Understanding Hypothyroidism in Dogs: Symptoms and Treatment

1. Introduction

Hypothyroidism is a common endocrine disorder in dogs, and a basic understanding of canine hypothyroidism and its management is important for all dog owners. The thyroid gland plays a critical role in maintaining the body's metabolic processes and thus is referred to as the body's "thermostat." Most of what the thyroid gland produces and releases throughout the body affects every body tissue and cell. In the event that the thyroid gland is underactive or not active at all, clinical signs of hypothyroidism become more or less obvious, depending on the duration of underactivity or inactivity and the severity of impairment. The most common form of canine hypothyroidism is "primary" hypothyroidism, in which the thyroid gland alone is affected.

to a reduction in a dog's metabolic state. Hypothyroidism is one of the most common hormonal (endocrine) diseases in dogs. It generally affects middle-aged dogs (average of 6–7 years of age), and it may be more common in spayed females and neutered males. A wide variety of breeds may be affected.

The thyroid gland produces hormones that regulate the body's metabolic rate. If thyroid hormone levels are increased (hyperthyroidism), the body's metabolism is elevated. If they are decreased (hypothyroidism), the metabolism slows down.

The thyroid gland is one of the most important glands in the body. It is located in the neck near the trachea or windpipe and has two lobes, one on each side of the trachea. The thyroid gland is controlled by the pituitary gland, which is located at the base of the brain.

Numerous surveys show the prevalence of hypothyroidism in various breeds. Over 258,000 cases were registered in a survey of 73 breeds. Golden Retrievers ranked first, mixed-breed dogs ranked second, and dachshunds ranked third in the total number of dogs tested in the same survey. However, by assessing the frequency of dogs tested with the number of resultant cases of hypothyroidism, the national reported breed hypothyroidism index was much higher and included the following breeds by order of rank: Nova Scotia Duck Tolling Retriever, Belgian Tervuren, English Springer Spaniel, Dogue de Bordeaux, Clumber Spaniel, Mexican Hairless, Norwegian Elkhound, and Cocker Spaniel. Thyroid disease in dogs must be managed correctly to get the best results. Veterinary awareness of the clinical signs and symptoms of this disease is vital to the diagnosis and treatment of hypothyroidism in dogs. This study reviews canine hypothyroidism, including its clinical symptoms of disease, diagnosis, and treatment, and discusses hypothyroidism ignored by clinicians.

2. Causes and Risk Factors

Hypothyroidism in a dog can have both genetic causes as well as factors that affect the environment in which a dog lives. Some affected dogs have inherited hyperthyroidism from their parents, while other dogs develop hypothyroidism due to a variety of diseases that destroy the thyroid gland, resulting in an underactive gland. Similar to humans, one of the main causes of an underactive thyroid gland in dogs is an autoimmune disease. This disease, known as lymphocytic thyroiditis, causes the immune system to produce antibodies that attack healthy thyroid tissue. Environmental as well as genetic factors can strongly influence the likelihood that dogs will develop this autoimmune-mediated hypothyroidism. Hypothyroidism is more common in older dogs aged between 4 and 10 years. Genetically, certain breeds and breeding lines are predisposed to the development of hypothyroidism. Chemical destruction of thyroid tissue, for example, after surgical removal of the thyroid gland or treatment with radioactive iodine, can also lead to hypothyroidism. Generally, after a dog's thyroid glands are removed or destroyed, they will require hormone replacements for the rest of their lives. The general inability to produce enough thyroxine in the body also increases the risk of hypothyroidism in dogs. Some researchers believe that thyroxine-deficient dogs may become more aggressive, have poor stamina, and may become generally weak. Some drugs and foods with certain chemicals can decrease thyroid hormone levels. Feeding fatty and poor-quality commercial dog food has a strong relationship with the incidence of hypothyroidism in dogs. Additionally, pesticides, hormones, and antibiotics in food or water can cause hypothyroidism. Dog hypothyroidism occurs in a small percentage of dogs visiting a veterinarian each year. Therefore, thyroid hormone deficiency is important to recognize for a longer lifespan and complete diagnosis and treatment.

3. Common Symptoms of Hypothyroidism

There are a variety of symptoms that may be seen in dogs with hypothyroidism, and while they can vary from one dog to the next, there is a consistent pattern of symptoms that regularly occur with this disease. One set of symptoms can be broken down into typical physical changes, and another set of symptoms includes changes in activity levels and behavior. The symptoms are included in two distinct categories for clarity and to make it easier for dog owners to recognize an issue in their pet. Physical signs include weight gain, changes in coat texture, and skin problems, as well as fluid retention. Behavioral signs are seen as a decrease in the range of activity and become worse as the disease progresses. It is important to remember that the severity of symptoms varies in dogs, and symptoms of the disease can also vary between breeds and between individuals of the same breed.

Hypothyroid dogs can show symptoms such as weight gain, excessive shedding, skin infections, a coat that is easily matted, hair loss, and dandruff. Weight gain can be the first sign noticed by pet owners, as human observation of weight is usually simple and also because it says something about how the dog feels. Weight gain can be accompanied by fat gain or by fluid accumulation. A loss of muscle mass can also occur. Reduced activity, minimal interest in physical exercise, inactivity, and sleepiness are typically seen, either on their own or along with a lack of attentiveness and a struggle to understand new commands. Inattention, sluggishness, and exhaustion are not uncommon. These symptoms will often increase slowly until they are ultimately observed. They can change with an external influence or be periodic. These symptoms may be indicators of various other conditions, so accurate and efficient diagnosis is important.

3.1. Physical Symptoms

One of the most common causes of hypothyroidism is autoimmune thyroiditis, in which the dog’s immune system attacks the thyroid. Lymph tissue blocks the thyroid gland and causes the normal thyroid tissue to be replaced by nonfunctional scar tissue. Typically, the first sign in dogs is skin-related problems, such as obesity, flaky skin, and hair loss, beginning about four months after they start to show hypothyroid signs. These dogs can be misdiagnosed as having a skin problem because nothing else may be wrong. Problems with hair can include a dry, brittle coat or 'rat tail' and a dull or matted coat. The hair loss will usually not be symmetrical as it is with Cushing’s syndrome. Many hypothyroid dogs will not lose hair, but their coats will feel 'different,' the so-called 'puppy fluff.'

A fall in the metabolic functions of the tissues occurs when the body doesn’t have enough T4 and T3. One effect of this decline is to weaken the skin’s resistance and make it more susceptible to infections by bacteria, fungi, and mites. It is important to note that many dogs refuse to show clinical signs for a period of time, and signs appear on initial examination only after the hypothyroid process is well underway and the body can no longer adequately compensate for the lack of thyroid hormone. Monitoring the T4 test helps find these later cases. Thus, since signs can be gradual and are hard to detect on physical examination early in the course of the disease, physical examination should be frequent when testing. Other health problems, like diabetes or Cushing’s disease, can overlap with or occur around the same time as hypothyroidism, increasing the possibility of finding signs consistent with the disease in a general health examination.

3.2. Behavioral Symptoms

When an otherwise spry and lively dog becomes listless and unresponsive, one possible culprit to consider is hypothyroidism, a hormone imbalance, or underactive thyroid, and one of the most common endocrine issues that occur in dogs. Like hyperthyroidism, dogs' symptoms of hypothyroidism can encompass a number of organ systems and mimic a variety of problems. Hypothyroidism can cause a wide range of behavioral changes. While these may be less obvious than physical symptoms, they are no less troubling to both dogs and their owners. Lethargy and irritability top the list, along with longer than necessary sleep times. Decreased exercise tolerance is also a symptom. A dog with a healthy hormone balance may play for hours, while a dog with hypothyroidism loses interest, wanting a nap instead. This is not due to a lack of energy, but to a drop in adrenaline, a hormone secreted when playing. Hypothyroidism may also lessen the pet's low-light vision and cause a decrease in alertness in general. Dogs under hormonal duress of any sort can become a little blue, but hypothyroidism has them down for the count.

Additionally, in older dogs, hypothyroidism can be mistaken for declining mental faculties, also called cognitive dysfunction. Unlike true aging, dogs affected by cognitive dysfunction tend to revert to puppyhood, which may appear to be a personality change. Dogs' decreased interest in interaction might be seen as a benign difference in preference. The monkey wrench in all of this is that because their pets are getting older, many owners of dogs with hypothyroidism expect to see changes. The overlap of symptoms with advanced aging often causes owners, and sometimes veterinarians, to delay treatment. The impression of the animal's behavior from having examined him or her can be reinforced when doctors fail to recognize the physical traits that have developed proper animal behavior.

4. Diagnosis and Testing

It is very important for your dog to get a thorough examination by a vet to determine the legitimacy of hypothyroidism. Ruling out other illnesses and possible ailments is necessary as well. Blood tests to measure the level of thyroid hormones in your dog are the primary way to diagnose canine hypothyroidism. They can be used to measure the levels of both T4 and T3. Hormone production may decrease while the pituitary is producing more of the hormone that tells the thyroid to make more hormone. Thus, the two values may be normal, but are still a reason to be concerned and may point to hypothyroidism. A decrease in hormone production may occur gradually and may take from six months to a year in order to reveal itself in blood tests. Conversely, an increase in hormone production may produce falsely low blood test results. It is also possible to obtain a false reading through the tests even though they may be acting and developing in the usual manner.

Considering both your pet’s health history and clinical signs are important factors in all testing procedures to diagnose any condition. In the event of a high TSH and a decrease in total hormone T4 production, an appropriate diagnosis is given. It is important that the pituitary gland is functioning correctly in order for some blood tests to be accurate. Additional confirmatory tests, which can involve the use of T3 on dogs, are even more accurate. In the event that T3 is deficient in relation to T4, the proper diagnosis is then made. If necessary, an ultrasound of the thyroid gland can be taken to determine signs of glandular atrophy and can thus result in a claim for hypothyroidism. Evidence of hypothyroidism should be sought based on the reversal of the majority of clinical signs if treatment is successful. Early diagnosis of hypothyroidism can greatly help to reduce future issues and return optimal results for pets.

5. Treatment Options

After a diagnosis is made, it should be noted that each dog should be treated as an individual case. Therefore, treatment recommendations should only be made following a comprehensive evaluation of each dog’s complete medical history and current condition. The main treatment for hypothyroidism in dogs primarily consists of giving a synthetic thyroid hormone supplement. Since the dog’s own thyroid tissue does not produce the correct amount of thyroid hormones, the thyroid hormone replacement is usually given as oral pills. The typical replacement drug is levothyroxine, although the natural thyroid product, desiccated thyroid hormone, is also sometimes used. Levothyroxine is the most widely used hormonal supplement. The supplements are usually given once or twice daily and can be supplied in pill or ointment form. The supplements are best absorbed on an empty stomach. During treatment, veterinarians will monitor your dog's hormone levels through regular blood work. The dosage of the medication may change based on these tests, and your veterinarian may recheck them six to eight weeks after therapy begins or the dosage is altered. It is important to give the medicine at the same time every day and not miss a dose. Treatment is lifelong.

There are no specific guidelines for canine hypothyroidism nutrition. However, certain nutrients that affect the skin and coat are essential. Essential fatty acids and B-complex vitamins help maintain a coat and skin. Nutrition is very important for overall health, and many dogs with hypothyroidism may benefit from prescription dog food. Hypothyroidism is completely treatable in dogs, although it may require lifelong management. Once treated, the hair usually grows back and the skin improves. Additionally, minor to moderate weight loss is possible, but better success is achieved when the condition is diagnosed and treated early. Left untreated, hypothyroidism will progress, negatively affecting the dog’s quality of life. Other related illnesses will become increasingly chronic and harder to treat. In some cases, dogs may develop heart disease, seizures, fluid in the chest or abdomen, or even a life-threatening form of heart failure. Always follow the veterinarian’s recommendations regarding medications and dog food when treating hypothyroidism because compliance plays a major role in the management of this disease. Be open to discussing any issues that may arise so treatment can be adjusted as necessary. Realize that there is no cure for hypothyroidism, but with medical care, most dogs can live a happy, healthy life. In fact, signs of lethargy, hair loss, and weight gain will likely improve within a few weeks. As such, owners should be advised to monitor their pets closely and report any unusual behavior during the treatment.