Hypothyroidism in dogs: causes, symptoms, diagnosis and treatment


Hypothyroidism occurs when the thyroid gland produces an insufficient amount of two essential hormones, namely triiodothyronine (T3) and thyroxine (T4).

Primary hypothyroidism is the most common form of this disease usually caused by inflammation and atrophy of the thyroid gland. For some reason, the dog’s immune system affects and damages thyroid tissues, as if they were foreign bodies. This reduces the amount of T3 and T4 hormones circulating in the bloodstream, triggering a cascade of metabolic problems. Environmental and food factors may also be involved.

Secondary hypothyroidism, rare in dogs, can be caused by systemic disease, iodine deficiency and pituitary gland tumors.

Hypothyroid dogs can exhibit a variety of symptoms, including sensory dulling, lethargy, weakness, exercise intolerance, weight gain for no apparent reason, cold intolerance, bilateral non-itchy symmetrical alopecia, ear infections, and dermatological changes. and the mantle.


L’primary hypothyroidism it is the most common form of this condition. Typically, it is caused by lymphocytic thyroiditis or idiopathic atrophy of the thyroid gland. These conditions lead to the immune destruction of the thyroid gland or damage it. For some reason, the immunological defense mechanisms of the affected dog affect the thyroid tissues as if they were foreign bodies. Over time, the levels of circulating T3 and T4 hormones decrease, causing a cascade of metabolic changes. Environmental and food factors can also contribute to the onset of hypothyroidism in dogs.

L’secondary hypothyroidism it is rare in dogs. When it arises, it is generally caused by congenital diseases, pathologies of the pituitary gland, food deficiency of iodine, pituitary tumors (the increase in tumor mass compresses the thyroid gland by altering its functionality) or other anomalies.



Hypothyroidism occurs more frequently in middle-aged dogs of both sexes and of any breed. The symptoms related to hypothyroidism are often nonspecific and have a rather gradual onset. They often vary according to the breed and age of the dog at the time of the onset of thyroid hormone deficiency.

Although the signs of hypothyroidism may be subtle, most affected dogs exhibit one or more of the following symptoms:

  • Dulling of the sensory
  • Exercise intolerance
  • Reluctance to carry out normal activities
  • Weight gain without a real increase in appetite or food intake
  • Symmetrical bilateral non-pruritic alopecia (symmetrical bilateral non-pruritic alopecia on the trunk. In general, the limbs and the head are less affected)
  • Excessive hair loss
  • Oily skin; scaly skin (seborrhea)
  • Skin pustules (pyoderma)
  • Hyperkeratosis, particularly on the face and forehead (myxedema), which gives the dog a swollen appearance called “tragic facies”.

Hypothyroidism can also affect the neuromuscular, reproductive, cardiovascular and / or gastrointestinal systems, causing one or more of the following symptoms:

  • Low heart rate (bradycardia)
  • Lack of coordination (ataxia)
  • Cardiac arrhythmias (irregular heart rhythm)

These more generalized symptoms are suggestive of hypothyroidism, especially if accompanied by the more specific primarily dermatological symptoms mentioned above.

Obviously, weight gain can be attributed to overeating and a lack of exercise. However, if the dog’s diet and physical activity have not undergone any changes and the animal gains weight for no apparent reason, hypothyroidism should be included in the list of possible suspected causes. The same applies when the dog develops inexplicable dermatological changes or hair load characterized by thinning of the hair on the back and around the tail, excessive hair loss, oily and scaly skin or other dermatological disorders.

Dogs most at risk

Hypothyroidism occurs with greater frequency in large and giant dogs and in middle-aged dogs (2-8 years) of both sexes and of any breed. Sterilized females appear to have a higher risk of developing hypothyroidism than whole females, although the reason for this association is unclear. Breeds reported to be prone to hypothyroidism include Doberman Pinscher, Golden Retriever, Labrador Retriever, English Greyhound, Greyhound, Great Dane, Irish Setter, Poodle, Cocker Spaniel, Shetland Sheepdog, Airedale Terrier, Schnauzer, Malamute, Boxer, Dachshund and other Terriers, Spaniels and Pointers.


Initial evaluation

Canine hypothyroidism is one of the most overdiagnosed diseases in domestic dogs, mainly because the symptoms related to this condition mimic those of many other ailments.

When a dog presenting some of the classic signs of hypothyroidism is presented, the vet will typically perform a complete blood count, blood count and a urine test as part of the initial data collection. The results of these tests may be suggestive of hypothyroidism.

Diagnostic procedures

There are additional tests that can help confirm the diagnosis, including measuring the concentrations of circulating thyroxine hormone. In the event that the first tests are inconclusive for diagnostic purposes, the veterinarian can conduct more advanced tests, including stimulation tests with TSH and stimulation tests with thyrotropin-releasing hormone (TRH). These tests are quite expensive and are not commonly used. Thyroid ultrasound and thyroid scintigraphy can also be useful from a diagnostic point of view, if performed by an experienced veterinary radiologist.

Currently, the best tests available to diagnose canine hypothyroidism are equilibrium dialysis (which measures the levels of the T4 hormone in the blood) combined with the measurement of circulating TSH. Almost all dogs with low levels of free T4 and high levels of TSH in circulation are affected by hypothyroidism.


Therapeutic goals

The objectives related to the treatment of canine hypothyroidism consist in restoring normal levels of circulating thyroid hormones and eliminating, or at least managing, the symptoms in order to guarantee the dog a symptom-free good quality of life.

Therapeutic options

The effects of hypothyroidism in dogs are caused by the inability of the thyroid gland to produce an adequate amount of thyroid hormones. Hypothyroidism is a chronic condition that normally does not require emergency care, but rather lifelong treatment. It is treatable with oral drugs for daily use, which reverse the clinical changes associated with the disease. By following this daily treatment on a constant basis, almost all dogs with hypothyroidism can lead long, healthy and normal lives.

Treatment generally involves the administration of daily doses of levothyroxine sodium (synthetic analogue of the hormone T4) orally. The effectiveness of the treatment depends on maintaining adequate levels of thyroxine in the dog’s blood. The frequency related to the dosage is variable, but most protocols provide for administration 1-2 times a day. The attending veterinarian will periodically examine the dog’s blood to ensure that circulating hormone levels are maintained at normal levels. These tests will probably be more frequent at the beginning (every 4-6 weeks approximately) and their frequency will decrease when the thyroid hormone levels have stabilized.

The general state and activity improve within 1-2 weeks, while dermatological and neurological signs within several months (about 4-8 weeks). The owner will notice hair regrowth, improved general condition of the skin and better exercise tolerance. Reproductive changes can take longer to resolve.

In the absence of a major clinical improvement within 3 months of starting treatment, despite the fact that blood levels of thyroxine are normal (monitoring of which occurs during treatment), there is a probability that the diagnosis of hypothyroidism is incorrect. Therefore, the dog will need to be re-evaluated to identify another underlying cause of the symptoms manifested.


The prognosis for dogs with primary hypothyroidism varies from very good to excellent once an appropriate hormone replacement protocol is established. Secondary hypothyroidism caused by pituitary gland cancer has a much more reserved prognosis.

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