What Are Signs of Low Thyroid in Dogs? (Hypothyroidism)

When to call the vet: low thyroid usually develops slowly rather than suddenly, so most dogs need a booked appointment rather than an emergency run. Call your veterinarian within a few days if you notice thinning hair, a dull or scaly coat, repeat ear or skin infections, weight gain without extra food, or a dog who is slowing down and seeking heat. Seek emergency care straight away if your dog collapses, cannot rise or stay standing, feels unusually cold, is shivering uncontrollably, becomes confused or unresponsive, has a seizure, is breathing with obvious effort, or develops sudden neurological signs such as a head tilt, stumbling or a dragging limb. Those signs are not typical of mild hypothyroidism, and they always warrant immediate veterinary attention whatever the underlying cause.

Signs of low thyroid in dogs show up in four places first: the coat, the skin, the waistline and the energy level. A dog with hypothyroidism often develops a dull, dry, thinning coat with hair loss that is roughly symmetrical across the flanks, trunk, tail and backs of the thighs. The skin may turn flaky, greasy, thickened or darker in patches, and ear and skin infections start recurring for no obvious reason. Weight creeps up even though the food bowl has not changed. Walks get shorter. The dog starts hunting for the sunny spot or the heating vent because cold bothers him more than it used to.

Hypothyroidism means the thyroid gland is no longer releasing enough thyroid hormone. That hormone sets the metabolic pace of nearly every tissue in the body: skin and hair follicles, muscle, heart rate, nerve conduction, fat metabolism, body temperature regulation. When the signal drops, everything downstream runs slow. That is why the symptom picture looks so scattered at first glance and so coherent once you understand the mechanism.

Low thyroid is diagnosed with bloodwork, not with a symptom list, but the symptom list is what gets your dog to the blood draw.

The four changes owners notice first

Coat changes. The hair cycle depends on thyroid hormone to push follicles out of rest and into growth. When hormone falls, follicles stall. Hair that is shed does not get replaced, so thinning appears in the areas with the most friction and the least stimulation: the flanks, the chest, the bridge of the nose, the tail. A tail stripped of guard hairs is sometimes described as a rat tail. The remaining coat looks dull, dry and brittle, and clipped hair is slow to grow back. Crucially, this hair loss is usually not itchy on its own, which helps separate it from allergy.

Skin changes. Slowed skin turnover produces scale and seborrhea, sometimes a greasy coat with an odour. Pigment can darken in affected areas. The skin may feel thickened or doughy. Because the skin barrier and local immune response are both metabolically dependent, bacterial pyoderma and yeast or bacterial ear infections keep returning after every course of treatment clears them.

Weight gain without more food. Owners describe a dog who is eating the same amount, or even less, and still putting on a layer. Some dogs also develop a pot-bellied look or lose muscle tone over the back and hind limbs at the same time as they gain fat.

Low energy, exercise intolerance and cold seeking. Mental dullness, longer naps, reluctance on stairs, lagging on the second half of the walk, shivering in rooms that never used to bother him. This is the symptom most often written off as the dog simply getting older.

Less obvious symptoms that still trace back to the thyroid

Not every dog shows the textbook picture, and some show almost none of it. These variants are worth knowing because they are the ones that delay diagnosis.

Facial changes. Fluid-attracting molecules can accumulate in the skin, producing thickened facial skin, drooping eyelids and a heavy, downturned expression that veterinary texts describe as a tragic facial expression.

Cardiovascular signs. A slower than expected resting heart rate and weaker heart contraction are recognised findings, and your veterinarian may note them on exam before you notice anything at home.

Neurological and muscular signs. Peripheral nerve dysfunction has been associated with low thyroid status in dogs, which can look like weakness, an unsteady gait, dragging a paw, facial nerve droop, or vestibular signs such as a head tilt. Some dogs are stiff and slow to warm up in a way that resembles arthritis.

Eye changes. Altered fat metabolism can produce lipid deposits in the cornea, visible as a hazy or cloudy film.

Reproductive changes. In intact dogs, irregular or absent heat cycles and reduced fertility can occur.

Behaviour. Some owners report a dog who seems flat, withdrawn or uncharacteristically irritable. Behaviour change alone is a weak signal, but combined with coat and weight changes it belongs in the conversation with your veterinarian.

Bloodwork findings. Elevated cholesterol and a mild, non-regenerative anaemia often appear on routine panels in hypothyroid dogs, which is one reason low thyroid is sometimes first suspected during a wellness screen rather than a sick visit.

What else looks like an underactive thyroid

Several conditions produce overlapping pictures, and more than one can be present at once. This is the core reason symptom-spotting cannot replace testing.

Condition	Overlapping signs	What usually sets it apart
Hypothyroidism	Symmetrical hair loss, dull coat, weight gain, lethargy, cold seeking, recurrent skin infections	Hair loss typically non-itchy; appetite normal or reduced despite weight gain
Hyperadrenocorticism (Cushing's)	Hair loss, thin skin, pot belly, lethargy, skin infections	Marked increase in thirst, urination and appetite; panting
Osteoarthritis and normal ageing	Slowing down, stiffness, reluctance on stairs, shorter walks	Coat and skin usually normal; stiffness worst after rest, eases with movement
Flea, environmental or food allergy	Hair loss, scaling, ear and skin infections	Itch comes first and drives the hair loss; pattern is often paws, face, belly
Non-thyroidal illness	Low thyroid hormone values on bloodwork	Hormone values are suppressed by another illness, not by thyroid failure

That last row matters more than owners expect. Serious illness elsewhere in the body can push thyroid hormone values down without the thyroid gland being diseased at all. Certain medications, including some antibiotics, steroid treatments, anti-seizure treatments and some anti-inflammatories, can also shift thyroid test results. A single low value on a single test is not a diagnosis, and this is precisely the kind of nuance your veterinarian is trained to untangle.

How a veterinarian confirms it

Testing usually begins with a total T4, often alongside a general chemistry panel and complete blood count. If total T4 is low or borderline, the next step commonly includes free T4 measured by equilibrium dialysis and canine TSH, sometimes with thyroglobulin autoantibody testing. Reading those values together, in the context of the dog's clinical signs, current medications and any concurrent illness, is what separates true primary hypothyroidism from a temporarily suppressed result.

The underlying cause in most diagnosed dogs is destruction of thyroid tissue, either immune-mediated inflammation of the gland (lymphocytic thyroiditis) or progressive idiopathic atrophy. It is most often identified in middle-aged dogs, and several breeds are recognised as predisposed, including Golden Retrievers, Dobermann Pinschers, Irish Setters, Boxers, Cocker Spaniels, Dachshunds and Beagles. Breed and age raise suspicion; they do not make the call.

When the diagnosis is confirmed, your vet may prescribe medication, and treatment is usually lifelong, with follow-up bloodwork to confirm the dose is landing where it should. The prescription, the schedule and every adjustment to it belong to your veterinarian. Nothing in this article should be read as a reason to delay, change or stop a prescribed treatment. If something about your dog's medication or monitoring schedule is unclear, that question belongs with the veterinarian who wrote the prescription.

What happens to a dog with untreated low thyroid

Untreated, the effects compound slowly. The coat continues to thin and the skin barrier continues to weaken, so pyoderma and ear infections recur in cycles and each one costs another round of veterinary treatment. Cholesterol stays elevated. Weight continues to climb, and extra load on hips, stifles and the spine makes any existing joint disease harder to live with. Muscle condition deteriorates because the dog moves less, and moving less accelerates the muscle loss, a loop that is far easier to interrupt early than late.

Exercise intolerance deepens. Some dogs develop the neurological or cardiac signs described above. In rare and severe cases, profoundly low thyroid function can progress to a medical emergency involving collapse, hypothermia and reduced consciousness, which requires immediate veterinary intervention.

The more encouraging part of the picture is that this is one of the more manageable chronic diagnoses in canine medicine. Once treatment is established and monitored by your veterinarian, owners commonly report that energy and attitude shift before the coat does, since hair regrowth has to follow the follicle cycle. Results vary from dog to dog, and your veterinarian is the right person to set expectations for yours.

What to watch at home

Hypothyroid signs creep in over months, which is exactly why written records beat memory. A few simple habits make the next appointment far more productive:

• Photographs. Take the same few shots every few weeks: both flanks, the tail, the face, any bald or darkened patch. Side-by-side images show change that daily contact hides.
• Weight. Weigh on the same scale, at the same time of day, and write the number down. Note whether the food amount has actually changed.
• Energy and stamina. Track how far your dog walks before lagging, whether stairs are being avoided, and how long recovery takes after activity.
• Warmth seeking. Note shivering, curling up tightly, or new attachment to vents, radiators and sunny patches.
• Skin and ears. Log every flare: when it started, which ear or which area, how long it took to clear, and whether it came straight back.
• Appetite, thirst and urination. Increased thirst and urination point away from a simple thyroid picture and toward other conditions, so these are important either way.
• Gait and comfort. Stiffness, a dragging paw, a head tilt, or wobbling deserve a note and a prompt call.

What to tell your vet

Bring the record, not just the worry. Useful details include when you first noticed each change and in what order, whether the hair loss itches or the dog ignores it, how many skin or ear infections there have been in the past year and how each was treated, the weight trend, and any change in behaviour or heat cycles. Just as important: a complete list of everything your dog receives, including medications, flea and tick products, topicals, chews and any supplement, because several of these can influence thyroid test results. Mention any recent illness, surgery or injury, since other illness can temporarily depress thyroid values. If your dog is already on treatment, bring the dosing schedule, the time of the last dose and the date of the last bloodwork, because timing affects how results are interpreted.

When it becomes an emergency

Go to an emergency veterinary clinic without waiting for an appointment if your dog collapses, cannot stand or walk, becomes unresponsive or severely disoriented, feels very cold to the touch, has a seizure, is breathing with difficulty, or develops sudden severe weakness or neurological signs. Severe untreated thyroid disease can cause this kind of crisis, and so can several other serious conditions, which is one more reason the assessment has to happen in person. Call your veterinarian promptly, rather than waiting for a routine recheck, if a dog already on prescribed treatment becomes restless or anxious, pants excessively, loses weight rapidly, drinks far more than usual, or seems to have a racing heart. Those changes should be reported and assessed by the prescribing vet rather than managed at home.

Key takeaways

• The classic signs are symmetrical non-itchy hair loss, a dull or scaly coat, recurrent skin and ear infections, weight gain without increased appetite, lethargy and cold intolerance.
• Less obvious signs include a thickened or drooping facial expression, a slow heart rate, weakness or an unsteady gait, corneal cloudiness and irregular heat cycles.
• Elevated cholesterol and mild non-regenerative anaemia often show up on routine bloodwork before the owner connects the dots.
• Cushing's disease, allergy, arthritis and ageing all overlap with this picture, so testing, not symptom matching, makes the diagnosis.
• Other illnesses and certain medications can depress thyroid values without thyroid disease being present.
• Confirmed hypothyroidism is managed with lifelong treatment your vet prescribes, plus follow-up bloodwork.
• Collapse, severe weakness, very low body temperature, seizures or sudden neurological signs are emergencies.

Your veterinarian owns the diagnosis and the treatment plan here: the bloodwork interpretation, the prescription, the monitoring schedule and every adjustment to it. Nothing on this page replaces that relationship, and the fastest path forward for a dog showing these signs is an exam and a thyroid panel. Good home observation does not diagnose anything, but it gives the veterinary team better information to work with, and that is the most useful thing an owner can contribute.

Related guides: Signs of a Dog With Thyroid Problems : What to Look For and What Causes Thyroid Problems in Dogs? (Breeds & Prognosis).