Behavioral Changes in Dogs?

Yes, hypothyroidism can cause behavioral changes. The most common behavioral sign is lethargy — dogs become less playful, less interested in activities, and may sleep more than usual. Some owners also report that their hypothyroid dogs seem mentally dull or less responsive. In rare cases, hypothyroidism has been associated with increased anxiety or even aggression, though these behavioral changes are less well-documented and may involve other contributing factors. Most behavioral changes resolve with appropriate thyroid supplementation.

Is Hypothyroidism in Dogs Related to Diet?

Hypothyroidism in dogs is not caused by diet. The condition results from immune-mediated destruction of the thyroid gland or idiopathic atrophy, neither of which is related to dietary factors. However, diet plays a role in managing hypothyroid dogs — appropriate nutrition supports weight management, skin health, and overall well-being during treatment. Feeding a balanced, high-quality diet helps support the dog’s recovery and long-term health.

Can Hypothyroidism in Dogs Cause Other Health Problems?

Untreated hypothyroidism can contribute to several secondary health issues. The impaired immune function makes dogs more susceptible to skin and ear infections, including chronic yeast infections. Elevated cholesterol increases cardiovascular risk over time. The neurological effects can cause weakness and balance problems. Reproductive problems can occur in intact dogs. However, these secondary issues typically resolve or significantly improve once thyroid supplementation is started and hormone levels are normalized.

How Is Hypothyroidism Different From Hyperthyroidism?

Hypothyroidism (too little thyroid hormone) is common in dogs but rare in cats. Hyperthyroidism (too much thyroid hormone) is common in cats but very rare in dogs. The two conditions produce opposite symptoms — hypothyroidism causes weight gain, lethargy, and cold intolerance, while hyperthyroidism causes weight loss, hyperactivity, and increased appetite. In dogs, hyperthyroidism is almost always caused by thyroid cancer, while hypothyroidism is caused by autoimmune destruction or atrophy of the gland.