If your dog was once a boundless source of energy but has slowed down to a snail’s pace and only wants to nap now, he or she may have developed hypothyroidism. Canine hypothyroidism is a disorder of low thyroid gland hormones in which treatment is needed. If your dog seems more sluggish than normal, it’s time to schedule a veterinary visit to assess your dog’s thyroid function. Read on to have your questions answered by The Meowing Vet and to learn why hypothyroidism can be so “tragic”.

Q: WHAT IS THE THYROID GLAND? WHERE IS IT LOCATED, AND WHAT IS ITS FUNCTION?

A: The thyroid gland is a small organ of the endocrine system (the organ system responsible for producing hormones to be released into the bloodstream). The thyroid gland has a “butterfly” appearance, consisting of two lobes that lie on either side of the trachea (windpipe) in the throat region. All vertebrate animals (i.e. those with a backbone, such as humans, dogs, and cats) possess a thyroid.

The thyroid gland produces thyroid hormones, primarily thyroxine (or T4) and triiodothyronine (or T3). These hormones are responsible for the body’s metabolism, meaning the vast number of chemical reactions that occur in each cell of each tissue of each organ. Additionally, thyroid hormones help regulate calcium levels in the body, the sleep cycle, as well as heart rate and strength. Thyroid hormones are also vital for the development of a healthy pregnancy and newborn. The thyroid gland is in turn regulated by other aspects of the endocrine system: the hypothalamus and the pituitary gland.

Q: WHAT IS HYPOTHYROIDISM?

A: Let’s dissect the word “hypothyroidism.” Hypo means “too little” or “decreased,” and thyroid refers to the thyroid gland. Thus, hypothyroidism is a disease in which the thyroid gland fails to produce a sufficient level of thyroid hormones. This causes the metabolic rate of the affected animal to be much lower than normal. The metabolism, or chemical reactions, of the body’s cells allow these cells to produce energy (in the form of a molecule known as ATP). If the metabolism slows down, then the amount of energy produced by the body also decreases, causing the affected animal to be less energetic than normal.

Q: ARE DOGS OR CATS AFFECTED BY HYPOTHYROIDISM?

A: In veterinary medicine, dogs are most affected by hypothyroidism. Cats, on the other hand, are more commonly affected by hyperthyroidism (or increased activity of the thyroid gland, resulting in a faster than normal metabolic rate). Both conditions require treatment to return thyroid hormones to a more normal level (or euthyroid state). However, when a dog with hypothyroidism begins treatment to manage this endocrine disease, thyroid hormonal regulation can take awhile, and the therapy may occasionally “overshoot” its goal, causing a hypothyroid dog to temporarily become hyperthyroid until the medication is better adjusted.

Click here to read more on feline hyperthyroidism.

Q: WHAT CAUSES Canine Hypothyroidism?

A: The most common cause of hypothyroidism is due to impairment of the thyroid gland (or thyroiditis), leading to primary hypothyroidism. Roughly half of these cases occur due an autoimmune disease of the thyroid gland (termed lymphocytic thyroiditis). Autoimmune diseases are when the body’s immune system malfunctions and goes into overdrive, falsely recognizing some of its own tissue as foreign (in this scenario, thyroid gland tissue); this leads to the formation of autoantibodies (a.k.a. antibodies against the body’s own tissue). In the other half of primary hypothyroid cases, a specific cause of thyroid gland damage cannot be found; these cases are known as idiopathic follicular atrophy in which the hormone-producing cells (follicles) of the thyroid gland waste away (or atrophy) for an unknown (i.e. idiopathic) reason.

Rarely, puppies (or kittens) may be born with a congenital defect of the thyroid gland, known as cretinism, in which the newborns are dwarf in stature with enlarged tongues and broad heads; these newborns often have abnormalities while walking and are difficult to train. Equally uncommon is hypothyroidism in adult dogs caused by cancer of the thyroid gland. Another rare cause of primary hypothyroidism is iodine deficiency. Iodine is a mineral acquired from food, and it is vital for thyroid hormone production. Commercial pet foods have more than enough dietary iodine to support thyroid gland health, but many homemade diets may not be properly balanced and may be lacking in iodine, leading to hypothyroidism.

Other rare causes of hypothyroidism may be diseases of or damage to the governing organs of the thyroid gland: the pituitary gland or hypothalamus (termed secondary and tertiary hypothyroidism, respectively). In cases of secondary or tertiary hypothyroidism, the thyroid gland itself is normal; it just doesn’t receive the proper signals from the pituitary gland (TSH, or thyroid-stimulating hormone) or hypothalamus (TRH, or thyrotropin-releasing hormone) that would normally tell the thyroid to produce thyroid hormones (T3 and T4).

Q: What dog breeds are more commonly affected? At what age are they typically diagnosed?

A: Medium and large-sized purebred dogs are the ones most likely to develop hypothyroidism. Breeds such as golden retrievers, Labrador retrievers, doberman pinschers, boxers, Shetland sheepdogs (shelties), old English sheepdogs, Irish setters, and great Danes are predisposed to this endocrine disorder. Other dog breeds such as poodles, cocker spaniels, dachshunds, and miniature schnauzers are also at an increased risk. Hypothyroidism generally occurs in adult dogs ranging from young adults to middle-aged (i.e. typically 4 – 10 years of age).

Q: WHAT ARE THE CLINICAL SIGNS OF HYPoTHYROIDISM?

A: As previously mentioned, you may notice that your dog fatigues more quickly than he once did and would rather spend his time sleeping rather than going on walks. If you notice these signs as well as any of the following described below, it’s time to schedule a veterinary appointment. Your vet may also observe other abnormalities, as listed below, during your dog’s physical exam.

• Changes that you may observe in your hypothyroid dog at home:
  • Tiredness (lethargy) and weakness
  • Forgetfulness: Mental dullness can occur with hypothyroidism. You may notice that your dog does not respond to commands and tricks as readily as he once did.
  • Possible “grumpiness” or newly displayed aggression
  • Avoidance of cold temperatures: Since your dog’s metabolism is sluggish, she does not generate as much body heat as normal, causing her to feel cold. You may observe her snuggling up to the fireplace or in a warm blanket more than usual.
  • Weight gain (despite a normal appetite)
  • Hair loss (alopecia): Your dog may start shedding excessively and experience areas of hair loss involving the neck (especially underneath the collar), tail (leading to a “rat tail” appearance), and along each side of the body.
  • Skin changes:
    • The skin may become either overly dry or greasy (seborrhea). It may also have darkened areas (hyperpigmentation).
    • Additionally, your hypothyroid dog may have recurrent skin infections (pyoderma) that are difficult to treat or which never fully resolve.
  • Recurrent ear infections (otitis externa)
  • “Spit up”: Your dog may occasionally hack up some clear fluid, or he may seem to throw up the entire contents of his meal, which appears undigested. While this may seem like “vomiting,” it is in fact likely “regurgitation” which occurs when food that never reached the stomach is expelled back up out of the esophagus. This can occur in dogs with megaesophagus, in which the esophagus (or tube connecting the mouth to the stomach) is persistently enlarged. This disorder can occur with neurologic dysfunction of the esophagus secondary to hypothyroidism. If not properly managed, your dog is at risk for aspirating food into its lungs, a highly dangerous condition.
  • Female dog infertility: As stated, thyroid hormones are needed to maintain a healthy pregnancy. Low levels can lead to infertility in intact (i.e. unspayed) female dogs. The fertility of male dogs with hypothyroidism is unaffected.

  
  

   

• Additional abnormalities that your veterinarian may detect upon physical examination:
  • Low body temperature
  • Obesity
  • “Tragic” facial expression (myxedema): You may have noticed that your dog’s face appears more puffy than it once did or that your dog looks sad. This is termed a “tragic” facial expression because your dog looks like it got dumped right before prom. 😉 This appearance is caused by a thickening of the facial skin and drooping eyelids.
    • Click here for a photo of the classical “tragic” facial expression of canine hypothyroidism as well as more information on this endocrine disorder from The University of Illinois College of Veterinary Medicine.
  • Possible heart abnormalities:
    • Slow heart rate (bradycardia) and weak pulses: Since your hypothyroid dog’s metabolic rate is lower than usual, less chemical reactions are occuring across its body, so its cells require less blood flow to obtain oxygen and nutrients and rid themselves of waste. Therefore, the heart does not pump as fast or as forcefully as it would in a patient with a normal metabolism. Additionally, the heart muscle does not produce as much energy for itself as normal due to this decreased metabolic rate.
    • Heart beats that sound muffled
    • Abnormal electrocardiogram (ECG or EKG)
  • Neurologic abnormalities: Your vet may detect abnormalities in the facial appearance or gait of your hypothyroid dog. These changes can be attributed to various nerve dysfunctions secondary to hypothyroidism. Affected dogs may have general weakness, poor muscle tone, foot “knuckling” (proprioception deficits), walk as if “drunk” (ataxia) or in an exaggerated fashion (hypermetria), decreased reflexes, a head tilt, or a droopy face with decreased facial sensitivity. Additionally, your dog may have developed megaesophagus, as discussed above.
  • Megaesophagus (as observed on thoracic radiographs, i.e. chest X-rays)

Click here to read our article on the basics of canine and feline heart disease. ❤️

Q: WHY DOES Hypothyroidism NEED TO BE DIAGNOSED AND TREATED?

A: The body of a dog with hypothyroidism gradually becomes more and more impaired because, due to its slowed metabolic rate, it is not being supplied with enough energy to sustain itself. This leads to nerve and brain dysfunction. Megaesophagus, for example, can lead to lethal aspiration pneumonia if not properly diagnosed and managed. Additionally, hypothyroidism increases the risk of atherosclerosis (plaque build-up inside the arteries) and stroke (blocked blood vessels to the brain) in your pet. Furthermore, if your pet should require anesthesia while untreated for hypothyroidism, he or she may have a prolonged recovery time or experience dangerously low heart rate, respiratory rate, blood pressure, and body temperature during the procedure. In the most extreme and rarest of circumstances, your hypothyroid dog can slip into a life-threatening coma (named a myxedema coma) if not treated. At the very least, the quality of life of your dog can suffer due to lethargy and recurrent painful skin and ear infections if hypothyroidism goes undetected and untreated. Very tragic indeed!

Come back later for a future article on megaesophagus.

Q: HOW IS HYPoTHYROIDISM DIAGNOSED?

A: Based on the physical exam findings and your description of your dog’s behavior at home, your vet will likely already be suspecting hypothyroidism (and so will you after reading this article!). To confirm the diagnosis of and assess any coinciding illnesses, your vet will run routine bloodwork (CBC and chemistry) in addition to a thyroid hormone panel. This specialized blood test, which measures thyroid hormone levels (total and free T4, total and free T3, and TSH) can be performed in-house in some veterinary hospitals while it may need to be sent to a special laboratory for other clinics. T4 and T3 will theoretically be low in hypothyroidism while TSH should be high. (Why? Whenever the pituitary gland detects low thyroid hormone levels, it produces TSH to try to stimulate the thyroid gland to make more hormones.) In cases of autoimmune thyroid gland disease (lymphocytic thyroiditis), special blood tests may be sent to a specialty lab to measure levels of abnormal antibodies (antithyroglobulin antibody or autoantibodies to T3 and T4).

Unfortunately, no single thyroid function blood test is perfect and must be interpreted carefully by your veterinarian. Normal daily thyroid hormone fluctuations may alter your dog’s hormone measurement at any given point, causing the thyroid panel to appear falsely decreased in normal dogs or falsely normal in cases of hypothyroidism. Therefore, tests may need to repeated to finally reach a true diagnosis. Additionally, some dogs with other non-thyroid diseases may have artificially low T4 and T3 values due to metabolic changes without having hypothyroidism; these cases are called nonthyroidal illness. Additionally, thyroid hormones may be decreased due to the use of certain medications (such as steroids, phenobarbital, clomipramine, and sulfonamide antibiotics). Furthermore, some dog breeds such as sighthounds (greyhounds, Irish wolfhounds, Afghan hounds, whippets, etc.) have normally lower T4 levels than other dog breeds. Moreover, some dogs with autoimmune lymphocytic thyroiditis may have detectable autoantibodies prior to the development of hypothyroidism (meaning that a dog’s thyroid gland may be attacked by the immune system yet not to the point of dysfunction and low hormone production).

Other common bloodwork changes occurring with hypothyroidism include a mild anemia (low red blood cells), mildly low sodium level, high cholesterol, and a mildly elevated creatinine level (due to decreased blood flow to the kidneys caused by a slower metabolic rate).

If blood tests are not aiding your veterinarian in reaching a confirmed diagnosis of hypothyroidism despite being suspicious of this endocrine disease, a trial of a thyroid hormone supplement may be suggested to see if your dog improves on this medication. If your dog’s clinical signs resolve with such a medication trial, the diagnosis of hypothyroidism is likely accurate. Read on to learn more about the treatment of hypothyroidism.

And click here to learn how to interpret your pet’s bloodwork.

Q: WHAT Is THE TREATMENT FOR HYPoTHYROIDISM? Can it be cured?

A: Hypothyroidism cannot be cured. Instead, it can only be managed with supplementation of thyroid hormones. Luckily, this therapy is relatively simple with a good response rate and minimal side effects. 😀 The thyroid hormone supplement is a synthetic form of T4 (thyroxine), known as synthetic L-thyroxine or levothyroxine. It is administered orally once or twice daily and requires consistent, life-long administration. Following the start of levothyroxine therapy, you should expect to see rapid improvements in your dog’s energy level within 1 – 2 weeks with full resolution within a couple of months. (Many owners are happily surprised that their “old” dog suddenly acts like a playful puppy again!) Skin and hair abnormalities may require several months to resolve, and any neurologic abnormalities may take even longer to improve. Megaesophagus may unfortunately never resolve, requiring proper feeding management to avoid aspiration and the development of potentially life-threatening pneumonia.

Q: WHY DOES MY VET RECOMMEND RECHECKS?

A: After starting levothyroxine, your dog’s bloodwork should be repeated in 6 – 8 weeks to re-evaluate thyroid hormone levels. Additionally, your vet will also want to repeat a physical examination to assess improvement in any previously noted abnormalities. If the hormonal levels are still low, your dog’s levothyroxine dosage may need to be increased. If the levels are higher than normal (meaning that your dog has become temporarily hyperthyroid), the levothyroxine may need to be backed down a bit. Hyperthyroidism (or thyrotoxicosis) is the only side effect of levothyroxine. Signs of hyperthyroidism include increased energy level, weight loss despite an increased appetite, increased urination and water consumption (respectively termed polyuria and polydipsia, or PU/PD), panting, and increased heart rate. (Remember, the goal in hypothyroidism is to find a dosage to make your dog euthyroid — in a state in which the thyroid hormone levels are normal.) Once at a normal level, your vet may recommend reassessing bloodwork every 6 – 12 months to ensure adequate therapy. Hypothyroid dogs that have developed long-standing megaesophagus or other neurologic consequences may require more frequent rechecks. Breeding animals of the at-risk dog breeds listed above should be screened every 1 – 2 years since hypothyroidism can affect fertility as well as fetal development.

This Q & A session has ideally addressed your major concerns about Canine Hypothyroidism — which, as you’ve learned, doesn’t have to be such a “tragic” endocrine disease after all. For an individualized diagnostic and treatment plan, ask your primary veterinarian for guidance for your dog should he or she be diagnosed with hypothyroidism.

Still have some questions on this metabolic disorder? Feel free to contact us and follow us on Facebook to have your dog and cat health or safety questions addressed. As always, remember to share with your pet-loving friends!

– Maranda Elswick, DVM

And for you U.K. readers out there, every time you hear “Tragedy” by Steps from now on, I hope you think of the tragic facial expression of Canine Hypothyroidism!