CONDITION

Hypothyroidism in Dogs

A condition where the thyroid gland produces insufficient hormone, affecting metabolism, energy levels, weight, and coat quality.

Why this matters now

Hypothyroidism is the most common endocrine disorder in dogs, typically presenting in middle-aged dogs between 4 and 10 years of age. Medium to large breed dogs are most frequently affected, with Golden Retrievers, Doberman Pinschers, Irish Setters, Dachshunds, Cocker Spaniels, and Airedale Terriers among the breeds with documented higher prevalence. Both sexes are affected, though neutered dogs of either sex may be at slightly higher risk compared to intact individuals. The condition most commonly results from immune-mediated destruction of the thyroid gland (lymphocytic thyroiditis) or from progressive replacement of thyroid tissue with fatty infiltration (idiopathic thyroid atrophy), both of which gradually reduce the gland's capacity to produce adequate thyroid hormones. Because thyroid hormones influence metabolic rate throughout the body, the effects of deficiency are widespread and often develop so gradually that they are initially attributed to normal ageing.

Hypothyroidism develops gradually as thyroid tissue is progressively destroyed or replaced, meaning that the transition from normal thyroid function to clinical hypothyroidism often occurs over months to years. During this slow decline, the pituitary gland compensates by producing increasing amounts of thyroid-stimulating hormone (TSH), maintaining thyroid hormone levels within the normal range for as long as possible — a phase sometimes described as subclinical hypothyroidism. Once thyroid reserve is sufficiently depleted that compensation is no longer possible, circulating thyroid hormone levels fall and clinical signs begin to appear. The insidious nature of the onset means that by the time a diagnosis is sought, the condition may have been present for a considerable period. With appropriate management, the prognosis is generally excellent, as thyroid hormone supplementation can effectively restore normal metabolic function.

Signals & patterns

Early signals

Gradual weight gain

Weight gain despite no increase in food intake is one of the most commonly recognised early signs. The reduced metabolic rate means fewer calories are burned at rest, leading to progressive fat accumulation. This change often develops so gradually — over months — that owners may not connect it with illness, attributing it instead to reduced exercise, ageing, or neutering.

Decreased energy and lethargy

A general reduction in energy levels, manifesting as increased sleeping, reluctance to exercise, and reduced enthusiasm for activities the dog previously enjoyed, often accompanies the metabolic slowdown. Affected dogs may seem content to rest rather than engage in walks or play, and their overall demeanour may shift towards passivity. This 'mental dullness' is frequently attributed to normal ageing rather than recognised as a metabolic sign.

Coat and skin changes

Early dermatological changes may include a dull, dry, or brittle coat that loses its lustre. Hair may thin, particularly in areas of friction such as the collar region, tail base ('rat tail'), and flanks. The coat may fail to regrow promptly after clipping for surgery or grooming, as the slowed hair follicle cycling delays regrowth. These changes often develop symmetrically, affecting both sides of the body equally.

Cold intolerance

Reduced metabolic heat production can make hypothyroid dogs noticeably sensitive to cold. They may seek warm resting spots, press against radiators, prefer to lie in sunlight, or shiver in temperatures that previously did not bother them. This heat-seeking behaviour can be one of the earlier signs noticed by observant owners, particularly during cooler months.

Recurrent skin infections

The immunomodulatory effects of thyroid hormone deficiency can predispose to recurrent bacterial skin infections (pyoderma) or ear infections (otitis externa). These infections may respond to antibiotic treatment but recur once therapy is completed, establishing a pattern of relapsing infections that prompts investigation into underlying causes. The skin's compromised barrier function and reduced immune surveillance both contribute to increased susceptibility.

Later signals

Pronounced bilateral alopecia

As the condition progresses, hair loss becomes more extensive and obvious, typically following a bilateral, symmetrical pattern that spares the head and extremities. The flanks, trunk, and tail are most commonly affected, and the remaining coat may become coarse, dry, and easily epilated. Unlike many hormonal causes of alopecia, hypothyroid hair loss is typically non-pruritic (not itchy) unless complicated by secondary infection.

Facial changes and myxoedema

In more advanced cases, accumulation of mucopolysaccharides in the dermis can produce a characteristic facial puffiness known as the 'tragic facial expression'. The skin may become thickened, particularly on the face and eyelids, and skin folds may become more prominent. This myxoedematous change is distinctive when present but does not occur in all hypothyroid dogs.

Neurological signs

Peripheral neuropathy can develop in some hypothyroid dogs, manifesting as weakness, proprioceptive deficits (knuckling of the paws), facial nerve paralysis, or vestibular signs (head tilt, circling, nystagmus). Laryngeal paralysis and megaoesophagus have also been reported in association with hypothyroidism, though the causal relationship is debated in some cases. These neurological manifestations reflect the dependence of nerve tissue on adequate thyroid hormone levels for normal myelin maintenance and nerve conduction.

Cardiovascular changes

Reduced thyroid hormone levels decrease cardiac contractility and heart rate, which may be detectable as bradycardia on clinical examination. In severe or prolonged cases, echocardiographic changes including reduced fractional shortening and chamber dilation may develop. Hypercholesterolaemia, which commonly accompanies hypothyroidism, can contribute to atherosclerotic changes in some individuals, though clinically significant atherosclerosis is less common in dogs than in humans.

Click to read about the biological mechanisms

How this is usually investigated

Diagnosing hypothyroidism in dogs involves integrating clinical signs with laboratory findings, as no single test is definitive in isolation. The investigation typically combines thyroid-specific testing with assessment of the broader clinical picture, as various non-thyroidal factors can influence thyroid hormone levels.

Total T4 measurement

Purpose: Total T4 is the most commonly used initial screening test for hypothyroidism. A low total T4 value in a dog with compatible clinical signs supports the diagnosis, while a normal total T4 effectively excludes hypothyroidism in most cases. The test provides a good starting point for the diagnostic process and is widely available.
Considerations: Total T4 can be suppressed by numerous non-thyroidal factors, including concurrent illness (euthyroid sick syndrome), certain medications (glucocorticoids, phenobarbital, sulphonamides, NSAIDs), and natural physiological variation. A low T4 alone does not confirm hypothyroidism, as up to 20-30% of euthyroid sick dogs may show suppressed values. Conversely, a normal or high T4 in a dog with suggestive signs may indicate that hypothyroidism is unlikely, though thyroglobulin autoantibodies can occasionally interfere with some T4 assay methods.

Free T4 by equilibrium dialysis

Purpose: Free T4 measured by equilibrium dialysis represents the unbound, biologically active fraction of thyroid hormone and is less susceptible to suppression by non-thyroidal illness compared to total T4. This makes it a more reliable test in dogs with concurrent conditions that might artificially lower total T4 values. It is considered a more specific marker for true hypothyroidism when results are interpreted alongside clinical findings.
Considerations: Free T4 by equilibrium dialysis is more expensive than total T4 and requires specific laboratory capabilities with longer turnaround times. While more resistant to non-thyroidal suppression, it is not completely immune to these effects and should still be interpreted in clinical context. Some dogs with genuine hypothyroidism may show free T4 values near the lower end of the reference range rather than clearly below it, requiring integration with other diagnostic information.

Canine TSH (cTSH)

Purpose: Thyroid-stimulating hormone from the pituitary should be elevated in primary hypothyroidism, as the pituitary responds to low thyroid hormone levels by increasing TSH production in an attempt to stimulate the failing thyroid gland. The combination of low T4 with elevated TSH provides stronger diagnostic evidence than either test alone.
Considerations: Canine TSH assays have limitations in sensitivity, with approximately 20-30% of hypothyroid dogs showing TSH values within the normal range despite genuine thyroid deficiency. This occurs partly because TSH secretion is pulsatile and can fluctuate, and partly due to assay sensitivity limitations. A normal TSH does not definitively exclude hypothyroidism, and an elevated TSH can occasionally be seen in recovery from non-thyroidal illness. The combination of low T4/free T4 with elevated TSH provides the strongest biochemical evidence for the diagnosis.

Thyroglobulin autoantibody (TgAA) testing

Purpose: TgAA testing detects antibodies directed against thyroglobulin, indicating the presence of autoimmune thyroiditis — the most common cause of hypothyroidism in dogs. A positive result confirms that an immune-mediated process is affecting the thyroid gland, though it does not by itself confirm that thyroid function has declined to the point of clinical hypothyroidism. TgAA testing can identify dogs in the early, pre-clinical phase of lymphocytic thyroiditis.
Considerations: TgAA positivity indicates autoimmune thyroid disease but may be present for months to years before clinical hypothyroidism develops. Not all TgAA-positive dogs will progress to clinical hypothyroidism within their lifetime. The test is most valuable for breed screening programmes and for identifying dogs at risk of developing hypothyroidism. In dogs already suspected of having clinical hypothyroidism, TgAA adds aetiological information but is not essential for diagnosis.

Complete blood count and biochemistry

Purpose: Routine blood work in hypothyroid dogs commonly reveals a characteristic pattern of changes including mild non-regenerative anaemia (reflecting reduced erythropoietin stimulation from decreased tissue oxygen demand), hypercholesterolaemia and hypertriglyceridaemia (from reduced hepatic lipid metabolism), and sometimes mildly elevated creatine kinase (from subclinical myopathy). These findings provide supportive evidence and help assess the broader metabolic impact.
Considerations: None of these routine blood work findings are specific to hypothyroidism, and they can be seen in various other conditions. However, the combination of characteristic clinical signs with compatible blood work abnormalities strengthens the diagnostic suspicion and supports proceeding with thyroid-specific testing. Hypercholesterolaemia is present in the majority of hypothyroid dogs and may be the most consistently identified routine blood work abnormality.

Options & trade-offs

Management of hypothyroidism in dogs is relatively straightforward compared to many endocrine conditions, centring on daily oral thyroid hormone supplementation. The primary decisions involve the specifics of supplementation and monitoring rather than choosing between fundamentally different management strategies.

Levothyroxine (synthetic T4) supplementation

Oral levothyroxine sodium is the standard treatment for canine hypothyroidism, providing synthetic T4 that the body can convert to the biologically active T3 as needed. The medication is typically administered twice daily initially, though some dogs can be managed with once-daily dosing once stable. The starting dose is based on body weight, with subsequent adjustments guided by therapeutic monitoring of serum thyroid levels and clinical response.

Trade-offs: Treatment is lifelong, requiring daily medication and periodic blood monitoring. Individual dose requirements can vary, and finding the optimal dose may take several weeks of adjustment. The timing of blood sampling relative to medication dosing affects interpretation of monitoring results, requiring coordination between the owner and veterinary team. Generic and brand formulations may have different bioavailability, so consistency in the product used is generally encouraged once an effective dose is established.

Liothyronine (synthetic T3) supplementation

Liothyronine provides synthetic T3 directly, bypassing the need for peripheral conversion from T4. This approach is occasionally considered when a dog appears to respond inadequately to levothyroxine despite appropriate dosing and monitoring, raising the possibility of impaired T4-to-T3 conversion. It is used much less frequently than levothyroxine and is generally reserved for specific clinical situations.

Trade-offs: T3 has a shorter half-life than T4, typically requiring more frequent dosing (two to three times daily). Monitoring is more complex, as T3 levels fluctuate more rapidly than T4 levels throughout the day. The evidence for conversion defects in dogs is limited, and most cases of apparent treatment failure with levothyroxine reflect dosing issues, compliance challenges, or incorrect diagnosis rather than genuine conversion problems.

Weight management alongside supplementation

Because hypothyroidism commonly causes weight gain, and excess weight carries its own health implications (joint stress, metabolic effects, reduced quality of life), active weight management alongside thyroid supplementation is often an important component of overall care. As the metabolic rate normalises with treatment, caloric requirements increase, and many dogs begin to lose weight without dietary restriction. However, some dogs may need a structured approach to caloric management, particularly if significant obesity has developed.

Trade-offs: Rapid weight loss following treatment initiation is generally well tolerated and expected, but the rate of change should be monitored. Some owners may continue feeding patterns established during the hypothyroid period, providing more calories than the now-normalised metabolism requires. Caloric intake may need to be adjusted once thyroid levels stabilise, and regular weight monitoring helps guide these adjustments.

Monitoring and dose optimisation

Ongoing monitoring is a core component of hypothyroidism management, typically involving blood sampling 4-6 hours post-medication to assess serum T4 levels at peak absorption. Initial monitoring occurs at 4-8 week intervals during dose establishment, transitioning to 6-12 monthly monitoring once the dog is stable and clinically well. Clinical response — improvement in energy, weight, coat quality, and resolution of skin problems — is evaluated alongside laboratory values.

Trade-offs: Monitoring requires periodic veterinary visits and blood tests, with associated costs and time commitments. The relationship between serum T4 levels and clinical wellbeing is not always linear, and some dogs feel well at different points within or slightly above the therapeutic range. Dose adjustments may be needed over time as individual requirements change, and concurrent medications or health conditions can influence thyroid hormone metabolism. The practical aspects of consistent daily medication and monitoring represent an ongoing commitment.

Common misconceptions

Misconception:

"Hypothyroidism only causes weight gain and laziness"

Reality:

While weight gain and lethargy are among the most commonly recognised signs, hypothyroidism affects virtually every organ system in the body. Dermatological changes (hair loss, recurrent infections, poor coat quality), neurological manifestations (peripheral neuropathy, vestibular signs, facial nerve paralysis), cardiovascular effects (bradycardia, reduced contractility), reproductive consequences (infertility, abnormal cycling), and metabolic disturbances (hypercholesterolaemia, anaemia) can all be part of the clinical picture. The breadth of potential effects reflects the fundamental role of thyroid hormones in regulating cellular metabolism throughout the body.

Misconception:

"A low T4 test result means a dog definitely has hypothyroidism"

Reality:

Total T4 can be suppressed by numerous non-thyroidal factors in what is known as the euthyroid sick syndrome. Concurrent illness of almost any nature, certain medications (particularly glucocorticoids, phenobarbital, and some NSAIDs), and even normal physiological variation can produce low T4 values in dogs with perfectly normal thyroid function. This is why the diagnosis of hypothyroidism relies on integrating clinical signs with multiple test results rather than any single laboratory value. Inappropriately diagnosing and treating hypothyroidism based on a single low T4 in a dog with a non-thyroidal illness is a recognised pitfall.

Misconception:

"Once treatment starts, the dog should improve immediately"

Reality:

While mental alertness and energy levels may begin to improve within the first one to two weeks of levothyroxine supplementation, many of the physical changes associated with hypothyroidism resolve more gradually. Weight loss may take several weeks to become apparent, skin and coat improvements typically require one to three months as new hair growth cycles through, and neurological signs, when present, may take several months to resolve and may not fully reverse in some cases. Setting realistic expectations for the timeline of improvement helps owners understand the gradual nature of recovery and avoid premature concern about treatment efficacy.

Understanding hypothyroidism involves recognising its characteristically gradual onset, which means many of its effects may have been present — and normalised — for some time before a diagnosis is reached. The good news is that hypothyroidism is one of the more manageable endocrine conditions, with thyroid hormone supplementation typically producing a noticeable improvement within weeks to months. The transformation that often accompanies appropriate treatment can be quite striking, with owners frequently commenting that they did not realise how much their dog had changed until the changes began to reverse. Ongoing monitoring helps ensure that supplementation remains at the right level, as individual requirements can shift over time.

Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS