Hypothyroidism

Hypothyroidism is the most common endocrine disorder affecting dogs, characterized by insufficient production of thyroid hormones by the thyroid gland. The thyroid gland, located in the neck alongside the trachea, produces hormones that regulate metabolism throughout the body, affecting virtually every organ system. When thyroid hormone production falls below the body's requirements, metabolic processes slow, leading to a wide array of clinical signs that can affect energy levels, body weight, skin and coat condition, and numerous other functions. This condition typically develops in middle-aged dogs, with most cases diagnosed between four and ten years of age.

The thyroid gland produces two primary hormones, thyroxine (T4) and triiodothyronine (T3), which travel through the bloodstream to regulate cellular metabolism throughout the body. These hormones influence how cells use energy, generate heat, and perform their specific functions, making them essential for normal health and vitality. In hypothyroidism, the reduced hormone levels cause cells to function at a slower pace, which manifests as the characteristic symptoms of lethargy, weight gain, cold intolerance, and various other changes. The condition develops gradually over months to years as thyroid function progressively declines.

The impact of hypothyroidism on affected dogs can range from subtle changes that owners may attribute to normal aging to significant alterations in health and quality of life. Dogs with hypothyroidism typically appear less energetic and may seem depressed or disinterested in activities they previously enjoyed. Weight gain occurs despite unchanged or even reduced food intake, as the slowed metabolism reduces caloric requirements. Skin and coat changes are common and can be quite striking, including hair loss, thickening of the skin, and increased susceptibility to infections. Untreated hypothyroidism can affect the heart, nervous system, reproductive function, and other organs, though serious complications are uncommon with appropriate treatment.

Fortunately, hypothyroidism is readily treatable with synthetic thyroid hormone supplementation, and most dogs return to normal health with proper medication. Daily oral administration of levothyroxine replaces the hormone the thyroid can no longer produce adequately, restoring normal metabolic function. Treatment is lifelong, as the underlying thyroid damage cannot be reversed, but the medication is relatively inexpensive and has minimal side effects when dosed appropriately. With consistent treatment and periodic monitoring, hypothyroid dogs typically enjoy normal lifespans and excellent quality of life, making early diagnosis and proper management well worth the effort.

The primary cause of hypothyroidism in dogs is lymphocytic thyroiditis, an autoimmune condition in which the body's immune system attacks and progressively destroys the thyroid gland. In this process, immune cells infiltrate the thyroid tissue and produce antibodies against thyroid proteins, leading to inflammation and gradual replacement of functional thyroid tissue with scar tissue. This autoimmune destruction typically occurs over one to three years before clinical signs become apparent, as the thyroid gland has substantial reserve capacity and symptoms only develop once approximately seventy-five percent of function is lost. The triggers that initiate this autoimmune process are not fully understood but likely involve a combination of genetic susceptibility and environmental factors.

Genetic factors play a significant role in susceptibility to hypothyroidism, with clear breed predispositions suggesting inherited risk factors. Golden Retrievers, Doberman Pinschers, Irish Setters, Dachshunds, Cocker Spaniels, and Airedale Terriers are among the breeds with elevated risk. The inheritance pattern appears to be polygenic, meaning multiple genes contribute to susceptibility. Some dogs inherit a tendency toward autoimmune disease in general, which may manifest as thyroiditis or other immune-mediated conditions. Research has identified associations between certain immune system genes and hypothyroidism risk, though genetic testing for susceptibility is not yet widely available. Breeders of predisposed breeds increasingly screen for thyroid function before breeding.

Idiopathic thyroid atrophy represents the second major cause of canine hypothyroidism, accounting for a smaller percentage of cases than autoimmune thyroiditis. In this form, the functional thyroid tissue gradually degenerates and is replaced by fat cells without evidence of immune-mediated destruction. The cause of this atrophy is unknown, hence the term idiopathic. The end result is the same as lymphocytic thyroiditis: insufficient thyroid tissue remains to produce adequate hormone levels. Whether this represents a distinct disease process or end-stage autoimmune disease where inflammatory evidence has resolved is debated among researchers.

Secondary and tertiary hypothyroidism, caused by problems with the pituitary gland or hypothalamus respectively, are rare in dogs but do occur. These conditions result from inadequate stimulation of the thyroid gland rather than thyroid disease itself. Pituitary tumors, congenital pituitary abnormalities, or other conditions affecting the brain regions that regulate thyroid function can lead to decreased thyroid-stimulating hormone production and subsequent thyroid underactivity. These forms of hypothyroidism require different diagnostic approaches and may indicate more serious underlying disease.

The mechanism of hypothyroidism development involves progressive loss of thyroid function until a critical threshold is crossed. The thyroid gland normally produces hormone in response to thyroid-stimulating hormone from the pituitary gland, which in turn is regulated by the hypothalamus in a feedback loop. As thyroid tissue is destroyed, the remaining cells work harder to maintain hormone levels, and the pituitary produces more TSH to stimulate them. This compensation can maintain near-normal thyroid hormone levels for an extended period. Eventually, the remaining thyroid tissue cannot meet the body's needs despite maximal stimulation, and circulating hormone levels fall. Once hormone levels drop below the threshold needed for normal cellular function, the clinical signs of hypothyroidism appear and progressively worsen without treatment.

The earliest warning signs of hypothyroidism are often subtle and develop so gradually that they may be attributed to normal aging or other causes. Owners frequently notice that their dog seems to have less energy than before, preferring to rest rather than engage in play or exercise. This decrease in activity often precedes other symptoms by months and may be dismissed as the dog simply slowing down with age. Mild weight gain despite stable food intake can occur early, as the slowing metabolism requires fewer calories. A slight dulling of the coat or increased shedding may be noticed by observant owners. These early changes are easily overlooked but can provide important diagnostic clues when recognized.

The most commonly observed symptoms of established hypothyroidism involve changes in metabolism, body condition, and energy levels. Weight gain is one of the most consistent findings, occurring in approximately forty to sixty percent of hypothyroid dogs. This weight gain happens despite unchanged or even reduced food intake, as the slowed metabolism dramatically decreases caloric needs. Lethargy and exercise intolerance are equally common, with affected dogs often reluctant to exercise and quick to fatigue when they do. Mental dullness or depression is frequently reported, with dogs seeming less alert, interactive, or interested in their surroundings. Cold intolerance develops as reduced metabolism decreases heat production, causing dogs to seek warm spots and shiver more readily.

Skin and coat changes are particularly characteristic of hypothyroidism and often prompt veterinary consultation. Hair loss occurs in approximately sixty to eighty percent of cases, typically in a bilaterally symmetric pattern affecting the trunk, tail, and areas of friction such as the neck under the collar. The classic rat tail appearance, where hair loss is particularly prominent on the tail, is highly suggestive of hypothyroidism. The remaining coat often becomes dull, dry, and brittle, losing its normal luster. The skin may thicken and become cool to the touch, developing a doughy texture. Hyperpigmentation, darkening of the skin, and comedones, blackhead-like lesions, frequently appear. Recurrent skin infections and ear infections are common secondary complications.

Less common but significant symptoms affect various organ systems and can provide diagnostic clues. Facial changes including a thickened, puffy appearance, sometimes described as a tragic facial expression, result from accumulation of mucopolysaccharides in the tissues. Reproductive abnormalities include infertility in females, failure to cycle, prolonged intervals between heat cycles, and in males, decreased libido and sperm abnormalities. Cardiovascular effects include slow heart rate and weak pulse, though these are often subclinical. Neurological manifestations range from mild dullness to more concerning signs including weakness, difficulty walking, facial nerve paralysis, and vestibular dysfunction causing head tilts and balance problems.

Symptom progression in hypothyroidism is typically gradual, occurring over months to years rather than days or weeks. The slow onset allows owners and even the dog to adapt to changes, which may delay recognition that a problem exists. As the condition progresses, symptoms become more numerous and severe. Dogs may become markedly obese, profoundly lethargic, and extensively affected by skin disease. Some develop myxedema coma, a rare but life-threatening complication of severe, prolonged hypothyroidism characterized by profound weakness, decreased consciousness, and dangerously low body temperature. More commonly, the cumulative effects of multiple symptoms significantly impair quality of life before diagnosis is made.

Emergency situations related to hypothyroidism are rare but do occur. Myxedema coma, though uncommon, requires immediate intensive care including thyroid hormone replacement, warming, and supportive treatment. More frequently, secondary complications of hypothyroidism may prompt urgent care, such as severe skin infections requiring aggressive antibiotic treatment. Dogs with hypothyroidism-associated laryngeal paralysis may develop acute breathing difficulties requiring emergency intervention. Hypothyroidism can also worsen other conditions, so any acute illness in a known hypothyroid dog warrants prompt veterinary attention. If an underlying hypothyroidism is discovered during workup for another condition, both problems should be addressed appropriately.

The diagnostic process for hypothyroidism typically begins when clinical signs prompt veterinary evaluation or when screening blood work reveals suggestive abnormalities. During the initial examination, the veterinarian will obtain a detailed history focusing on changes in energy, weight, appetite, skin and coat condition, and behavior. Physical examination may reveal obesity, cool skin, thickened skin texture, symmetric hair loss, a dull coat, a slow heart rate, or other signs consistent with hypothyroidism. The thyroid gland itself is rarely palpable in dogs, unlike in cats where thyroid nodules can often be felt. The combination of compatible clinical signs and signalment, including breed and age, raises suspicion for hypothyroidism and guides diagnostic testing.

Blood testing forms the foundation of hypothyroidism diagnosis, with thyroid hormone levels and related parameters providing essential information. Total T4, the most commonly measured parameter, is decreased in hypothyroid dogs but can also be lowered by non-thyroidal illness, certain medications, and normal fluctuations. Free T4 measured by equilibrium dialysis is less affected by these factors and provides more reliable results. Thyroid-stimulating hormone measurement helps differentiate primary thyroid failure from other causes of low T4. In primary hypothyroidism, TSH is typically elevated as the pituitary attempts to stimulate the failing thyroid. Thyroglobulin autoantibodies may be detected in cases of lymphocytic thyroiditis, confirming autoimmune disease. A comprehensive thyroid panel including multiple parameters provides the most accurate diagnostic information.

Complete blood count and chemistry panel results often show changes consistent with hypothyroidism and support the diagnosis. Mild, non-regenerative anemia is present in approximately thirty percent of cases, resulting from decreased erythropoietin production and reduced oxygen demands. Elevated cholesterol and triglycerides occur in approximately seventy-five percent of hypothyroid dogs due to altered lipid metabolism. Mild elevation of a muscle enzyme, creatine kinase, may be seen. These findings are not specific for hypothyroidism but add to the overall clinical picture. Normal results do not rule out hypothyroidism, as many affected dogs have unremarkable routine blood work.

Differential diagnosis requires distinguishing hypothyroidism from other conditions causing similar symptoms or laboratory findings. Non-thyroidal illness syndrome, sometimes called sick euthyroid syndrome, can suppress thyroid hormone levels in dogs that do not have primary thyroid disease. Any significant illness, including infections, organ failure, and cancer, can cause this phenomenon. Certain medications, particularly corticosteroids and phenobarbital, lower T4 levels in dogs with normal thyroid function. Cushing's disease produces similar symptoms of weight gain, hair loss, and lethargy while also suppressing thyroid hormone levels. The timing of testing relative to recent illness or medication use must be considered. When diagnosis remains uncertain, a therapeutic trial of thyroid supplementation with careful monitoring of response may be considered, though definitive diagnosis before treatment is preferred.

Treatment for hypothyroidism is straightforward and highly effective, consisting of oral thyroid hormone replacement administered daily for the remainder of the dog's life. Levothyroxine, synthetic T4, is the standard medication and is available in various strengths to allow precise dosing. Treatment typically begins with a dose calculated based on the dog's weight, usually administered twice daily initially. Some dogs are eventually maintained on once-daily dosing, which improves compliance for some owners, while others require the twice-daily schedule for optimal control. The medication is inexpensive, widely available, and can be given with or without food, though consistency is recommended.

Dosing optimization occurs over the first few months of treatment through monitoring and adjustment. Initial follow-up typically occurs four to eight weeks after starting treatment, at which time a blood sample is taken four to six hours after the morning dose to measure peak T4 levels. This timing captures the highest hormone level achieved after each dose. The goal is to achieve T4 levels in the upper half of the normal range or slightly above at this peak time. If levels are lower, the dose is increased; if they are higher than necessary, a reduction may be considered. Further adjustments are made based on subsequent testing until stable, appropriate levels are achieved.

Clinical response to treatment is often dramatic and gratifying for both owners and dogs. Increased energy and improved demeanor are typically among the first changes noticed, often within the first one to two weeks of treatment. Mental brightness returns, with dogs becoming more alert, interactive, and interested in their surroundings. Weight loss begins once metabolism normalizes, though this process takes weeks to months depending on the degree of obesity. Skin and coat improvements occur more slowly, with initial changes visible at six to eight weeks and full coat regrowth taking three to six months or longer. Some changes, such as facial puffiness, may resolve within weeks.

Supportive care during treatment addresses secondary issues and optimizes recovery. Dogs with skin infections require appropriate antimicrobial therapy, which may need to continue until thyroid levels are normalized and skin immunity improves. A gradual increase in exercise as energy levels improve helps with weight management and reconditioning. Dietary adjustments may be appropriate, as caloric needs increase with normalized metabolism; dogs should not be severely calorie restricted during treatment as their requirements are changing. Regular grooming helps remove loose hair as the coat regenerates. Ear infections, if present, should be treated appropriately.

Long-term treatment considerations include ongoing monitoring, medication sourcing, and recognizing signs that adjustments may be needed. Periodic thyroid level checks, typically every six to twelve months in stable dogs, confirm continued adequate control. Some owners notice the return of symptoms when hormone levels become suboptimal, providing an early warning that reevaluation is needed. Weight changes may necessitate dose adjustments, as dosing is weight-based. Different levothyroxine products have varying bioavailability, so consistency in medication brand or formulation is recommended. The medication should be stored properly and given consistently for optimal results.

Treatment success rates for hypothyroidism are excellent, with the vast majority of dogs responding well to appropriate hormone replacement. Dogs typically return to their normal energy levels, achieve appropriate body weight, and regrow healthy coats with consistent treatment. Owners often comment that their dog seems years younger after treatment. Side effects from levothyroxine are uncommon when dosing is appropriate but can occur if doses are too high, manifesting as signs of hyperthyroidism including restlessness, panting, increased thirst and urination, weight loss, and rapid heart rate. These resolve when the dose is reduced appropriately.

The recovery timeline for hypothyroidism varies by symptom, with some improvements appearing within days and others taking months to fully resolve. Energy and demeanor typically improve first, often noticeably within one to two weeks of starting treatment as hormone levels rise and metabolism begins normalizing. Mental clarity returns during this early phase, with dogs appearing more alert and engaged. Appetite may decrease as the abnormally increased hunger associated with metabolic dysfunction normalizes. These early improvements are encouraging for owners and confirm appropriate response to treatment.

Post-treatment care during the recovery phase focuses on monitoring response and gradually reintroducing normal activities. Owners should observe for improvements in energy, demeanor, and activity levels, reporting their observations at follow-up appointments. As energy returns, dogs can gradually increase exercise levels, starting with short walks and building duration and intensity as conditioning improves. Weight loss should be gradual and steady; crash dieting is inappropriate as metabolic needs are changing. Follow-up blood testing at prescribed intervals confirms appropriate thyroid levels and guides any needed dose adjustments. Most dogs achieve optimal control within three to six months of starting treatment.

Prognosis for dogs diagnosed with hypothyroidism is excellent with appropriate treatment. The condition does not affect lifespan when properly managed, and most dogs return to completely normal health and function. Quality of life is typically excellent, with well-controlled hypothyroid dogs indistinguishable from healthy dogs. The underlying thyroid damage is permanent, requiring lifelong medication, but this is usually a minor inconvenience once routine is established. Dogs that were severely affected before diagnosis may have some permanent effects, such as residual neurological changes, but this is uncommon with reasonably timely diagnosis.

Long-term outlook for treated hypothyroid dogs is highly favorable, with most enjoying normal lifespans and excellent quality of life. Ongoing medication is required indefinitely, but the pills are easy to administer, inexpensive, and have minimal side effects. Periodic monitoring ensures continued appropriate control, with adjustments made as needed based on blood levels and clinical signs. Weight management may require attention, as previously hypothyroid dogs may be prone to obesity. Secondary conditions such as skin infections typically resolve completely with treatment of the underlying thyroid disorder and any needed supportive care.

Primary prevention of hypothyroidism is not possible given the autoimmune nature of most cases, but awareness of risk factors and early detection can minimize the condition's impact. Owners of predisposed breeds should be familiar with hypothyroidism symptoms and report any changes in energy, weight, skin, or behavior promptly to their veterinarian. Routine wellness examinations that include thyroid screening can detect abnormalities before clinical signs become severe. While the disease cannot be prevented, early treatment prevents the development of more advanced symptoms and associated complications.

Breeding considerations play a role in reducing hypothyroidism prevalence in future generations. Responsible breeders of predisposed breeds increasingly include thyroid testing in their health screening protocols. Dogs with confirmed hypothyroidism should not be bred, as the genetic susceptibility may be passed to offspring. Testing breeding stock for thyroglobulin autoantibodies can identify dogs in early stages of thyroiditis before clinical disease develops, allowing them to be removed from breeding programs. Organizations such as the Orthopedic Foundation for Animals maintain registries of thyroid test results, allowing breeders to make informed decisions. Puppy buyers seeking breeds with elevated hypothyroidism risk should inquire about thyroid testing in the parents and their lines.

Nutritional considerations for thyroid health are relatively straightforward, as the thyroid gland requires adequate iodine to produce hormones. Commercial dog foods provide appropriate iodine levels, making deficiency-related hypothyroidism essentially nonexistent in dogs eating complete and balanced diets. Excessive iodine supplementation is not beneficial and could potentially cause harm. Some goitrogenic foods, which can interfere with thyroid function, are occasionally mentioned as concerns, but normal feeding of commercial diets or balanced homemade diets does not pose risks. Overall nutritional adequacy supports general health, including thyroid function.

Regular veterinary care enables early detection and intervention for dogs developing hypothyroidism. Annual or biannual wellness examinations provide opportunities for assessment of changes that might indicate thyroid disease. Baseline thyroid testing in young adults of predisposed breeds establishes normal values for that individual, making subtle decreases easier to detect later. Any dog showing compatible symptoms should be evaluated promptly rather than attributing changes to aging. Monitoring dogs on medications known to affect thyroid values, such as phenobarbital, helps distinguish medication effects from true thyroid disease.

Early intervention when hypothyroidism is detected prevents progression of symptoms and preserves quality of life. Beginning treatment as soon as diagnosis is confirmed prevents further deterioration and starts the recovery process. Dogs treated early experience faster and more complete resolution of symptoms compared to those with long-standing untreated disease. Early treatment may also prevent secondary complications such as severe obesity, extensive skin disease, or neurological manifestations. The excellent treatability and favorable prognosis of hypothyroidism make early detection and intervention a worthwhile investment in the dog's health.

Daily management of a hypothyroid dog primarily involves consistent medication administration, which most owners find straightforward to incorporate into their routine. Levothyroxine tablets are typically given once or twice daily, with many owners finding it convenient to administer medication at mealtimes or other consistent daily events. The tablets can be given with food if this improves compliance, though some sources recommend consistent timing relative to meals. Setting phone reminders or using pill organizers helps prevent missed doses. The medication is generally very well tolerated, making daily administration uneventful for most dogs.

Home environment considerations for hypothyroid dogs are minimal once treatment has normalized thyroid function. During the initial recovery phase, providing a warm, comfortable resting area accommodates any residual cold intolerance. As energy returns, dogs can resume normal activities and often appreciate increased opportunities for play and exercise. Feeding may need adjustment as metabolism normalizes; dogs previously fed reduced amounts due to weight concerns may need appropriate increases, while those that gained significant weight may benefit from a gradual weight loss program. Once stable on treatment, hypothyroid dogs require no special environmental accommodations.

Maintaining quality of life for hypothyroid dogs involves monitoring for any return of symptoms and ensuring continued medication compliance. Well-treated dogs should have normal energy levels, maintain appropriate body weight, and have healthy skin and coats. Any recurrence of lethargy, weight gain, or skin problems should prompt reevaluation of thyroid control. Owners sometimes become complacent about medication administration once their dog appears healthy, but consistent treatment remains essential. Missing doses or discontinuing medication will result in return of symptoms, as the underlying thyroid deficiency persists.

Ongoing monitoring through home observation and veterinary visits ensures continued optimal thyroid control. Owners should note any changes in energy, weight, appetite, or coat condition, as these may indicate the need for dose adjustment. Regular weigh-ins help track body condition. Veterinary rechecks with thyroid level measurement, typically every six to twelve months in stable dogs, confirm appropriate hormone levels. Dosing may need adjustment over time due to changes in weight, absorption, or medication formulation. Annual wellness examinations assess overall health and screen for other conditions that may develop with age.

Caregiver considerations for hypothyroid dog owners are generally manageable, as the condition is highly treatable and maintenance requirements are modest. The main ongoing commitment is daily medication administration, which becomes routine for most owners. Medication costs are typically low, and the expense of periodic blood monitoring is reasonable. Owners traveling can easily bring medication along or arrange for pet sitters to administer it. Some owners benefit from connecting with online communities of hypothyroid dog owners for support and information. The excellent prognosis and relatively simple management of hypothyroidism make it one of the more manageable chronic conditions in veterinary medicine.

Medium to large breed dogs are disproportionately affected by hypothyroidism, with several breeds showing markedly elevated risk compared to the general dog population. Golden Retrievers are among the most commonly affected breeds, with studies suggesting prevalence rates significantly higher than mixed breed dogs. Doberman Pinschers demonstrate similarly elevated risk and may be particularly prone to the autoimmune form of the disease. Other breeds with well-documented increased risk include Irish Setters, Great Danes, Boxers, Dachshunds, Airedale Terriers, Old English Sheepdogs, and Cocker Spaniels. The consistent breed patterns across multiple studies strongly indicate genetic susceptibility factors, though the specific genes involved have not been fully characterized.

Moderate-risk breeds and categories warrant heightened awareness for hypothyroidism symptoms. Many of the large sporting and working breeds beyond those mentioned above show moderately elevated risk. Beagles and Miniature Schnauzers are sometimes included in at-risk lists. Mixed breed dogs can inherit susceptibility from purebred ancestors. The condition affects both males and females, with some studies suggesting slight predominance in spayed females, though this may relate to age at diagnosis rather than true sex predisposition. Middle-aged to older dogs are most commonly diagnosed, with peak incidence typically between four and ten years of age.

Screening recommendations for at-risk breeds include baseline thyroid testing in young adults and periodic retesting as dogs age into the higher-risk period. Breeders of predisposed breeds should test breeding stock for thyroid function and thyroglobulin autoantibodies. The Orthopedic Foundation for Animals provides thyroid evaluation certification for dogs that pass screening criteria, and results are maintained in a public database. Puppy buyers can inquire about thyroid testing in the puppy's parents and relatives. Early identification of thyroiditis, even before clinical signs develop, allows monitoring and prompt treatment initiation when needed. Owners of at-risk breeds should familiarize themselves with hypothyroidism symptoms and report any concerns promptly.

Hypothyroidism frequently coexists with other conditions, and understanding these relationships improves comprehensive management. Obesity is present in the majority of hypothyroid dogs at diagnosis and may persist if not specifically addressed during treatment. The metabolic slowdown of hypothyroidism dramatically reduces caloric needs, leading to weight gain that often continues until treatment normalizes metabolism. Skin conditions including bacterial infections, yeast infections, and seborrhea are common secondary findings resulting from impaired skin immunity and altered skin oil production. These typically resolve with thyroid treatment but may require concurrent antimicrobial therapy. Hyperlipidemia, elevated blood fats, accompanies most cases and can contribute to other problems including pancreatitis in susceptible dogs.

Several conditions produce symptoms that overlap with hypothyroidism, requiring differentiation for accurate diagnosis. Cushing's disease causes lethargy, weight gain, and skin changes similar to hypothyroidism, and the two conditions can coexist. Non-thyroidal illness can suppress thyroid hormone levels without true thyroid disease, complicating diagnosis in sick dogs. Medications including corticosteroids and phenobarbital lower measured thyroid levels. Alopecia from other causes, including seasonal flank alopecia, pattern baldness, and hormonal imbalances, may be confused with hypothyroid hair loss. Careful diagnostic evaluation distinguishes hypothyroidism from these conditions, allowing appropriate treatment.

Complications and associated conditions extend the impact of hypothyroidism beyond metabolic and skin manifestations. Megaesophagus, enlargement of the esophagus causing regurgitation, has been associated with hypothyroidism in some dogs and may improve with thyroid treatment. Laryngeal paralysis, affecting the larynx and causing voice changes and breathing difficulties, occurs with increased frequency in hypothyroid dogs. Vestibular disease causing head tilts and balance problems may be more common in affected dogs. Facial nerve paralysis, causing facial drooping, is an occasional neurological manifestation. Reproductive abnormalities affect fertility and breeding success in intact dogs. Most of these complications improve or resolve with appropriate thyroid treatment, though some neurological changes may be slow to reverse.